Details

Snapshot 1: After a completely inelastic head-on collision both spheres move together.

Snapshot 2: In an elastic collision, the trajectories of two identical spheres in any angular collision are exactly perpendicular.

Snapshot 3: However, if the collision is inelastic the angle between the trajectories is smaller than 90°.

Snapshot 4: Here is the same collision as in snapshot 2, viewed in center-of mass frame.

Snapshot 5: The same collision as in snapshot 3, viewed in center-of mass frame.