# GRE Momentum question debate

## Homework Statement

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!

Doc Al
Mentor
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.
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)?

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

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.
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.