When I say centrifugal, I mean centrifugal!

By Phil Plait | August 30, 2006 9:16 pm

In various places on my site and in this blog, I have mentioned centrifugal force. Invariably when I do so, someone pedantically comes along and says "That’s not a real force! It’s fictional! It’s really just centripetal force".

Let me clear and blunt here: that’s wrong. Centrifugal force is every bit as real as centripetal force. It’s just in a different frame. "Centripetal" means "center-seeking", and "centrifugal" means "outwards-seeking" or, more literally, "center-fleeing". You’d think these are opposites, but they are in fact the same thing! It just depends on your point of view.

If you are standing outside a spinning object, and then draw a diagram of the forces, yes, you’re better off using centripetal force. The math works out more easily. But if you’re on that spinning object, then the forces are easier to draw assuming a centrifugal force. Really!

Think of it this way: watch a car make a right turn. The people inside have inertia, and they "want" to stay moving forward. The car pushes them to the right, toward the center of the circle it is making. Centripetal.

Now sit inside that car. When the car turns right, which way do you feel yourself leaning? Toward the left, away from the center. You feel a force towards the outside of the circle.

Tadaaa! Same thing, different viewpoints.

Centripetal = centrifugal. Got it? They’re the same thing. If one is real, then so’s the other. Once more, just to make sure:

Centripetal = centrifugal.

Cripes, I hope I don’t ever have to explain that again. If I run across this non-issue again, I will link the perpetrator to this page and make them read it three times.

And if you don’t get it, don’t feel bad. James Bond didn’t either.

Comments (107)

  1. elgarak

    YES! Finally someone who agrees with me.

    I’m a physicist, and I hear this all the time. Now I have something to direct all the people who disagree with my explanation (along the same lines as yours).

  2. Ah xkcd is such a brilliant webcomic!

  3. csrster

    Dead right.

    However there is one big difference between inertial and
    non-inertial forces. Non-intertial forces do not obey Newton’s
    Third Law. Imagine firing a gun along a line of latitude. The bullet
    veers off-straight because of the Coriolis force. But the bullet doesn’t push anything in the opposite direction – there is no reaction.

  4. Michael

    After mentioning this on a different post in your blog, I did some searching, and discovered that several sites (including Wikipedia) have two definitions of “centrifugal force”.

    The first is the real force (and the definition I always knew), which is the reactionary force to the centripetal force. As such, centripetal != centrifugal, but |centripetal| = |centrifugal|. The bucket is held in a circle by the centripetal force of the string pulling on it, while the string is held taut by the centrifugal force of the bucket pulling right back.

    The second is the pseudo force, which is the perceived force flinging you to the outside while inside a rotating reference frame. It isn’t a true force (under classical mechanics, anyhow), but can be easily modeled as such and makes more sense based on experience, so it’s logical to use it in many places. As such, centripetal != centrifugal, but non-rotating centripetal = rotating centrifugal and rotating centripetal = non-rotating centrifugal. The centripetal force is the reactive force keeping you in your seat, while the centrifugal force is the active force pushing you against it.

    The key is to remember what reference frame you’re using, and stick with it. In either case, they are opposite forces, but it doesn’t usually matter–whether you say centripetal or centrifugal, in 99% of discussions everyone understands what you are talking about. Only when you are trying to model both directions at the same time does it become important to distinguish which means what.

  5. Tim G

    Keep centrifugal force out of our High School textbooks (or at least warning label on them). They make things unnecessarily complicated, Phil. Is centrifugal force real? Fine, but only in a rotating frame of reference (except if you’re discussing reaction to centrifugal force). Students would have a much easier time understanding forces if they analyze them in a frame that does not accelerate or rotate. The laws of physics are the same for all such frames.

    Remember the lesson on the trajectory of a projectile being a parabola? Well, it’s actually an ellipse because it is in orbit about the center of the Earth. But the whole point of assuming unidirectional gravity is to show how forces can be broken down into components and analyzed independently to come up with a solution. Why complicate things by deriving the shape of orbits? By the same token, why complicate things by including terms unnecessary for a fundamental understanding of physics? Undergraduate physics students can take a course in analytical mechanics in which they deal with rotating frames of reference. Everybody else should stick with non-rotating, non-accelerating frames because that is all engineers and laypeople need.

  6. I’ll agree with Phil that centrifugal and centripetal are the same force, but the name is really misleading in several cases. If we’re talking about the standard example of a sphere being twirled around with a rope, centripetal force is easy to understand cos you feel it in your hand, but applying the name “centripetal” to an orbitting spacecraft is more tricky, wouldn’t you say?

    The only force exerted on a spacecraft is Gravity (which you’ll agree that is centripetal by definition) and it also has a tangent velocity. Trying to explain Gravity as a centrifugal force, just creates unnecessary problems in comprehension of a circular motion in a force field (such as Gravity).

  7. csrster

    Michael: I’m not sure I’m with you. In a rotating frame, all bodies experience a centrifugal force. So how can you write “non-rotating centripetal = rotating centrifugal”? The non-rotating centripetal might
    be zero. In any case, the centripetal force, being a “real” force, is
    the same in all reference frames.

  8. “Keep centrifugal force out of our High School textbooks (or at least warning label on them). They make things unnecessarily complicated, Phil. Is centrifugal force real? Fine, but only in a rotating frame of reference (except if you’re discussing reaction to centrifugal force). Students would have a much easier time understanding forces if they analyze them in a frame that does not accelerate or rotate. The laws of physics are the same for all such frames.

    … Everybody else should stick with non-rotating, non-accelerating frames because that is all engineers and laypeople need.”

    I respectfully beg to differ. Considering how often the question comes up and how much physicists LOVE to make an issue of centripetal vs centrifigual force — usually delivered with a knowing smirk — I think it SHOULD be at least discussed in high school physics classes, even if it’s not part of the required curriculum. Students don’t necessarily need to master the calculations, but they do need to know that the calculations they are doing are for highly simplified situations. Letting them know there is something beyond the limited parameters they’re being taught is critical — and might even encourage more of them to take a few more science classes to figure out what they’re missing. It’s not a question of what they “need” to know; this is something they OUGHT to know.

    Frankly, I think students can make the distinction beween rotating and non-rotating frames of reference… And Phil, I’m clipping this for my own future reference, since I get asked the question a great deal. You know, by curious laypeople who don’t “need” to know the difference…

  9. Berkeley

    For every force, there is a counterforce of equal size. Isn’t that Newton?

  10. Joe

    I think the confusion all stems from the fact that in an accelerating frame the apparent force (actually comming from the acceleration of the frame) is known as a ‘fictitious force’. So the centrifugal force is a fictitious force.

    This does not mean it doesn’t exist (and thats where people go wrong, assuming it means the same as fictional), it does. It’s just a technical term.

    Wikipedia has more:

    http://en.wikipedia.org/wiki/Accelerated_reference_frame

  11. Think of it this way: watch a car make a right turn. The people inside have inertia, and they “want” to stay moving forward. The car pushes them to the right, toward the center of the circle it is making. Centripetal.

    Now sit inside that car. When the car turns right, which way do you feel yourself leaning? Toward the left, away from the center. You feel a force towards the outside of the circle.

    Tadaaa! Same thing, different viewpoints.

    Err… not really. You oversimplify and obfuscate

    The force of the seats in the car pushing on you exists in both frames. It is a “real” force in that it exists even in an inertial frame. It comes from the friction of the seats on your butt, or from the pushing of the side of the car on you through (ultimately) the electromagnetic interaction.

    The centrifugal force certainly is “real” in that in the accelerated frame of reference of the car, you are being pushed towards one side of the car. However, that force is present only in the accelerated frame of reference; it is not present in an inertial frame. (If you want to get all deep an Einstein Equivalence Principle about it, you could say that it is indistinguishable from *gravity*, but it’s not indistinguishable from the centripetal force you’re talking about.)

    There is a clear asymmetry here.

