gravity vs. centrifugal force

jadibartolomeo · Oct 13, 2005

it seems the basis of my question is, how does gravity work and differ from centrifugal force? from what i understand, it's the rotation of mass that creates gravity, but why and how does that differ from centrifugal force? in his book accompanying the theory of relativity, einstein used the example of a man sitting eccentrically on a rotating disk, and that the centrifugal force he feels could be mistaken for gravity in the realm of the general theory of relativity. why is it not centrifugal force? since the earth spins, why are we not flung off of it? thanks!

igorsboss · Oct 13, 2005

from what i understand, it's the rotation of mass that creates gravity Nope. The mere presense of mass is enough to warp space. Rotation is not required for gravity. Mass warps the space around it, so that any other nearby mass will be attracted towards it. If the two bodies start from rest, they will simply fall towards each other and collide. That's the very simplest case, with no rotation, and no orbit.

drwayne · Oct 14, 2005

Note that there really is no such thing as a "centrifugal force" - it is, in physics terms a "pseudo force", it is the product of being in a non-interial (accelerating) reference frame. Did you realize that objects in circular motion have an acceleration associated with them? The acceleration of an object is by definition the time rate of change of the velocity vector. For an object in circular motion (on object in a circular orbit, a car going around a banked track, a person inside a "roundup"), the direction of the velocity vector is always changing, but not its magnitude. The magnitude of the acceleration (a) for such motion is given by a = v^2/r where v is the speed of the object, and r is the radius of the circle it is moving in. You can think of this equation as a "recipe" for circular motion. Why did I go to all this trouble to talk about the acceleration associated with circular motion? As you noted, one thing that Einstein stated was that to an observer, all inertial (non-accelerating) reference frames are equivalent. But, if you are spinning inside a dryer - you are not in an inertial reference frame - you are in fact accelerating. Wayne <div class="Discussion_UserSignature"> "1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher </div>

Saiph · Oct 15, 2005

centrifugal force, fact or fiction? Lets not get into that (it involves rotating and accelerated reference frames...ugh). But, gravity is not caused by rotation. And one can not tell the difference between "mechanical" acceleration and gravitational acceleration. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

jadibartolomeo · Oct 15, 2005

thanks for the replies! you helped clear it up a bit.

siarad · Oct 15, 2005

You have to be connected or directed to feel centrifugal force & change position. Weight on a string, connected externally. Turning, directed internally. Gravity is felt without either by a mysterious means requiring no change in your position.

rogerinnh · Oct 15, 2005

Saiph stated: "one can not tell the difference between "mechanical" acceleration and gravitational acceleration. " Ah, but yes you can. Well, at least under certain circumstances. Let's say the "gravitational" gravity (i.e. caused by the presence of a spherical mass) is experienced inside a box on the surface of a planet and the "mechanical" gravity is experienced inside a box in space that is being accelearated, by a rocket engine, perhaps. Since you're inside a box in each situation and cannot look outside to see whether you're on a planet or being pushed by a rocket, can you tell the difference? Yes you can. Get two strings and two weights. Tie each string to a weight and tie the strings to the ceiling of your box, one at each corner of the box, so that the weight hangs down like a plumb bob. If you are in the box that's being accelerated by the rocket, both strings will hang exactly parallel. If you are inside the box on the surface of a planet the two strings will not hang parallel but rather will have a slight (excrutiatingly slight) angle between them, since each one hangs towards the center of the planet. In other words, the accelaration within the rocket-propelled box is the same throughout the box, while the acceleration (due to mass) on the planet is different at every single point on the planet's surface, and in the box, in that it's direction (its vector) is different.

electronman · Oct 16, 2005

Ok wise guy. Just kidding. Don't get twicked out I was just trying to call your attention. Now that you've made a beautiful unification of gravity and centrifugal force; is there a way to associate these forces for the tendency of all planetary bodies at least with sizeable masses not to take other shapes other than a sphere? I'm a common layman but can take some scientology beating if you will. I'm afraid this is another one of those dumb questions that you guys probably find boring but of mysterious observation to an Appreciation of Physics 101 sounding but hopelessly curious dude like me. Thanks to your indulgence. Curiosity killed the cat.

drwayne · Oct 16, 2005

Actually, the premise of the elevator experiment is that you are in fact in a uniform gravitational field (there was a discussion on this some time ago) - the planet that your elevator is sitting on is VERY large if you will. Wayne <div class="Discussion_UserSignature"> "1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher </div>

drwayne · Oct 16, 2005

"Ok wise guy. Just kidding. Don't get twicked out I was just trying to call your attention" I am confused. You have not posted to this particular thread, yet your post seems to indicate that you have, and that you found something I said perturbing. In answer, I think, to your subsequent question, large masses tend toward the semi-spherical (note that the Earth for example is an oblate spheroid) because that is a minimum energy configuration - one in which everyone gets as close to the center as possible. Wayne <div class="Discussion_UserSignature"> "1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher </div>

Saiph · Oct 17, 2005

drwayne has the right of it. You can't tell the difference between a uniform gravitational field and uniform acceleration. One neat thing is that you can tell the difference between gravity and rotational acceleration. I caught a professor on that one. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

drwayne · Oct 17, 2005

You can essentially get around the rotational issue by assuming a very large radius of rotation - as long as you don't worry about how fast you are spinning. <img src="/images/icons/wink.gif" /> Wayne <div class="Discussion_UserSignature"> "1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher </div>

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