I always liked Winston Churchill!

*Quickly in astronomical terms. I don’t know how fast the disk material is orbiting.

You mentioned that the apparently nearly circular disk of Vega’s system is evidence that we are looking nearly pole-on to a circular system — taken from the fact that if we looked obliquely at a circular system we’d see an ellipse.

Okay, so how do we know we aren’t looking at an elliptical system obliquely, yielding an almost-circle?

Mixing pounds with newtons, that was waay bad!

I also corrected my math error. I didn’t convert miles to km! Amazing. Good thing I don’t plan any Mars probes.

Back near the middle of the last century, I gave many talks on the subject of measuring the acceleration of gravity and I used to always use the 0.5% approximate value with about a 50/50 split between the two effects. It was not quite true.

I thought it interesting that the producers even put a debris field around the star.

- Jack

PS – If you have the DVD of the movie, watch the special feature on the making of the opening “pullout” sequence. On the image of Mars, the FX guys embedded Hogland’s face. Well, not his actual face; it’s the image he’s been making a living off of for the past decade or so. Just another in-joke.

As I learned it back in high school physics (slightly after the Earth cooled), centrifugal force is cause by centripital acceleration. The radial acceleration is towards the center causing a force away from the center.

- Jack

Your weight at sea level at the equator is about 0.5% lower than it is at the poles. About half of the difference is due to the centrifugal effect and about half is due to the spheroidal shape of the earth, also caused by the centrifugal effects. Thus, because you are farther from the center of the earth, the effective force of gravity is lower at sea level on the equator than at the poles. The acceleration of gravity (which by convention, includes centrifugal effects) changes by about one part in a million for every every ten feet change in altitude. Thus, you also weigh about 0.3% less at the summit of Mt. Everest (not that I’ve been there) than you do at sea level at the same latitude. Gravity provides the centripetal force toward the center of the earth. This, of course, is far larger than the force needed to overcome the tendancy for the rotation to throw you off the earth.

Using Google as a calculator, and Nineplanets.org for dimensions:
275 km/sec = 615157 mph
Earth’s circumference = 24901 mi
615157 / 24901 = 24.7 revolutions per hour
1 hour / 24.7 = 146 seconds per revolution, not 90.

According to http://www.solstation.com/stars/vega.htm, the actual rotational period is 12.5 hours, which “suggest that the star is rotating at about 92 percent of the speed (angular velocity) that would cause it to physically fly apart”.

The orange “+” represents the “subsolar point”, according to the paper at the link Phil posted. Doesn’t that mean a line between the core of Vega and us passes through that point? Which in turn means it’s the center of the disc, as seen from here?

Finally, about the centripedal/centrifugal thing: Not sure how it’s “officially” defined, but it seems logical to me to use them for the correct forces. Since centripedal means “to the middle”, it should refer to the major body pulling inward on the minor body (whether major/minor be star/planet, planet CoG/planet particle, or person/bucket of water). Since centrifugal means “away from the middle”, it should refer to the minor body pulling outward on the major body. Both are real, opposite forces that are actually present in any reference frame. The direction may seem to change (so which is which might be hard to define without a defined reference point), but they are still different forces. The inertial effect is what gives the forces meaning (otherwise they’d both just instantly accelerate each other to infinity–or impact).

Like in a car: the centripedal force is the tires pulling the car towards the center of the corner, the centrifugal force is the car pushing the tires towards the edge of the road. The inertial effect determines how much force the tires must overcome to keep you from smashing into a tree.

I haven’t taken a classical mechanics class here in France, but since the physicists in this neck of academia seem to use the same concepts as those back in the United States, I expect they’d think about la force centripète and la force centrifuge the same way.

The details get horribly messy, and no two books describe rotating bodies in exactly the same way. The “centrifugal” or “centripetal” forces are not the only things at work. For example, imagine standing at the center of a merry-go-round and trying to walk to a point on the outer edge. In a reference frame which rotates along with the carousel, a point on the edge is fixed with respect to the center: walking from the center to the edge would look like a straight-line path. But to a person standing outside the carousel, feet planted firmly on the ground, you would be moving in a spiral trajectory.

According to Newton’s Laws, if a body in motion changes its speed or direction of movement, a force must be acting on it. Joe, standing beside the carousel, sees Moe on the carousel changing his direction of motion, and so Joe deduces that a force must be acting on Moe, a “Coriolis force”.

Things can get even more fun when you consider motion on a spinning planet. There are centripetal effects and Coriolis effects, and like I said, no two books want to talk about it the same way. This is a ready source of problems used to torture college physics students: “A cannon at the North Pole fires a projectile at 500 meters per second. If the planet were not rotating, the acceleration of gravity would be 9.8 meters per second squared, but the planet is rotating with an equitorial velocity of 1600 kilometers per hour. Where will the cannonball hit the surface of the planet?”

Really, too much brainpower has already been wasted arguing back and forth between “centripetal” and “centrifugal” this-or-that. If demon gnomes snuck into all the libraries one night and swapped the two words everywhere they appear, it wouldn’t make a difference. (People who care about etymologies might get a little confused, but we already have plenty of words in science which go “the wrong way”: positive and negative electric charge, for example. Cathode and anode are also turned around, in consequence.) We should be allowed to use whichever word we prefer when speaking in loose or qualitative terms, since everybody knows perfectly well what we’re talking about.

The dispute over petals and fugals is the science-class equivalent of the arguments about splitting infinitives or ending sentences with prepositions. It’s all the sort of nonsense up with which we should not put.

College physics was a long time ago for me. In an example of a child spinning a rock on a string, I distinctly recall that centripetal force was the force keeping the rock on the string from flying away–ie it was the force exerted on the rock by the string. That’s the opposite of centrifugal force, which is the force pushing the rock away from the child.