    I agree with you that centrifugal force is real, just like the Coriolis force is real, in that it’s there and has an effect if you’re working in a rotating frame of reference. However the two things you mention are not the “same thing” from different viewpoints at all. I also think it is very misleading to say “centrifugal=centripetal”. *IF* you make exactly the right frame transition, yes, the magnitude of the centrifugal force in one is the same as the magnitude of the centripetal force in the other. But it’s not really a Newton’s 3rd law sort of thing, even.

    -Rob

  12. BB

    No, centrifugal force is not a real force, since there is nothing pushing you to the outside of the car. The outside of the car is accelerating inward, while your inertia causes you to continue forward. The only reason that you perceive a force is because in the frame of reference of the car, you are accelerating; however, since that is not a valid newtonian frame of reference (only frames which are not accelerating are valid), F != ma (within that frame), and thus, you can’t assume that just because you have an acceleration that there is a force in the same direction.

  13. In fact… let me add one thing.

    “Centipetal” force, is in a sense less real than centrifugal force. If you working in a rotating frame of reference, you will feel the centrifugal force.

    Centripetal force doesn’t arise at all. It is what is needed to keep you rotating. When you’re in a car going around a curve, you feel the centrifugal force. You need a balancing centripetal force — the seats and the car pushing you the other way — to keep you from being flung out of the car by that centrifugal force.

    In the inertial frame of an outside observer, the car is turning, and there needs to be a force on you to make you turn with the car; otherwise, you will continue in a straight line, and diverge from the car.

    Centripetal force is just the name for the needed force; something has to physically supply that needed force. Centrifugal force, however, is always there if you’re in a rotating frame of reference.

    I think your basic point is right (centrifugal force is “real”), but I think you are making a big mistake by trying to say centripetal and centrifugal force are the same thing from different points of view.

    -Rob

  14. BB

    Look at it this way: If you’re in a car that happens to be accelerating forward, you feel a “force” pushing you backwards. Is that force real? Of course not, it’s just that your inertia is keeping you still while the car is accelerating forward. In the frame of reference of the car, you have an acceleration backward due to your inertia, but that doesn’t mean there is a backwards force acting on you, since again you are not in a valid inertial frame of reference. That “backward force” when you accelerate forward, and the “forward force” when you brake are no different from the “centrifugal force” when you turn, and none of them are real.

  15. Look at it this way: If you’re in a car that happens to be accelerating forward, you feel a “force” pushing you backwards. Is that force real? Of course not, it’s just that your inertia is keeping you still while the car is accelerating forward.

    So are you or aren’t you being pushed towards the back of the car?

    If you’re in that frame of reference (the inside of the car), the force is as real as the force of gravity — and, indeed, according to GR, it is indistingushable from gravity!

    It is also correct that inertia is keeping you stlil while the car pushes you forward… that’s the description you’d use if you’re an observing sitting on the ground looking at the car. But if you’re working in the frame of reference of the accelerating car, that force is there. Indeed, if you wall off the car so you can’t see outside, you could not tell the difference, in principle, between the “fictitious” force of the acceleration and the force of gravity if the car were tilted upward and you were on a planet with a stronger gravitational field.

    If you want to call forces that you get in noninertial frames “not real”, you also have to call gravity “not real”. And, there is some headway to be made with that point of view; General Relativity doesn’t consider gravity a force, but rather the curvature of spacetime. However, in the standard Newtonian Mechanics version that we use for everyday situations, gravity is a force, and in an accelerating reference frame you will feel forces like the one you describe and like centrifugal force.

    -Rob

  16. Brant D.

    I have always enjoyed the “centrifugal force is not real” claim. It is like telling hurricane victims “You were not hit by a severe storm, but rather you experienced an Incorrect Reference Frame Event.” I would pay some serious dough to see a government official claim this.

    Also, gravity is not a centrifugal force. In an orbiting satellite’s reference frame, gravity is counterbalanced by the centrifugal force, thus the satellite does not accelerate in its frame. Gravity itself is not the outward-pointing force.

  17. TR

    “Now sit inside that car. When the car turns right, which way do you feel yourself leaning? Toward the left, away from the center. You feel a force towards the outside of the circle.”

    Speak for yourself: when I ride in a car that turns right, I feel the force from the seat pulling my butt to the right!

    If I want to keep my back straight, I must use my muscles to pull my shoulders to the right also, but this does not mean that there is a force pushing my shoulders to the left! (The equal but opposite force is applied at the other end of the muscle, not on my shoulders.)

    If I don’t bother to keep my back straight, I’ll lean to the left until the door exerts the necessary centripetal force by pushing my shoulders to the right!

  18. Greg Fuchs

    First the ‘lighter than air’ thing and now this.

    Phil sure knows how to get a discussion going!

    Greg

  19. Isaac Newton shakes his head in amused agreement.

  20. Miss Physics

    Just had to add my two cents:

    Centrifugal force is not the reaction force to centripetal force. For example, the centripetal force that holds Earth in orbit around the Sun is the force of gravity exerted BY the Sun ON Earth. The “reaction” force is the force of gravity exterted BY Earth ON the Sun. The reaction force is always acted on the other object.

  21. owlbear1

    If the car door wasn’t there and there was no friction would still be sitting there? NO. You would undergo ‘Centripedal Force’ and fly out at a tangent. Centrifugal force is nothing more than short hand for the summation of all the friction forces.

  22. icemith

    Don’t you people use seat belts when in the car? Or am I really complicating things unnecessarily, with regard to distribution of forces?

    Ivan.

  23. Geoff

    It’s about time someone stood up for centrifugal force. Next to gravity, it’s my favorite force of all time. I once saw a bike with an automatic transmission that used centrifugal force to change gears.

  24. The “centrifugal” illusion worked in Kubrick’s 2001. All these physics blogs would look better if you included Equations and Graphics.

  25. Christian Burnham

    Is this the same as saying that the Sun revolves around the Earth in our preferred reference frame?

  26. LOL, hehe ;) I agree to you

  27. Antares Richard

    With me being a nervous driver, I’d be more worried about the forces of the Law, what with all those right turns, accelerations and brakings! Love the talk.

  28. Well, according to Phineas J. Whoopee (Tennessee Tuxedo) centrifugal force is the “old english term” of what is now known as centripital force. Yes very much the exact same thing.

    You see? I did learn something from all of those cartoons I watched as a kid!!

    –SjN

  29. Roy Batty

    My take on this?
    Centripetal force = Centrifugal EFFECT.
    Disappointed with this blog post BA :(

  30. Dan Gerhards

    Christian: Maybe I’m misunderstanding your post, but the sun DOES revolve around the earth in our reference frame. The motions of the rest of the solar system can be worked out by assuming that too, it’s just complicated. It’s much easier to describe everything by assuming a heliocentric system.

  31. I know I’m late on this, but my high school physics teacher had the best way of explaining this. The people above who were using the linear analogy were on the right track. Centrifugal force is the result of Centripetal acceleration. Think F=ma where “a” is the Centripetal acceleration towards the center and “F” is the Centrifugal force forcing you to the outside. Simplistic, but it’s kept me straight lo these many decades.

    - Jack

  32. Michael

    I was wrong several times in my earlier comment, so pretend I didn’t say that. I guess I shouldn’t write things when I’m that tired. :) (As a self ego booster, I did have the right idea, but made some dumb errors, like assuming the “center” of the circle switches sides when you go from rotating to non-rotating.)

    All the forces pulling towards the center are centripetal, and all the forces pulling away from the center are centrifugal (by definition!). In a rotating frame of reference (FoR) the centrifugal forces are the active forces, and the centripetal forces are the reactive forces. In a non-rotating FoR the centripetal forces are the active forces, and the centrifugal forces are the reactive forces. Depending on how you look at it, it’s possible that there’s no definable “center”, so there can’t really be centrifugal or centripetal forces. In no case are the centripetal and centrifugal forces anything but opposites.

