Comments on: Q & BA Episode 4: The Gravity of the Situation

By: Astrolink [Global Edition] » Double dipped supernova | Latest astronomy news in 11 languages

Fri, 15 Jun 2007 02:47:09 +0000

[...] blobs of gas — massing as much as the Sun! — at high velocity, and was temporarily the second brightest star in the sky, even though it’s 7000+ light years [...]

By: Amanda

Amanda — Wed, 28 Feb 2007 21:39:00 +0000

You rock. Seriously. Whenever I hear things like this, it reminds me that there are soooo many things to think about and learn and understand… thank goodness for teh intarwebs, so that somebody like you, who is so very good at explaining complicated things in a way that nearly everyone can understand, can teach so many people about things like this.

Your website is a science teacher’s dream. Fun, informative, relevant. Amazing.

By: bloo

bloo — Tue, 27 Feb 2007 19:03:08 +0000

Who’s the guy with the fish?

By: Garrett

Garrett — Tue, 27 Feb 2007 17:29:32 +0000

This topic brings to mind a great xkcd cartoon

http://www.xkcd.com/c89.html

By: Heraclitean Fire » Links

Heraclitean Fire » Links — Tue, 27 Feb 2007 11:28:03 +0000

[...] Bad Astronomy Blog Â» Q & BA Episode 4: The Gravity of the Situation ‘Just how strong is the gravity from the Moon compared to someone right next to you?’ – I found the answer surprising. via Sandwalk. (tags: astronomy moon gravity) [...]

By: BC

BC — Tue, 27 Feb 2007 08:22:39 +0000

I’m confused. Doesn’t that mean that the tides should be far more dependent on the Sun than the Moon? In other words, tides should be very dependent on day/night and the moon should have very little effect in comparison, right?

Oh wait. I just looked at wikipedia. I might as well post my comment for others to see. http://en.wikipedia.org/wiki/Tides#Tidal_physics

By: Aristotle Pagaltzis

Aristotle Pagaltzis — Tue, 27 Feb 2007 01:55:30 +0000

Hey Phil, nice hairdo this time around (or was it simply masked by the cap in the last couple of episodes?). I reckon the change to you nick for that oneâ€™s gonna be miniscule, you only need to add a single L to â€œThe Bad Astronomer.â€ :)

By: Melusine

Melusine — Mon, 26 Feb 2007 23:37:51 +0000

BTW, it's another Virgin Mary sighting in Houston - this time on a baking pan. You can see the photo where they compare the pan on the left to an actual picture of a VM outlined. I think it looks like a fat bowling pin. It's embarrassing.

By: The Bad Astronomer

The Bad Astronomer — Mon, 26 Feb 2007 19:38:10 +0000

I worried a bit over the fact that humans are not point sources, but realized that in a 5 minute vidcast that would be hopeless to cover. I also considered tides, but again I didn’t have time. If you work out the tides from astronomical objects, the nearby human wins easily, because you get an extra factor of distance in the denominator (1/distance³), so celestial objects get drastically reduced in their influence.

Cindy– the video/auido synching is making me crazy. It looks fine here at home, but when I upload the video it gets out of synch. That caused me many nightmares last week; the Google video was awful and I had to reupload a different version. I have to make a WMV and QT version, as well as audio, and upload it to YouTube, and and and. I would love it if I could make one version for everything, but there is no standard yet.

By: Gary Ansorge

Gary Ansorge — Mon, 26 Feb 2007 17:45:48 +0000

It’s generally much easier to consider a gravitational acceleration as coming from a point source. ie, consider earth as a singularity with the center of mass 6400km below your feet. That produces an acceleration of 9.8 meters/sec/sec at your feet.

Much easier to do these calcs. from this point of view,,,

GAry 7

By: ioresult

ioresult — Mon, 26 Feb 2007 15:30:44 +0000

And thanks atr also. These posts are arriving at great speed!

By: ioresult

ioresult — Mon, 26 Feb 2007 15:29:07 +0000

David SD, thanks.
Disregard my last paragraph about integral calculus! heh heh

By: atr

atr — Mon, 26 Feb 2007 15:27:45 +0000

And I clearly spent too long writing that, as did ioresult who had his bit about spheres answered just before he posted.

By: atr

atr — Mon, 26 Feb 2007 15:26:06 +0000

I suspect it’s not actually quite 1/r^2 because a person is not a spherical body (except in certain special cases…). It’d probably approximate to a lower power of r (given that that’s what an infinite wire or infinite sheet would be) so the person would lose even more though.

By: ioresult

ioresult — Mon, 26 Feb 2007 15:24:52 +0000

I haven’t seen the video yet (being at work), but I made an interesting calculation:

To feel the same pull as the moon, you’d need to be touching a 30m diameter sphere of iron.

