Non-frictionless ball-ball collision

[wxmodz]

when is Elastic, instantaneous coming? been waiting for it from long

[ekiefl]

Context from https://ekiefl.github.io/2020/04/24/pooltool-theory/#section-ii-ball-ball-interactions:

This section is dedicated to the collision physics between two balls.

When physically modelling a phenomenon, the sky is the limit in terms of how real you want to get. In consideration of the ball-ball interaction, a complete classical description would entail treating the balls as compressible objects–perhaps even modelling the pressure waves that emanate within each ball during a collision. Perhaps this treatment is most necessary during the break shot, and I would be very interested to know how the degree of realism of such a treatment compares to the more pragmatic approaches I will be taking. Speaking of which, here are the two models I will present:

Elastic, instantaneous, frictionless
Elastic, instantaneous (TODO)

In each of these models, multi-ball collisions are not considered, i.e. each interaction is pairwise.

Eventually I want to focus on improving the physics, but I have no time estimate for you. Pull requests are welcome.