OK
all the 'reflection' stuff I mentioned earlier?
thats for when Balls hit Walls; the wall doesnt move so a perfect reflection of the balls position and velocity is good there
Its a special case.


for when balls hit each other, we have a different situation.

Where they collide, you find the plane of collision - the plane that is tangent to their contact point.

If the balls are equal size(so mass is not a factor) we just trade velocity components along the collision normal.

If they are not equal size, the momentum is a factor and Im not sure how that case works.. but I'd assume you trade momentum (like a weighting factor when doing the velocity trade).


So that should settle their velocity change...
You'll also need position changes; since the actual collision occured inbetween frame updates, you'll need to calculate the actual time and location that it happened, then using their new velocities and the remaining time to next frame- find their new bounced locations.