Thanks, you were right about the astronaut moving backwards, that solved that problem.
As for the cannonball one, I think I'm supposed to assume its a frictionless surface/system, since I'm given no information about that. With that in mind:
Solving for time: (.4 m) / (12 m/s) = .0333 s...
1. A 160.0Kg cannon initially at rest shoots an 8.00Kg cannon ball at a speed of 240.0 m/s. The cannon moves backwards 0.40 meters. What is the force on the cannon after firing the cannon ball?
I've solved for the velocity of the cannon, getting 12 m/s, but don't know exactly where to go from...