# Homework Help: Conservation of momentum problem

1. Nov 3, 2013

### bookerdewitt

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

An object with mass m and an object with mass 3m collide in one dimension. The object with mass m is moving at 3 times the velocity of the object with mass 3m. What can you say about their velocities after the collision?

2. Relevant equations
m1vi1 + m2vi2 = m1vf1 + m2vf2

3. The attempt at a solution
Well first I tried to use the equation for conservation of momentum and solve for the final velocities but it got quite messy and I wasn't getting anywhere. But if they collide head on wouldn't the velocities after the collision have to be 0 since the total momentum before would be 0? The only thing is it doesn't say theyre colliding head on.

2. Nov 3, 2013

### Nick O

It says that they collide in one dimension, which is a way of saying that they collide head-on. The velocities do not have to be zero after the collision, but the momentum has to remain zero. This is probably supposed to be an elastic collision, so kinetic energy must remain unchanged (i.e., velocity can not be zero).

To start, try changing m1 and m2 to m and 3m, and then vi1 and vi2 to 3v and v. After you do that, you can simplify the equation considerably.

3. Nov 3, 2013

### bookerdewitt

I did try that but I must have messed up somewhere... ill try it again