A naive person I deal with says that the Spin of an planetary object... ?

Question

... This person says that the Speed of the Spin of a heavenly body dictates at large its capacity in gravity. I know this is not true and we had a bet.
If you know for certain whether me or the other is right please illustrate your understanding hitherto, thanx.

Answer 1

Let's clear up our terms a little bit.

Gravity only generally occurs between things of mass. And it is pretty much independant of what the mass is doing. And I say pretty much because there are things that can create or destroy mass... but in a general sense it all comes down to the mass.

Now, as astronomical bodies form, they tend to get a spin. Because of the conservation of angular momentum, things that suck up more mass tend to have more spin than others. These are, however, tendancies, NOT necessities.

Case in point - the Earth. Because of the tides, the spin of the Earth is slowing down. We used to have twelve-hour days millions and millions of years ago, but now they're twice as long. Has this affected the force of gravity? Not a bit. The same mass is present, and it's causing the same amount of gravity.

What your friend may be confused by is what is sometimes called centrifugal force (a physicist would probably just call it inertia, though). Although gravity pulls us down to the centre of the Earth, the motion of spin carries us away from the centre, counteracting gravity. This means if you take a tool to measure gravity near the equator, you'll get a lower reading then you will when you measure it at the axis. The force of gravity hasn't changed at all, but the spin is creating another force that is mitigating gravity's effects. On Earth, this force is pretty small, but it's concievable that it could amount to a huge amount on a fast-enough spinning object... say, a space station with almost no gravity, but which was spinning fast to simulate gravity but pulling out instead of in.

So in a literal sense, you are completely right. Gravity has ONLY to do with mass (well, mass and distance). In a squishier sense, your friend is sort of right, in that spin can change the APPARENT force of gravity in local areas and that objects with large gravities also tend to have large spins.

Hope that helps!

Answer 2

After reviewing some of the other answers, I think some clarification is necessary.

Both of you are right! - (How's that for clarification?)

What I mean is, it depends on which model of Gravity you're using.

If using Newton's Law of Universal Gravitation, the rotation of a non-spherical object only influences it's Gravitational Field in a trivial way. That is, the shape of the object will modulate the attractive and tidal forces felt at an observation point. I call this trivial because Newton's Gravity does not include any dynamic terms, like Maxwell's Equations or General Relativity. It is strictly "action-at-a-distance." Hence, in Newton's model the rotation of a perfectly spherical mass has a no effect on its Gravitational Field.

In General Relativity (GR), the story is much, much more complex. Main reason, GR includes dynamic terms like Maxwell's Equations, hence a rotating perfect sphere does have a different Gravitational Field compared to a non-rotating perfect sphere. In fact, GR introduces an effect completely analogous to magnetism, called Gravitomagnetism, which cannot be described in any way by Newton's Universal Law of Gravity.

So, depending on which version of Gravity you're talking about, you're both right.

PS - Gravitomagnetism is also called "Frame Dragging."

Answer 3

You win! An object's mass determines its gravitational pull. It's spin has nothing to do with it.

Tidal forces may encourage a moon to rotate around a planet at the same rate of rotation (like the moon - this is why the moon always faces the earth the same direction) but that depends on the composition of the planet and moon, among other things.

Answer 4

the other person is right... the angular speed of massive object affects the value of gravity.
A non-rotating massive object and a rotating object (both with the same mass) have different gravity field, but the differences are not big values.
i.e. the earth has a theoretical gravity (after applying Laplace's equation for gravity field) defined by:
g = 9.78048*(1 + 0.0052884 sin²(theta) - 0.00000059 sin²(2*theta))
where theta is the latitude of the place on earth's surface.

Answer 5

This isn;t true. Gravity is dependent on the mass of the object and not on its rotational speed. This is given by the equation F = G(m1)(m2)/ r^2.