# Homework Help: Gravitation Problem

1. Mar 5, 2008

### Pho3nix

[SOLVED] Gravitation Problem

1. The problem statement, all variables and given/known data
The moon has a mass of 7.7times10^22kg and radius 1.7times10^6m. Calculate:
a) The gravitational potential at its surface.
b) The work needed to completely remove a 1.5times10^3kg space craft from its surface into outer space.
c) What is the minimum speed which a body must have to escape from the moons gravitational field?

2. Relevant equations
Vg=-GM/r
Ep=GMm/r
Ek=.5m(vsquared)
Vg is the gravitational potential
Ep is potential energy

3. The attempt at a solution
a) Vg=GM/r
G=6.67times10^-11
M=7.7times10^22kg
r=1.7times10^6m
Plug in the numbers and I get -3.0times10^6
b)Ep=Gmm/r
M=7.7times10^22Kg
m=1.5times10^3Kg
r=1.7times10^6
Plug in numbers and get -4.5times10^9 Joules
c)In need help on this one.

2. Mar 5, 2008

### Shooting Star

Equate the KE to the work done in (b). That gives you the minimum v, which is called the escape velocity, which is the minimum speed that has to be given to the body remove it to infinite distance.