    The confusion is that we are looking at different forces, then being astounded that they appear to be different forces. To simplify, let’s look at a median FoR–rotating, but not as fast as the car’s FoR.

    In a right-hand turn, the car is accelerated to the right by a real centripetal force (caused by tire friction), the passenger is accelerated to the left by an apparent centrifugal force (caused by inertia), and the two will eventually impact. At this point, the passenger is pushed against the car with a centrifugal force, and the car is pushed against the passenger with a centripetal force.

    The debate of “is the centrifugal force real?” stems from whether it’s legitimate to call an apparent force real, since we know it’s really just inertia. Originally, I thought it wasn’t, but I guess since it’s not possible to tell the difference from inside, it’s no less real than gravity.

    However, the force you *feel* is always real, and is always the centripetal force. Consider gravity: you are pulled down by gravity, but can you feel gravity? No! Just look at the micro-gravity on the space station and you can see that. What you feel is the reactive force of the ground pushing up on you. Likewise, whether the inertial “force” is a real force or a pseudo force, you never feel it–just the reactive force of the car pulling inward on you.

    Also, note that I am simplifying things a bit; in reality, you feel both forces throughout your body. Your skin feels the centripetal force of the seatbelt/seat/car body and the centrifugal force of your muscles. Your muscles feel the centripetal force of your skin, and the centrifugal force of your bones, internal organs, and extremeties that aren’t belted in (like TR talked about using his torso muscles to keep his shoulders and head from moving sideways relative to his seat). And you can continue this type of analysis for every molecule in the system. Obviously, the reality is that these tides are what kills people in crashes, but as a whole person, you feel a net centripetal force and nothing else.

    Again, it’s not terribly important, since in 99% of discussions we know what is meant. But saying they are the same force is absurd–it goes against common sense and any amount of linguistic logic. They are, by definition, opposite forces. The only time they might be the same is if they are both zero, which means there’s no rotating FoR to begin with so it’s a moot point.

    Once more, just to be sure:
    1. The centrifugal effect caused by inertia may or may not be a real force, depending on your frame of reference.
    2. The force you feel inside a turning car is always the centripetal force (assuming we are only talking about the interaction between your body and the car)
    3. The centrifugal force is always the opposite of the centripetal force.

  33. Kyle_Carm

    No no ruidh Newton shakes his fierce wig and b!tch smacks those who don’t understand the reality. ;-)

  34. In a rotating frame of reference (FoR) the centrifugal forces are the active forces, and the centripetal forces are the reactive forces. In a non-rotating FoR the centripetal forces are the active forces, and the centrifugal forces are the reactive forces.

    Wrong.

    In the non-rotating frame, the centripetal force is supplied by something. Say you’re swinging a ball on a string around on your head. The tension in the string supplies the centripetal force that keeps the ball rotating in the circle. The “action/reaction” force is the ball pulling back on the string. As for which is the “active” and which is the “reactive,” that’s just whatever you want to call it. The fact is that there is a force of the ball on the string, and a force of the string on the ball.

    There is only one force on the ball– the string pulling on the ball. Thus, the ball accelerates– and that’s what keeps it moving around in a circle. If you are working in the inertial (non-rotating) frame of reference, there is no centrifugal force present.

    In the rotating frame of reference, the ball feels two forces. It feels a centrifugal force in one direction, and it feels the force of the string tension pulling it towards your hand. These are not reactive forces of each other, they’re just two forces present. One of the forces is “fictitious”, and is the result of working in the accelerated frame– but it’s there. The ball doesn’t move in this accelerated frame because the two forces acting on it balance.

    The reactive force to the string pulling on the ball is, once again, the ball pulling on thes tring.

    -Rob

  35. Addendum : Miss Physics, above, already said what I was trying to say about the active/reactive forces. Read her comment.

  36. keiths

    Jack Hagerty wrote:
    Centrifugal force is the result of Centripetal acceleration. Think F=ma where “a” is the Centripetal acceleration towards the center and “F” is the Centrifugal force forcing you to the outside. Simplistic, but it’s kept me straight lo these many decades.

    Jack,

    The problem with that approach is that the “F” and “a” in “F=ma” are both vectors, with “m” as a scalar multiplier. Since m is positive, the F and a vectors both point in the same direction: toward the center. The force is centripetal.

  37. csrster

    Ok, Suddenly I’m confused. Consider a stationary ball in a stationary reference frame. There are no forces acting on the ball. Fine.

    Now look at the same system in a rotating frame. The ball now appears to be
    circling the axis of rotation. But, in the rotating frame, the only force acting on
    the ball is the _outwardly directed_ centrifugal force. Huh?

  38. TR

    If the car door wasn’t there and there was no friction would still be sitting there? NO. You would undergo ‘Centripedal Force’ and fly out at a tangent.

    I would not “fly out” on the tangent. I would continue to travel in a straight line as the car pulled out from under me. As Newton would say, I would be an object in motion staying in motion because I would not feel a force!

    A centripetal force is needed to pull the car towards the center of the turn; no force is needed to explain why I keep going in a straight line as the car turns.

  39. TR

    How ’bout this, Phil (Considering your Bond-cartoon example):

    I’ll agree that there is a centrifugal force if you’ll agree that Bond is not the one that feels it. In order to make him move in a circle, the wheel must exert a inward (critical) force on Bond. He, in turn, exerts an outward (centrifugal) fore on the wheel. (Newton’s III)

    Is this a “real” force? Well, it is as far as the wheel in concerned. But saying that the Bond will be killed by the centrifugal force would be like saying that a suicide jumper is killed by the fore his body exerts against the pavement when he lands!

  40. TR

    Sorry for the 3 posts in a row, I guess I should read everything before I respond to anything. (That might also save me from the embarrassment of miss-spellchecking “centripetal” into “critical” in my last post.)

    csrster says:

    Ok, Suddenly I’m confused. Consider a stationary ball in a stationary reference frame. There are no forces acting on the ball. Fine.

    Now look at the same system in a rotating frame. The ball now appears to be circling the axis of rotation. But, in the rotating frame, the only force acting on the ball is the _outwardly directed_ centrifugal force. Huh?

    Not exactly. If the object is stationary, there is no net force acting on it. Consider an apple resting on a table: it is pulled down by gravity, but held up by the table. Those two forces cancel each other out, and the apple at rest remains at rest.

    If you consider the ball on a string whirling in a circle, you can’t discount the tension exerted on it by the string, that is a real force. (If you were a flea living on the ball, even if you didn’t realize that the ball was in motion, you could still go thump on the string and observe that it really is under tension.) But if the ball is considered from the stand-point of a certain rotating reference frame, it is seen to be at rest, so (in that frame) we must posit the existence of some force which is countering the tension and keeping the ball from accelerating in the direction of the string (like the table keeps the apple from falling down).

    This new, made-up, force is the “centrifugal” force that got this whole thing going. The reason centrifugal force it is often called a “pseudo-force” (and people get all in a twist when Phil mentions it) is that it is only needed to account for the motion of the ball if it is observed from that particular reference frame. If we use an inertial reference frame, then the tension in the string is all that is required to account for that ball’s behavior.

    This is the crux of it: The string really is under tension (put a spring scale on it and everyone in every reference frame can agree that the scale shows a non-zero value) so there really is a centripetal force. But the centrifugal force which cancels out that tension is only needed in certain non-inertial reference frames to explain why the ball is not seen to be accelerating. Many would argue that a force which only exists as a mathematical expedience in a certain reference frame, is (at the very least) less real than a tension in a string which can be measured by a spring scale.

    And I think I’ll leave it at that. It’s not quite as pithy or emphatic as “centripetal = centrifugal”, but I do think it’s correct.

  41. csrster

    Ok. But in my example there is no string and no force at all on the ball in the inertial reference frame. In the rotating frame, the only force on the ball is the centrifugal force. But the ball’s apparent path is a circle, ie it appears to be accelerating inwards.

