Hi everyone, I've got a problem with solving this one. Two spheres X and Y (no mentioning about them being identical in mass but drawn in equal sizes) are about to collide head on. Their initial velocities are +5ms^-1 and -5ms^-1 initially, moving towards each other with X being towards east. Assuming completely elastic and head-on collision, what happens to the spheres after collision? The answer I was given was that X comes to a rest while Y moves off in the reverse direction, that is, east with 10ms^-1. Why is this so? I don't really understand why is it that Y would move off and not the other way around where X moves off towards west with 10ms^-1? Why is all momentum transfered to Y and not X? I apologise that I don't have any workings with me when I should have but I'm confused with the principles here. Also, does the relative speed of approach only applicable to elestic collisions? Thanks again to all for the constant help.