# Homework Help: Gravitational potential energy of satellites

1. Oct 8, 2008

### pbumper1

1. The problem statement, all variables and given/known data
Two Earth satellites, A and B, each of mass each of 980kg, are launched into circular orbits around the Earth's center. Satellite A orbits at an altitude of 4500kg, and satellite B orbits at an altitude of 13600km.
What are the potential energies of the two satellites?
What are the kinetic energies of the two satellites?
How much work would it require to change the orbit of satellite A to match that of satellite B?

2. Relevant equations
GmM2/r^2

3. The attempt at a solution
I have tried this problem several ways adding the radius of the earth, not adding the radius. I have been working on this problem for the past 3 days and can't seem to get any of it. Please help!!!

2. Oct 8, 2008

### Staff: Mentor

Show what you've done so far.

Hint: The "r" you need is the radius of the orbit, which is the radius of the earth plus the altitude.

3. Oct 8, 2008

### pbumper1

R=6378.1+4500=R1 R=6378.1+13600=R2
R1=10878.1 R2=19978.1
U1=-6.6*10^-11x980x5.974*10^24/R1 =-3.59*10^13
U2=-" "/R2 =-1.96*10^13 got them both wrong
v^2=6.6*10^-11x980/R1
plug v^2 to 1/2mv^2 i got 2.94*10^9 and 1.6*10^9 got them both wrong

W=GmM(1/R1-1/R2)=1.60*10^13 got that wrong too

4. Oct 9, 2008

OK.