  42. TR

    Wait a minute: Are you talking about a ball sitting at rest, say on a table, as seen from the standpoint of a reference frame rotating with respect to that table?

  43. PK

    Be careful: don’t confuse a rotating reference frame with a rotating physical system.

  44. Tukla in Iowa

    You people are making things too complicated. Angels just push objects in the direction God deems most holy, and Jesus holds atomic nuclei together with his hands. Haven’t you people heard of Ockham’s Razor?

    (Sorry, been reading old Chick tracts lately.)

  45. TR

    If you are talking about a ball sitting on a table as seen from a reference frame which rotates with respect to the table, you do not need to resort to centrifugal force to explain the object’s circular motion. That’s the problem with centrifugal force; it comes and goes depending on the way you look at the situation.

    (Man, I love those Chick Tracts. Wouldn’t it be great to make a skeptic’s version of those and pass them out on street corners? If only I could draw…)

  46. yep–it’s the fault of certain textbooks–my HS physics textbook made a big deal about there being no such thing as a centrifugal force. The author needs to spend some time in a centrifuge….

    but if there is no centrifugal force, then by similar reasoning, there is no gravitational force; it’s all space curvature; and no electromagnetic or nuclear forces–just particle interactions.

  47. Gary Ansorge

    This is the last time I’m telling you kids! Quit riding that centepede around in a circle,,,it makes him ditzy!

    GAry 7

  48. So … you’re saying that Newtonian mechanics is augmented by recognition of relative frames of reference?

    Hmm, you might be on to something there.

  49. SpikeNut

    Eschew Obfuscation!

  50. Michael

    Rob Knopp says: “The reactive force to the string pulling on the ball is, once again, the ball pulling on the string.”

    That’s exactly what I’m saying. It depends on which way is “toward the center”, but whichever force pulls toward the center is, by definition, centripetal, and whichever force pulls away from the center is, by definition, centrifugal.

    Rob Knopp says: “If you are working in the inertial (non-rotating) frame of reference, there is no centrifugal force present.”

    Wrong. The force of the ball pulling outward on the string is centrifugal.

    Rob Knopp says: “In the rotating frame of reference, the ball feels two forces. It feels a centrifugal force in one direction, and it feels the force of the string tension pulling it towards your hand. These are not reactive forces of each other, they’re just two forces present.”

    Mostly correct. First, there is still the real centrifugal force of the ball pulling outward on the string. *That* centrifugal force is always present (you didn’t say otherwise, but you didn’t mention it either). Second, the ball doesn’t feel the centrifugal force pulling it outward. If you cut the string, the ball suddenly stops feeling anything, just like you’d stop feeling anything if the ground disappeared out from under you right now. It experiences this force, but if it had human-like pressure sensors, it wouldn’t feel it–only the force of the string holding it back.

    csrster says: But in my example there is no string and no force at all on the ball in the inertial reference frame.

    That means it’s stationary in the inertial FoR, and moving in a circle relative to the non-inertial FoR (non-inertial=rotating, right?). In the non-inertial FoR, it would appear to be accelerating by a pseudo force. The force would be centrifugal for half the circle, and centripetal for half the circle, if “towards the center” was a fixed angle relative to the non-inertial FoR; if “towards the center” was a fixed point, the ball wouldn’t experience centrifugal or centripetal forces, since it’s not moving relative to that point.

    Miss Physics says: Centrifugal force is not the reaction force to centripetal force. For example, the centripetal force that holds Earth in orbit around the Sun is the force of gravity exerted BY the Sun ON Earth. The “reaction” force is the force of gravity exterted BY Earth ON the Sun. The reaction force is always acted on the other object.

    I finally figured out what you’re saying (it was so obvious I missed the point), and you’re right; I missed a step. Gravity pulls us downward, and the Earth upward. Because of the Earth-pulls-us force, when we stand on the ground, we put a downward force on the ground, and it puts a reactionary upward force on us. Because of the we-pull-Earth force, the ground puts a negligible, upward force on us and we put a negligible, downward force on the ground.

    In the rotating FoR, a fictitious, centrifugal pulls on us, but since it’s not a real force, there’s no real reactionary force (if there were one, it would be centripetal). Because of the fictitious, centrifugal force, we put a real, centrifugal force on the inside of the car and the car puts a real, centripetal force on us.

    TR says: But saying that the Bond will be killed by the centrifugal force would be like saying that a suicide jumper is killed by the fore his body exerts against the pavement when he lands!

    Exactly. The force his body exerts against the pavement puts a dent in the ground (if he hits hard enough); it is the reactionary force the pavement exerts on his body that kills him. (In reality, it’s the tidal force caused by one side of his body feeling the upward force and the other half not feeling it until he’s flat already.)

    Basically, the problem is that some of you trying to relate the pseudo-centrifugal force to the real centrifugal force, and then you’re forgetting which one we feel. The pseudo-centrifugal force causes the real centrifugal force, and this real centrifugal force is paired with a real centripetal force that we feel. If we move to a non-rotating FoR, it is inertia causing the real centrifugal force and this is paired with a real centripetal force.

    And I’m done–last time I got on here and tried to discuss this type of stuff I ended up nowhere 2 months later. So no more of these posts by me. :D

  51. Second, the ball doesn’t feel the centrifugal force pulling it outward.

    You are very wrong, and will remain wrong no matter how many times you say it.
    If the ball *didn’t* feel the centrifugal force pulling it outwards in the rotating frame, then it would accelerate inwards, for there would be no force to balance the tenison of the string!

    In the rotating frame of reference, the ball absolutely feels that force just as much as a ball held in the air feels the force of gravity. If you have squishy liquid stuff in the ball, it will be squished and separated outwards just like different liquids will separate in a glass standing on a counter under the force of gravity.

    If you drop a ball in a gravitational field in a vacuum, it doesn’t feel any forces, and there’s no squishing going on inside the ball; it’s in free-fall, but in the frame of reference of (say) the Earth where the ball is falling it accelerates.

    Similarly, if you cut the string, the ball flies off, and doesn’t have any squishing or something, but in the rotating frame of the ball before you cut the string, it accelerates away.

    Centrifugal force is, in a rotating frame, absolutely as real as the force of gravity, and indeed the effects are quite similar.

    Rob Knopp says: “If you are working in the inertial (non-rotating) frame of reference, there is no centrifugal force present.”

    Wrong. The force of the ball pulling outward on the string is centrifugal.

    Well, it may be “center-fleeing” in its direction, but it is not the “centrifugal force”, which is the name given to a certain term that shows up in dynamical equations when you are working in a rotating reference frame. That’s what I mean when I say there is no centrifugal force present. You are not using the terms in the standard manner that is meant when physicists really talk about rotating reference frames.

    -Rob

  52. cyber_rigger

    If you have a spinning ring of ice

    that melts

    what force makes the water drops disperse?

  53. Mike

    I know some empirical observations might be asking too much for all of the armchair physicists, but has anyone here actually gotten inside a centrifuge to make some observations?

    Has anyone been on the Gravitron at their local county fair? You know, the ride that is just a giant enclosed centrifuge? Less likely, how about those NASA centrifuge tests that the astronauts go through in the movies and on TV? Do air force pilots get to do that too?

    If so, what did it feel like? I know I felt like i was being pulled back against the outer wall of the centrifuge. I would lift my hands away from the outer wall, directly toward the center of the centrifuge, and they would be “pulled” right back against the wall. I was feeling a “force” pulling me directly away from the center of the ride.

    So for all the people inside that centrifuge, would you deny that this “force” existed? If that was their world, all they knew, and they needed to do some calculations about how objects in their world would act, would you come to them with a complicated explanation of “inertia” and alternate frames of reference, insisting that they use your formulas, or would you allow them to perform simpler calculations based on the assumption that there was a force pulling them outward toward the wall of the centrifuge?

