# Fluid problem.

1. Apr 13, 2008

### shenwei1988

my professor gave us this problem on the homework assignment about Fluid.

Planet X is a sphere made entirely of the mythical metal exoferrum, which has density 2810 kg/m3. What is the period of orbit for a satellite in circular orbit just above the surface of Planet X? (That's right: I did not give you the mass or the radius of the planet!)

i have no ideal which equation i should use.

2. Apr 13, 2008

### Shooting Star

This has got nothing to do with fluids, and you have shown no work.

Could you have done it if r had been given?

Assume that some value has been given to you for the radius r, and just see how far you can proceed using 'r' as a symbol (or a number, if you like). If you get stuck, we are here.

3. Apr 13, 2008

### shenwei1988

using newton's law of gravity, T^2= (4π^2*r^3)/GM

volume of sphere is V=4/3πr^3
ρ=M/V

put in newton's law of gravity, T^2=3πV/GM T^2=3π/Gρ

4. Apr 13, 2008

### shenwei1988

thanks a lot, the hint you gave is so helpful. i got it right.