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### Understanding Elastic Collisions

This applet animates a collision between two balls, moving horizontally on a frictionless table. It's assumed that the balls are also friction free, so the collision does not cause spin, which would absorb kinetic energy.

*Points to note*: in the center of mass frame, the momenta are equal and opposite at all times. The velocities scale as the inverse of the masses in this frame of reference.
In the so-called laboratory frame (one ball initially at rest), note that the center of mass moves at constant velocity throughout (why?), and *for equal masses*, the final velocities are always at right angles. (That follows immediately from comservation of (vector) momentum, conservation of energy, and Pythagoras!)

You can estimate relative velocities by looking at the distances between the (equal times) dots.

* Code by Morgan Locks*