    I believe that the whole point here is being able to simplify the model based on your frame of reference, and therefore being able to simplify the calculations required to obtain some meaningful result.

  54. I hate to call Phil Plait wrong, but I must. Centrifugal and centripetal are not the same, because you can have one without the other.

    Consider a rotating reference frame, and an object in free fall that is momentarily stationary in the rotating reference frame. As seen from the rotating frame, it will be accelerating away from the center. This acceleration is explained by the centrifugal force, but there is centriPETAL force to be found anywhere for this situation, in any frame.

    Conversely, in an inertial frame there are no centrifugal forces at all. However, if you’re in an inertial frame and observe an object move along a circular path, you know that there is a net centripetal force on the object. (In this case, however, you can choose a rotating frame where the centripetal force, still present, is cancelled by the centrifugal force).

    To get THINGS to rotate, you must supply a centripetal force. If you let your FRAME rotate, you get centrifugal forces. The two concepts are only equivalent when you consider things that happen to follow your frame.

  55. jose hevia

    Finally, someone that agrees with me. I have 25 years, engineering sudies, and have been thinking the same since my phisics teacher told as:(at 16)
    There is not such a thing as centrifugal force.(but in spanish)

  56. icemith

    Rob Knop, at 0835am you said,” …but in the rotating frame of the ball before you cut the string, it accelerates away.”

    I’m a bit confused. Did you mean *after*, not *before*? And secondly, the ball could never *accelerate* after it was cut free. It would move in a straight line – tangentially – unless acted on by some other force, gravity, wind resistance or the ubiquitous brick wall. Its motion would only be equal to whatever it had in the previous circular frame (?), or less.

    The other general question that I have concerns the, shall we say, pivot or hook, you know whatever the string is tied to that rotates the whole shebang. It has a rotational force that the string is tensioned by, and that must be at a small angle, away from the line directly between that point and the center of mass of the ball. (Perhaps the acceleration part is integral in the original assertion. That is , the ball has to be *swung* to get, and keep, it going. It is this part that I am also curious about, the imparting of that motion. And exactly what it is.

    Someone more capable of understanding the maths involved, will be able to express it more eloquently. Any ideas?

    Ivan.

  57. Ken

    “Now sit inside that car. When the car turns right, which way do you feel yourself leaning? Toward the left, away from the center. You feel a force towards the outside of the circle.”

    Have you ever been in a car? What are you smoking?

    When my car turns right, my body shifts left relative to the car, but I feel pressure on the left side of my body, pushing me to the right.

    If I walked up to the left side of you while you’re sitting in a chair, and pushed, would you say that you felt a force pushing you leftwards?

    You’re confusing “motion relative to” with “force in the direction of”. If I feel pressure on one side of my body, it’s never because I’m being pushed in that direction.

  58. MIKE: When I wore a flightsuit, I went through the centrifuge in Pensacola. On the door was painted a Disney E-ticket. It was really fun! We called it “Spin and Puke”!

  59. JRY

    A question I would like answered: If centrifugal is a real action force in a given frame of reference, then, By Newton’s 3rd, what is the reaction force? In other words, if I take a sharp left turn and experience this force pushing my to the right, what am I pushing against in return?

  60. “Why complicate things by deriving the shape of orbits? By the same token, why complicate things by including terms unnecessary for a fundamental understanding of physics? Undergraduate physics students can take a course in analytical mechanics in which they deal with rotating frames of reference.”
    Because we’re tired of people from every level of education smugly writing to us to tell us that the word we just used isn’t real!
    “A question I would like answered: If centrifugal is a real action force in a given frame of reference, then, By Newton’s 3rd, what is the reaction force? In other words, if I take a sharp left turn and experience this force pushing my to the right, what am I pushing against in return?”
    Well, there doesn’t seem to be one (you’re not necessarially pressing against the door just yet). But by the same token, in the rotating reference frame, you are a mass experiencing an acceleration. So therefore there IS a force, by the very definition. And that’s really the law I like better, anyway.

  61. Phil wrote: “Centripetal = centrifugal. Got it? They’re the same thing. If one is real, then so’s the other. Once more, just to make sure:”

    Not quite true, though. As another poster pointed out, you can have one without the other. A puck on a spinning ice rink will slide outward due to centrifugal force with no centripetal force stopping it. And if we go back to the stationary frame, there’s still no centripetal force holding it in place.

    Also, has anyone looked into holding hockey matches on spinning rinks? Because I’d pay to watch.

    Actually, any kind of sport on a spinning field would be okay by me.

  62. Nigel D

    Maybe it comes down to whether you think that F=ma *defines* force, or whether it expresses the connection between two physical quantities (force and acceleration) that have an existence independently of each other. If F=ma is viewed as a definition, then anyone (inertial observer or not) is trivially entitled to claim that there’s a force acting on any accelerating object. But if force is regarded as something with a real physical cause – such as gravity or the electromagnetic interaction – then centrifugal “force” isn’t a force, and the equation “F=ma” doesn’t hold in a rotating frame.

    If I wanted to study motion in a rotating reference frame (e.g., Foucault’s pendulum on the rotating Earth) I’d write “F + (centrifugal / coriolis terms) = ma”, where F includes all the “forces with physical causes”. Should I call the centrifugal / coriolis terms forces, or not? They affect the observed acceleration in the same way as “real” forces; on the other hand, they don’t have “real” physical causes. Is it worth losing sleep over? I don’t think it’s possible to say that one choice is “right” while the other choice is “wrong”. All that matters is to be completely clear about what is meant by a force being “real” before spending a lot of time arguing! (Arguing is fun, though!)

  63. Michael

    Rob Knopp says: You are very wrong, and will remain wrong no matter how many times you say it.

    Ah yes, now I remember why I quit the last debate; everyone kept trying to argue the lesser, often symantic, points, and missed the main point I was trying to make. I got tired of trying to make people understand what I was talking about, much less my view on the subject, so I finally gave up. Let’s see if I can clarify this time around.

    Forget about whether my definitions are as correct as yours, and forget about whether the centrifugal force (by your definition) is real or just convenient. I understand the BA was trying to explain that it’s real and not just convenient, but he missed the same basic premise of physics that you have:

    *** It’s not the fall that kills you; it’s the sudden stop at the end. ***

    I don’t know who Mike is calling an armchair physicist, but every building I’ve jumped off of, every tree I’ve fallen out of, every rope I’ve climbed, every swing I’ve swung in, every bicycle I’ve wrecked, every set of trampoline springs I’ve landed in, every corner I’ve apexed, every guardrail I’ve hit, every plane I’ve flown (I only got to pilot one, but it’s included), every textbook I’ve read, and every professor I’ve talked to all say you are wrong. I know this because my life has depended on such knowledge many times. I know this because the physics that your theories and hypotheses try to describe is fundamental to my hobbies. I know this because I have empirically observed my environment to see how it stacks up against what scientists say is true, and have done so since before I could read (I do not claim to understand all of it, but this I understand quite well).

    You can freefall forever, but if there’s no wind, no branches, no ground, no water to resist you, you’ll never feel like you’re moving. And when the ground comes up to meet you, the force you’ll feel is most assuredly *not* gravity; the force you’ll feel is the millions of grains of grit and sand embedding themselves into your skin, the twigs and rocks tearing through your flesh, and the Earth itself impacting you with such terrible momentum you’ll think you’ll never breath again.

    When you skid sideways into the side of a mountain and the car comes to a very rapid halt (or, it is stationary relative to a rapidly rotating frame of reference, and you are accelerating rapidly out the door), it’s not the centrifugal force you feel; it’s the violent, centripetal force of your seatbelt yanking you back so hard that your skin burns and the muscles bruise; it’s the force of the window smashing your helmet into your head so aggressively you’ll swear there’s a God and He just punched you; it’s the force of the steering wheel nearly breaking your fingers in its haste to leave your hand, and the force of the door panel succeeding where the wheel left off.

