# Homework Help: Cannon shot on airless planet with x gravity

1. Oct 22, 2008

### Mugen112

1. The problem statement, all variables and given/known data
A huge cannon is assembled on an airless planet. The planet has a radius of 5.0 \times 10^{6}\;{\rm m} and a mass of 2.6 \times 10^{24}\;{\rm kg}. The cannon fires a projectile straight up at 3800 m/s.

An observation satellite orbits the planet at a height of 700 km. What is the projectile's speed as it passes the satellite?

2. Relevant equations
1/2mv22 - GMm/r+y2 = 1/2mv12- GMm/R

3. The attempt at a solution

I'm trying to use that equation and solve for y2. I keep coming up with a negative number.

v2= sqroot[v12 - (2GM/R) - (2GM/R+y2)]

you can see that when u subtract, you will get a negative under the root.... im going to go crazy. any help is appriciated.

2. Oct 22, 2008

### alphysicist

Hi Mugen112,

I don't believe this equation is correct. When you moved the potential energy term from the left side to the right its sign should have become positive.