Gravity and mass

[foodchain]

Purely hypothetical.

 

If you had a body, a planet sized one with a planetary orbit like in our solar system for instance, and you had two of them, with identical orbits, such as one is just polar of the other in the orbit(if that’s possible?) and they both had the same mass and velocity for instance in the orbit, when to composition of such have any impact on the gravity they might posses, such as if one was made of carbon or one was made of oxygen for example. Now baring density from this, just balls of matter of those two types, with exact mass and velocity, would they hold a different degree of gravity, such as more or less gravity?

[Spyman]

Density has no impact on the strenght of gravity, but distance has.

 

The planets would attract each other or a spacecraft with the same strenght, but a person visiting the surfaces would feel a difference since with different densitys the planets would also have different radius.

[Rocket Man]

i think the question is where is the centre of gravity?

obdviously the centre of mass is in the middle but considering the inverse squared taper, how would you calculate the perceived position of that mass as a point source?

[swansont]

i think the question is where is the centre of gravity?

obdviously the centre of mass is in the middle but considering the inverse squared taper, how would you calculate the perceived position of that mass as a point source?

 

Gauss's law tells you that as long as you are outside of the mass (assuming it's spherically symmetric), it looks like a point source.

[Rocket Man]

thanks

http://en.wikipedia.org/wiki/Gauss's_law

It is interesting to note that the gravitational flux, like its electromagnetic counterpart, does not depend on the radius of the sphere.

i didn't expect that.

[foodchain]

So then, the composition of mass such as mass from carbon of mass from oxygen deriving a body such as a planet has no factor in gravity as long as mass and velocity were equal in comparison of the planets? Its a question that has been bothering me for about two and half months now.

[Rocket Man]

basically, if you have two different spheres of the same mass but different density, the gravity will be the same at the same distance from the centre.

this only works for spheres of uniform density.

 

a planet made from oxygen is likely to have gaseous layers etc which mess around with the density so the inverse squared taper will be a little distorted. but it holds mostly true that you can assume a planet of any radius is a point source of gravity with that same mass.

[swansont]

basically, if you have two different spheres of the same mass but different density, the gravity will be the same at the same distance from the centre.

this only works for spheres of uniform density.

 

Uniform density isn't required, just spherical symmetry (no variation with theta and phi). The density can vary radially (smoothly or not — you can have a shell), and whatever mass is closer than you looks like a point source. Anything outside of you contributes nothing