    It doesn’t matter what frame of reference we’re in. It doesn’t matter which object is accelerating which direction, nor which is stationary. The reality is that you and I feel the impact and resistance forces–nothing more, nothing less. When we hit the ground, the ground hits back, regardless of what caused us to hit the ground in the first place.

  64. icemith

    You say ‘centrifugal’, and I say ‘centrifugal’,
    You say ‘centripetal’, and I say ‘centripetal’,
    Centrifugal, centrifugal, centripetal, centripetal,
    Let’s call the whole thing off!

    Ivan. …..(With apologies to the Gershwins).

  65. this is a load off bull. centrifugal force is not a force that can accurately discribe the motion of objects. the centrifugal force doesnt accuratly map the motion of an object when the gravity or connecting force is removed from both objects. the object will move in a tangent of 90 degrees to the direction of travel, not directlty out from the middle of orbit like the centrifugal force model discribes.

  66. Rich Hearn

    Interesting thread. From the FWIW Dept, I’ve always found it helpfull to think of it as a bucket of liquid water (as opposed to frozen) on a rope. The bucket is held by centripetal force (rope) and the water stays in by the outward force (centrifugal) (refering to the “top” of the bucket facing “center”) to the “pivot” point where the rope is attached.

  67. Jim

    Sounds to me like you’re all going around in circles. And each has their own spin on it (pun intended)!

  68. Physicist Bob

    You are absolutely wrong. The term centrifugal is made up to describe the feeling of the reaction of a real force. After 23 years of work in the physics field, you won’t convince me or the rest of the scientific field that centrifigal force is a bonifide force. use the term because it’s what everyone understands, but don’t try to pass it off as correct.

  69. Hello Folks:
    Some thoughts on the subject of centrifugal force. It is often measurable by scale as the acceleration/Reaction force providing the Newton’s LAW III required support for the centripetal acceleration/Action force. It is always caused by the centripetal a/A force.

    It is always a force internal to each component of the accelerating object’s matter. Now here is the good part. If the centripetal a/A force is an external force (contact force) then the centrifugal forces stack up through the object’s matter in the direction opposite to the acceleration. On the other hand, if the centripetal force is also an internal force such as gravitation, then there is no stacking-of-forces effect leaving the orbiting object in internal equilibrium or a state of weightlessness as experienced by an orbiting astronaut.

    In a linear acceleration event as already pointed out, the a/A force causing the linear acceleration, like the centripetal a/A force, also causes to exist the supporting a/R force directed opposite to the acceleration just like the centrifugal a/R force. Again, with the a/R force always being an internal force, if the a/A force is an external force then the a/R forces stack up in the direction opposite to the direction of acceleration. This is like when yourself sinking back into the accelerating seat or when your eyes want to pop out while your body reactively bears against the seat belt during hard braking. Yet if the linear acceleration/Action force is also internal, such as gravitation, then you will again experience internal equilibrium with no stacking-of-forces effect present as when you accelerate at the rate of a race car while falling down inside a deep well. It will seem like you are not accelerating at all but instead like the bottom of the well is rushing up to meet with you! This is an example of weightless acceleration. Once you understand the difference between internal and external forces it all begins to make sense.

    Newton’s LAW III tells the truth that it is simply not possible to apply an acceleration/Action force without there being immediately present (not hundreds of thousands of mile away) an equal and opposite acceleration/Reaction force of support. Centrifugal force is the name we call this a/R force that is present in every event involving centripetal acceleration. We don’t have any particular name for the same a/R force present in linear acceleration. Currently our best minds ignore the presence of both measurable example of the common everyday acceleration/Reaction forces. Tension and compression scales can display their presence yet the experts deny the truth of the readings. No wonder there is such confusion surrounding the centrifugal a/R force. Yes it is real. No it is not capable of acting as the cause of any event. It always reacts in the direction opposite to the event’s direction. If you think a reaction force that is by nature incapable of causing any event is not a real force, think again. Newton wrote of reaction forces in his LAW III and in his whirling bucket of water event where he referred to them as “forces receeding from the axis of circular motion.” Reaction forces are real measurable force experienced by every accelerating object. To claim otherwise is to take a hard stand against Newton.

    A force is a push or a pull experienced by an object. You are the object in the car turning right. The presence of a centripetal acceleration/Action force is required to cause the same rate of centripetal acceleration enjoyed by the car. You are counting on the door latch to hold while the door applies the centripetal a/A force against the left side of your torso. If the latch holds then your body’s rate of centripetal acceleration will match the car’s rate. The a/A and a/R forces between your body and the door are mutual as in equal and opposite just as predicted by Newton’s LAW III. To claim that the centripetal a/A is present while the centrifugal a/R force is absent is contrary to the measurable evidence.

    If the latch fails then you will experience less centripetal acceleration than the car meaning your body will perhaps travel more of a straight line. Some confused folks think this straight-line departure from the turning car has a cause which they term “inertia”. Actually a straight line path is your default motion for which no cause exists. Aristotle tried to tell us that a “mover” was the cause of our straight-line motion. Later Newton tried to tell us the same thing, namely that “inertia” was the cause of our straight-line motion. But today we all understand that in the absence of acceleration, an object has no other option but to continue on with its default state of motion which can either be observed as rest or equally valid as motion in a straight line. No imaginary “inertia” or “mover” required. Only Galileo got this important point right.

    If you would like to read more about Newton’s error, visit http://www.UniversalPhysics.org/article_1.html The Reality of Newton’s Inertia

    For a look at internal centrifugal a/R forces visit
    Article XI: Reaction Forces
    http://www.universalphysics.org/article_11.html

    Finally for an analysis of centrifugal force read
    Question 5: Is centrifugal force real?
    http://www.universalphysics.org/question_5.html

    and Question 5: Addendum
    http://www.universalphysics.org/question_5_addendum.html

    Good evening to all,

    Ethan Skyler

  70. Johan Couder

    CENTRIPETAL = CENTRIFUGAL ?
    That sounds a bit too simplistic ! At best:

    real centripetal force in inertial frame of reference = – virtual centrifugal force in (rotating) non-inertial frame of reference

  71. alex

    You should not have a spinning frame of reference!
    Centrifugal force only exists if you do us that sort of frame, which you shouldn’t. Those who say it doesn’t exist are totally correct.

  72. Geraint Lewis

    Phil is wrong.

    Centrafugal force is not “real” in the sense that there is no source for it.

    It is fictional – and with all fictional forces, you cannot feel it.

  73. Kristin C

    OMFSM!

    Can we have a definite answer already? o.O

    I want to be properly educated here. *confused sniffle*

  74. G Oas

    In the case of being in a car, there is the sense of an outward
    force that you feel but there is no such force.
    The problem here is with the use of the word “feel”.
    When people say they feel gravity, are they really feeling gravity?
    No, they are feeling the normal force acting upon them in the opposite direction (a real force). The only time you would “feel” gravity is when
    it is the only force acting upon you and that is in free fall. This
    is a state that people usually say “I don’t feel gravity, I’m weightless.”
    Thus we must divorce the notions of “feeling” from
    quantitative analysis of forces.
    When one feels centrifugal force it is the same as the normal
    force, it is the inward force that is felt. There is no outward
    force on the person in this situation.

    Again, I think this is a pedagogical issue because you can read many
    studies where the use of centrifugal force leads to misconceptions
    in introductory physics. You can talk about centrifugal forces in
    non-inertial frames but then Newton’s laws don’t apply in their
    simple form -that should
    be saved for later. Trust me, first time learners and the lay-public
    are not prepared for that discussion.

  75. Montie

    Hello all!
    I would like to begin by saying thank you to everyone who has commented (*without* resorting to personal attacks) on the discussion. It is clear that there is a great deal of misunderstanding about this topic and I personally appreciate everyone’s comments because it forced me to re-evalute my understanding of the issues. Since I actually teach physics, I have found it to be an extremely useful exercise and I believe it is important in Physics (and by extension, Science) to promote such discussion, not quash it. I have always taught that there is no such thing as centrifugal force – although in my defence, I only used a non-rotating frame of reference. And for the record, I believe anyone can make a valid contirbution to a discussion, not just “experts.”

