# GRE Momentum question debate

1. Sep 28, 2013

### PsychonautQQ

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

http://grephysics.net/ans/0177/55

The question hasn't been typed up yet but it's pretty basic. A particle of mass m runs into a particle of mass 2m. After the collision, the particle with m is at rest and the particle of mass 2m has split into two particles each with mass that have both y and x components to their velocity, separated by θ.

If you look at the people that commented on this question there seems to be a bit of debate. One quite intelligent and confident poster wrote that the velocity in the x direction for each particle is equal to v/2, since the initial velocity was v and horizontal momentum must be conserved. Another poster wrote that the x component of velocity for each particle does NOT equal v/2 because they aren't moving along the x-axis.

Who is correct and why?

Thanks Physics Forums!

2. Sep 28, 2013

### Staff: Mentor

A full statement of the problem would help. The angle θ is the angle between the two split particles or is the angle they both make with the original direction of the first particle (as one comment assumed)?

Why couldn't the x-components be v/3 and 2v/3? Unless the split is symmetric.

I don't know what that is supposed to mean. I assume that the x-axis is the direction of motion of the first particle.