So one particle hits another particle and they both go off at certain angles from the original path. If you are trying to find the change in initial kinetic energy would you use the velocities along the new paths or the components in the direction of the original velocity?

Question

So one particle hits another particle and they both go off at certain angles from the original path.  If you are trying to find the change in initial kinetic energy would you use the velocities along the new paths or the components in the direction of the original velocity?

anonymous · Answer

For momentum conservation, you are working with vectors and need to use components. 

Energy is a scalar and thus uses only the velocity magnitudes.

anonymous · Answer

I think you would need both

You have KE of ball 1before impact, so you need to get the direction, mass and velocity of ball 1. 

And then the KE of ball 1 = KE of ball 1 [after impact] + KE of ball 2.
So you need to determine the direction and velocity of the after impact to get KE of ball 1 and you need the mass of ball 2 to determine what velocity and direction it will move after impact with ball 1