Collisions - Elastic and Inelastic - Center of Mass Frame of Reference
http://videolectures.net/mit801f99_lewin_lec16/
1. 1D Elastic Collisions:

A mass with given speed collides with a second mass (initially at
rest) in a one dimensional collision. Momentum is conserved. If kinetic
energy is also conserved, the velocities of both objects after the
collision can be calculated. Three limiting cases are explored
analytically, and then demonstrated. The equations are used to predict
the outcome of some air track experiments.

2. Brain Teaser - Elastic Collision with a Wall:

A tennis ball bounces off a wall elastically. The momentum of the
wall changes, but the kinetic energy of the wall remains zero. How is
that possible? Something to think about!

3. Center of Mass (CM) Frame of Reference:

A 1D elastic collision is considered as seen from the CM frame of
reference (where the total momentum is zero). Using the velocity of the
CM in the Lab frame, you can transfer between the two frames.

4. 1D Inelastic Collision and Internal Energy:

A 1D inelastic collision is considered from the laboratory and the CM
frame. The kinetic energy is calculated in both frames and it is shown
that the initial KE in the CM frame is the maximum KE that can be
converted to heat (this is called the internal energy of a system). The
equations are used to predict the results of an air track experiment.

5. Newton's Cradle Demonstration:

Professor Lewin solicits an analytical proof of his demo showing a lineup of colliding balls.