[VRWCtaz]

Particles bouncing off a surface, even at a shallow angle, impart a force perpendicular to the surface, not at the angle of the incoming particle (my summary). I had to think about that, as it is counter-intuitive unless you think it through, but sure enough, that objection is valid. The change in the particle’s direction uses up the rest of the momentum (approximate statement).

It took me many Saturday afternoons in a dark pool hall and a few forfeited dollars before I fully appreciated that concept.

[steve86]

Hadn’t thought about that application, not being a billiard player. So it really works, eh?