# Gravitational question (spacecraft)

1. Nov 1, 2007

### ccsmarty

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

An unmanned spacecraft is in a circular orbit around the moon, observing the lunar surface from an altitude of 52.0 km. To the dismay of scientists on earth, an electrical fault causes an on-board thruster to fire, decreasing the speed of the spacecraft by 27.0 m/s.
If nothing is done to correct its orbit, with what speed (in km/h) will the spacecraft crash into the lunar surface?

2. Relevant equations

mass of moon = 7.36*10^22 kg
radius of moon = 1.7374*10^6 m

3. The attempt at a solution
This is what I tried, but I got the wrong answer. Can someone please tell me what I did wrong.

2. Nov 1, 2007

Can you justify why you did this?
GM/R^2 - vf^2/R = a

And later on, why is v0 = 0?

3. Nov 2, 2007

### ccsmarty

Well, the first equation I got when I asked an online tutor for help.
I made v0 = 0 because I thought that the instant after the thruster fired, the velocity would be zero.

I tried doing the problem a different way today and here's my work:

However, this did not yield the right answer :(.
Any help is greatly appreciated.

4. Nov 3, 2007

### learningphysics

What you did the second time around looks right to me. only thing I'm concerned with is the exact numbers, and how accurate your result needs to be...