# Density of a planet

1. Jul 17, 2008

### lempkema

1. The problem statement, all variables and given/known data

A satellite is in orbit very close to the surface of a spherical planet. The orbital period is 2.58 hours. What is the density of the planet? Assume the planet has a uniform density.

2. Relevant equations

T^2=(4pi^2/GM)*r^3

Density=m*v

3. The attempt at a solution

I've posted in several other forums, but no one can seem to help me with this problem. I can't find a way to solve for radius or mass, and i need both of those to ultimately find the density of the planet.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 17, 2008

### alphysicist

This formula is incorrect; density is mass/volume.

So what is the formula for the volume? Since you have two equation, have you tried putting them together (solving for one and plugging into the other)? What does that give?

3. Jul 17, 2008

### lempkema

that would give me:

T^2=(4pi^s/(G*(Density*v))*r^3

but that would give me three unknowns instead of just two. i just feel like i need more information to solve this problem.

4. Jul 17, 2008

### alphysicist

What is the formula for the volume? Plug that into the equation. Do you see how that helps?

5. Jul 17, 2008

### lempkema

or it would give me:

T^2=(4pi^2/(G*density*(4/3pi)*r^3))*r^3, which is T^2=(4pi^2/(G*density))

thank you so much, i think i've got it from here :)

6. Jul 17, 2008

### lempkema

haha i mean (4pi^2/(G*(4/3)*pi*density))

7. Jul 17, 2008

### alphysicist

That sounds right to me.