Momentum Problem: Jack and Jill on a Crate

[robbondo]

Homework Statement

Jack and Jill are standing on a crate at rest on the frictionless, horizontal surface of a frozen pond. Jack has a mass of m1, Jill has a mass of m2, and the crate has a mass of m. They remember that they must fetch a pail of water, so each jumps horizontally from the top of the crate. Just after each jumps, that person is moving away from the crate with a speed of v relative to the crate. What is the the final speed of the crate if jack and jill jump off the crate simoltaneously in the same direction.

Homework Equations

The Attempt at a Solution

I know that this has something to do with momentum

I solved the total momentum of the system equal to zero and solved for the v of the crate Vc.

m1v+m2v+mVc=o and solved for Vc to get

Vc=(m1v+m2v)/(-m)

This is incorrect. I don't really know if I should be using momentum or conservation of energy in this problem. thanks for all your help.

 

[robb_]

which conservation to use? well, when does each apply?