Hello,

I have a question about the following question:

'Two particlesAandBare moving on a smooth horizontal plane. The mass ofAiskmwhere 2 <k< 3, and the mass ofBism. The particles are moving along the same straight line, but in opposite directions, and they collide directly. Immediately before they collide the speed ofAis 2uand the speed ofBis 4u. As a result of the collision the speed ofAis halved and its direction of motion is reversed.'

There are some questions that follow this statement, which I have answered. The question I have is about knowing what happens to the direction of motion of two particles after they collide. In the last question I had about colliding particles it stated that the direction of both particles was reversed. I answered the question by assuming that the momentum of the two particles before the collision must be the same, which gave me the correct answers. But the question above has me questioning that assumption.

Taking the direction of motion ofAto be positive then the momentum ofAbefore the collision is 2kmuNs and the momentum ofBis -4muNs.

Since 2 <k< 3, 2kmu> 4mu. I'm probably missing something, but I would assume that since the momentum ofAis larger then it is particleBthat should change direction. It turns out, according to one of the other questions, that they both change direction.

My question is, can you tell from the relative momentum of two colliding particles what the direction of motion of each particle will be after the collision?

Thank you