    Some defintions & abbreviations:
    1. a “real” force is a force which can be measured.
    2. FoR = Frame of Reference
    3. RFoR = Rotating Frame of Reference
    4. NRFoR = Non-Rotating Frame of Reference

    In my humble opinion, a lot of confusion comes as a result of the frame of reference. I would suggest the following:
    1. In an NRFoR, there is only one force – the centripetal force. centripetal force is the force that makes the mass move in a circle.
    2. In a RFoR, there is only one force – which you can call the “centrifugal” force, if you like.
    3. Both cannot be in the same FoR.

    It suddenly became clear to me when Mike (September 2nd, 2006 at 2:09 am) was describing the NASA centrifuge. In that RFoR they only (really) experience a force which we could arbitrarily say was to the left (not down or towards the centre of any circle), and could (for instance) simulate gravity. It would be measurable using any device capable of measuring force (spring, etc) and therefore it actually would be “real”, but there would be no centripetal force in this example. Remember, that in their RFoR, they do not realize that they are moving in a circle – only someone in the NRFoR would know that. For someone in the RFoR, they are in the same physical situation as a linear accelerating system. In the end, most people solving the problems usually prefer a NRFoR, as it usually easier to describe and explain (the “God” view).

    In fact, calling it “centifugal” may actually be part of the problem. In their RFoR, they do not know they are moving in a circle, so how could it be “centre leaving” if you do not know you are moving in a circle? It is just a force being exerted on the mass(es). I believe the magnitude of the centrifugal force in the RFoR would be equal to the magnitude of the centripetal force in the NRFoR.

    Another side note: I believe Newton’s Third Law can only be used when there are two objects/masses. You do not use it to describe the action/reaction pair on the same object. For example, while standing on ice, when you push a wall, the wall exerts an equal but opposite force back. So while you may use Newton’s Third Law to explain why a person ends up moving with a car that is accelerating (in either FoR), it will not address the centripetal/centrifugal issue.

    So to sum up, “centrifugal” force is a “real” force in the RFoR, but there would not be any centripetal force. Centripetal is a real force in the NRFoR and there would not be any centrifugal force.

    I would suggest.

    Thanks.

  76. Andy

    Centrifugal force is a shorthand way of working out problems. In a pure physics sense, there is no such thing. Centripetal force is the only true force to be considered of the two, although for many systems centrifugal force can be used as a shorthand, for use by engineers and the like. Physics will never teach centrifugal force along with centripetal force as equivalents or opposites.

  77. David F

    As a student myself, I agree with what Tim said back on August 31: It just makes things unnecessarily complicated. I agree that they are the same thing, it just depends on one’s frame of reference, but to introduce a concept like that to a student who is struggling to even learn the most basic theories is just a little cruel. There is no reason for them to learn such topics, because they aren’t necessary. I totally agree with teaching it in a graduate-level class, but not to someone who is going to forget virtually all of it within a year. All it would do is create more work for the teachers involved.

  78. Cassiopeia

    A frame of reference is a very simple concept to children, not so for adults that were educated without such a concept. It’s a simple concept that is made complex by not teaching it early enough. If you lie and say that the sun goes around the earth, you’re making a similar mistake (though not the same). Relativity is easy enough to understand if you don’t have to fight stone. A few simple videos from the Space Station will show how it works in free-fall (don’t say zero-gravity, it’s not that). Ask the children why, if you play ping pong or tennis on a ship, the ship’s movement doesn’t affect it. Ask them to test it in a car.

    Then ask them to test with a helium ball, what does it do in a moving car, and ask for solutions. The helium ball is lighter than the air in the car, so the air pushes it away, that’s the best explanation you can say. If you’ve never done it, do it. It’s fun.

  79. Cassiopeia

    I think it is very necessary for a student to learn relativity at an early point. Just use the ship and tennis/ping pong example. It is not a complicated concept nor difficult. For children, it will be as clear as day. You don’t have to test them on it, but you have to teach them. You’ll light up the bright minds there to physics and science. They’ll learn at a later age why people age at different rates depending on their relative accelerations, it’s just a nice thing to show children in physics. Relativity is just that when you look at things from a different point of view it looks different, that’s not only important in physics, but everything children need to make best of life.

  80. e

    Neither force is real. They are both illusions. They are just inertia being affected by the bond of the object that is spinning as a whole, and whatever parts are attached to it. If not for the bond, (whether it be due to gravity or another force), each part in motion would just move in a straight line in the very direction it was going when released from the bond.

    Neither actually exist. They the combination of 2 forces made to look like one.

  81. Brandon M. Sergent

    Sergent’s Law: The Length of total comments on a “intellectual” post varies proportionally with the perceived prestige of the blog’s host.

    Or put another way this is intellectual masturbation designed to distract the participants from real issues that require personal discomfort to solve, such as how you all would live if WalMart weren’t selling you products built with slave labor, while conversely making you feel oh so much better than the uneducated wretches “beneath” you. Ironically those very same slaves oh whose backs you live.

    This is as absurd to me as a picard vs kirk debate. I mean really, how is this not just yet another idiotic human faction thing? What is at stake here other than ego? Just hire cheerleaders and complete the obvious pointlessness.

    And deep down you all know it, which is why you’re having this debate on a discover magazine blog, if this were scientific American the comments would read like the library of congress.

    Ra Ra Centripetal ! ! *backflip*

  82. Robert Addams

    I see a lot of these, why complicate it posts and people agreeing with what Tim said in post 5.

    While you might not like learning new things, science and math is complicated. Instead of worrying about making things easier for students why don’t we focus on advancing science and technology. Yes is new, yes its different from what was though before but its happened all through out the progression of learning. If what we have now is engineers and laypeople use maybe our next advancement will come from what we don’t use

    Change is scary but that doesn’t make it wrong

  83. sir,
    if we are observing the rightward turning car from road, and car is somehow made invisible only pessenger are visible, don’t we then see the passenger leaning leftward. how can we say centrifugal forces are applied only while observing in car’s (non inertial) frame. secondly how weightlessness can be expained in an orbiting shuttle

  84. Chris Barnhart

    I don’t have time to read all these lengthy comments agreeing and disagreeing. Let’s use the simple example of the car accelerating around a corner. In one direction, a passenger appears to be “pulled” toward the “center”. In the other direction, a passenger appears to be “pushed” away from the center. In both cases, it is inertia, which is not a force at all, as you know, but a property of matter, which causes the passenger to move with respect to the center of mass of the car, but the actual FORCE causing the passenger to re-adjust acceleration has nothing to do with the center of mass of the car, so is neither centripetal or centrifugal. The force exists between the passenger and the seat belt, or the passenger and the car door. Or the frictional force which exists between the passenger’s butt and the car seat. If you drew a force vector diagram, the center of mass of the car wouldn’t be involved at all.

    Same thing with a centrifuge. A test tube in a centrifuge pushes against the wall of the centrifuge, just as the wall of the centrifuge pushes against the test tube. Again, the forces acting on the test tube have nothing to do with the center of the centrifuge. The fact that the centrifuge is rotating is just a red herring. The exact same principles apply if you push against a wall, the wall pushes back. The fact that you happen to be standing in a round room at the time is completely superfluous.

    Same thing with a yo-yo on a string. If you swing a yo-yo in a circle, the forces acting on the yo-yo are all center seeking. If the string were cut, the yo-yo would move in a straight line in the direction it was moving in when the string was cut. It would NOT move directly outward away from the center of rotation. Nor would it continue to travel in a circle. This is another example of inertia, NOT FORCE.