Math involved:
Mass of iron sphere = (mass of moon) * (distance to center of iron sphere)^2 / (distance of moon)^2
Mass of iron sphere = 4/3*pi*(radius of iron sphere)^3 * 7860 kg/cubic meter (density of iron)

Solved for when distance to center of sphere is equal to radius of sphere.

I’m not taking into account that parts of the sphere are nearer than other parts. I’d need to re-study integral calculus for that. Not just now.

By: David S-D

David S-D — Mon, 26 Feb 2007 15:22:00 +0000

2 things:

First, Josh Zucker makes a good point that if you’re in free fall around an object, then you only feel tidal forces. For instance, the sun’s gravitational field causes us to accelerate towards it. But it causes our head, and our arms, and our feet, and the floor under us, and the furniture around us to accelerate towards it at almost exactly the same rates. We, and everything around us are in free fall around the sun, so by the equivalence principle, we don’t observe any change in nearby physics. This picture gets modified somewhat because there are minute differences in the strength of the sun’s gravitational field between our head and our feet. These differences cause some parts of us to accelerate at different rates than other parts, and we effectively feel tidal forces, which as Josh noted go as 1/R^3, and are quite small. The only objects we aren’t in free fall around are the earth and the obstetrician (in both cases, the floor is providing the opposing force), so their effects go as 1/R^2.

The other thing I wanted to mention is less important: calculating a gravitational field using GM/R^2, where R is the distance to the center of mass only works for far-away objects (when the distance to the object is much greater than the size of the object), or for spherically symmetric objects. Strictly speaking, to get the gravitational field from a human, you have to sum up all the little fields from each part of the human. This is complicated, but here’s a quick estimate of the size of the effect:

When you’re very close to a human, they look more like a cylinder than a sphere, so their gravitational field goes like 1/R (where R is the distance to the axis of the cylinder — the line running from the top of your head to the bottom of your feet). As your distance becomes greater than the person’s height, they look more like a sphere, and the gravitational field begins to drop off as 1/R^2. The dimensional quantity that sets the scale for this change is the person’s height, h, so the gravitational field will probably look something like

1/R(R+h)

which is about 1/Rh when R is small, and more like 1/R^2 when R is big. Regardless, 1/Rh is the relevant law for two people who are close together. Since a person’s height is about 10 times their width (which is what Phil used in the 1/R^2 law), Phil probably overestimated a nearby person’s gravitational pull by about a factor of 10.

By: Cindy

Cindy — Mon, 26 Feb 2007 13:56:34 +0000

Thanks, Phil, now I know of a great problem to give my students this spring when we cover gravity! And something else to think about when I’m in labor this spring.

Just a note, the sound synchronization is a bit off from the video on the version you have above.

By: Samuel

Samuel — Mon, 26 Feb 2007 13:28:17 +0000

An esoteric group invited me once to talk about the moon in one of the their meetings. Beautiful place, beautiful moonshine night, interesting people. But they were a bit upset when I said the gravitational and “atomic radiation” influence of mountains were a lot bigger than the Moon’s.

Joshua have a good point about the tidal force. It is more complicated because there have to be a differential distance with respect to the center of the Earth (Moon, Sun, Jupiter etc) and the direction matters.

Thus, a person in top of another would “feel” Earth’s tidal force but not side by side. But then, gravity is not a big issue in those moments. [;)]

By: Joshua Zucker

Joshua Zucker — Mon, 26 Feb 2007 10:02:54 +0000

I think that there’s another possible issue: tidal forces. After all, the gravitational influence doesn’t really affect you — you and the Earth together are essentially freely falling around the moon, right?

So, how about the tidal forces? They’re 1/R^3 so now the person (millions of times closer) easily wins over the Moon. And maybe the OB beats the Rocky Mountains, too.

By: Johan Couder

Johan Couder — Mon, 26 Feb 2007 09:23:21 +0000

no no no, you’re getting away with this too easily. The proper theory to use is General Relativity of course. And as Einstein was well aware, people do not distort space-time the same way starts or planets or moons do: “Sit with a beautiful girl for two hours and it seems like two minutes. That’s relativity”. I looked at the moon for two hours, and it really felt like two hours.

By: Nathan Hinman

Nathan Hinman — Mon, 26 Feb 2007 07:38:32 +0000

another great video Phil.

You really should do more educational programing on a good educational channel like Discovery (TLC doesn’t count since they started doing home reno and make over shows all the time).

By: Lauren

Lauren — Mon, 26 Feb 2007 05:51:00 +0000

I like the Skepdude Calendar in the background. I always look forward to seeing what is going to be sitting there.

I found this episiode to be really interesting. That was a great question! My first response to the question was, “Of course the person has a bigger pull,” then actually thinking about it, it makes more sense that it’s the other way around. After all, the moon does have an effect on the tides.

Great show, once again. I look forward to these videos, for the info, the humor, and whatever is going to be on the table.

Just a note, I subscribed in iTunes, and I think the quality there is a lot better than youtube. I’ve also noticed that the videos appear there before they do here, haha.