    Gravity is an example of centripetal force because the FORCE exists between the center of mass of two objects. Someone above misquoted (if I read correctly) Newton’s Third Law to point out that there must be an equal and opposite force…centrifugal force. In fact, if you were to draw a force vector diagram, the equal and opposite forces are the objects pulling each other toward the combined center of mass. A “centrifugal” force would BREAK Newton’s third law, because the “opposing” forces would be pointing in the same direction.

    Arguing that one “feels” oneself move away from the center of mass “proves” that there is a force acting to “push” someone away from the center of mass is such a blatant misunderstanding of Newton’s laws that I doubt your qualifications to teach any physics whatsoever.

    That’s the same as saying that one feels pushed back into a plane seat when a plane accelerates for liftoff. That feeling is a basic result of inertia. What force is pressing the passenger into the seat? None. Let’s state Newton’s First Law of Motion for all to understand: An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

    Once again, inertia is NOT a force.

  85. Chris Barnhart

    I misstated myself in my previous comment. When I said one direction or another, what I meant was one perspective or another. (inside or outside the “circle” using a car in motion). This was a blatant and completely false misstatement and I apologize for misstating, and thus obfuscating, my own position.

    Look, let’s take a slightly different angle to this. It’s true that F=ma. But it’s more true to say that net F = ma, or, in other words, the net of all forces acting on an object causes its acceleration. Sorry, I don’t know how to make a sigma on my keyboard.

    When one is inside a car and “feels” pushed toward the center the of motion going through a turn, what one is actually experiencing is acceleration. This is the result of ALL forces being exerted on the passenger. And, of course, inertia, which is still NOT a force. Did Phil say that it was? No. But he certainly seemed to imply it.

    In fact, the passenger of the car is NOT being pulled toward the center, he is being pulled/pushed tangentially, around the arc of motion. If one breaks it down to look at the door pushing against the passenger, then there IS a force pushing the passenger toward the center of the circle of motion. This is A centripetal force.

    When an object circles another object, like the moon around the earth, the centripetal force that causes the moon to be attracted toward the center of mass, or the central of circular object, is gravity.

    There is no force that pushes an object away from the center in the car example. It may be useful to think of a force pushing away from the center, but you can’t show me one. It does NOT exist. What one feels going into a corner is acceleration, the net result of ALL forces acting on an object, according to the law of inertia, and those forces ACT on an object to change it’s direction. Despite one’s point of view, the actual forces in action do not change and NONE of them act to push the object away from the center of motion/rotation, however you want to look at it.

    I’m sorry my first comment was so badly worded. Several bad examples and a couple of flat out WRONG statements. Mea culpa, mea maxima culpa. Seriously. But that doesn’t change the facts. Centrifugal force is NOT the same thing as centripetal force and, in Phil’s example, he is flat out wrong.

  86. Chris Barnhart

    Oh look. Here’s the misunderstanding. And I apologize to Phil for not getting it. Sorry, Phil. The problem is we’re not differentiating between “real” and “fictitious” forces, and how they can be used.

    I don’t have my physics text books at hand, so I had to resort to Wikipedia. This article on Reactive Centrifugal Force, when and where it can be used, and again, the difference between real and magical/fictitious forces – or pseudoforces.

    Read the article here.

    http://en.wikipedia.org/wiki/Reactive_centrifugal_force

  87. Chris Barnhart

    Man, lesson learned. NEVER comment on a blog about ANYTHING with a fever of 102. Sorry for my bloviating idiocy, Phil.

  88. Loretta

    “Centrifugal” force is simply the act of inertia.

  89. jacob

    thanks loretta , that one line almost explains it all , inertia is the key here, its reaction is with space time quantum physics , and does not fit in with ( newtonian laws ) which have to be updated to include quantum physics , but that may take a lot work , it has been taught for as long as people remember, but the term centrifugal can not really be explained with newtons law , no different than the electron spinning around the nucleus , nobody anywhere can explain that , basically if it cannot be explained in school, or university it will not be in a curriculum, regards jacob

  90. Wendy

    Holy cannolli, people are really passionate! So is that thing the lab puts blood in a centripete? From the bloods point of view it is experiencing a force pushing it outward and the force of the test tube keeps it in place but does not force it toward the center. Huh? It still sounds centrifugal to me.
    also…
    I teach 7 – 18 year old oval race car drivers that centrifugal force tries to send their race car into the wall so to help fight this they put larger tires on the outside, the racetrack is banked and they need to have strong arms. Am I right or wrong? The only thing centripetal here is the fact that the track is an oval the car always wants to go out and away from the center. The right tire budget should prove this.

  91. Acceleration, vectors, scalars, inertia, jerk ,jounce, fair ground rides maximize same forces, ….constants, rate of change, all real. Car turning to right more about vectors, scalars, change in angular momentum. On mobile here so might read a bit brief as only got small box.

  92. Dave

    This reminds me of people who say astronauts are not in zero gravity, it’s just that they are continuously falling around the earth. Annoying.

  93. Curiosity Geek

    If RELATIVITY has frames of reference, why not these 2 forces? I think think the GRAVITRON at the fair is a great example. Standing outside, I don’t feel the force, but I can see its effect.

    Regarding the BULLET, isn’t air hitting it creating friction?

    Regarding CHICK TRACTS, search for anti-chick tract online. Skeptic versions do exist.

    Lastly, it’s troubling that scientists disagree on the basic Forces of Nature. I look to Science for knowledge and understanding. If scientists can’t agree on what is real or not real, then how am I to find correct data to feed my brain?

    To the best of my limited knowledge, BA has satisfied my curiosity in the past, if not always my complete understanding. My experience and his explanation of these forces gel. Moving with the BA on this one.

  94. Andres Minas

    A misunderstanding of choice of frame of reference to a frame of reference. In the end, Phil is right. Centrifugal = Centripetal.

  95. Tom Concannon

    It’s amazing how the comments on this blog post continue to grow.

    It’s also amazing how many people don’t remember elementary physics. There is no such thing as “centripetal force” as an actual physical force (i.e., arising from one of the four fundamental forces of nature). Centripetal force is the VECTOR SUM of all the physical forces (each arising from one of the fundamental forces of nature) keeping an object moving in circular motion. No good student (or teacher) of physics will draw a free-body diagram with a centripetal force vector on it.

    On the other hand, centrifugal force certainly “feels real” in a rotating reference frame, but, as other commenters have noted, this effect arises from inertia. There is no “centrifugal force” but in a different way than there is no “centripetal force”; there are no physical forces pointing in the “outward” (away from the center of rotation) direction that you can sum, even in the rotating reference frame.

    Centrifugal force is a ficticious force in the sense that you need to include it to make Newton’s second law covariant (of the same form in the rotating frame of reference). It’s ficticious because there none of the four fundamental forces of nature that provide the source for it. As Pauli once noted, “It’s not even wrong!”

    So, sorry Phil, centrifugal != centripetal.

    (I meant to comment on this years ago, when it was still a hot topic, but Phil’s recent post of the very cool hula hoop camera video led me back again to here.)

  96. Kristian

    Maybe you could answer this question for me then – I am more confused that I have ever been about rotational forces!!

    My daughter was spinning on a pole today with bearings top and bottom. She noticed that when she straightened her arms (making radius of the circle greater) she span slower, and when she then bent her arm she traveled faster again. My wife explained it as being “centrifugal force” to which I replied (spouting the words of my high school physics teacher) “No it is centripetal force, there is no such thing and centrifugal force”, and then went on to explain the reasoning.

    My daughter then worked out that if she were to spin as fast as she could with straight arms, then bend them she would go super fast. However when she tried this she didn’t have the strength in her arms to bend them. This kind of proved my wife’s argument right. I had no reply to her question then what force is working to in to the center of the circle?

    My only answer was to quote Newton’s third law of motion, and therefore if centripetal force exists centrifugal force MUST exist.

    I thought I had a better grasp of physics than my wife but she has just upstaged me! I don’t mind being upstaged with a decent explanation backed up by scientific study. Help!!

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