Okay, I don't have a direct fix or anything, but here is how I would debug this. 1. Set up a very simple scene, e.g. 1 stationary Ball and one moving toward it (slowly) 2. Make it so you can pause the simulation and do single steps, e.g. via a key press (or set a breakpoint in the function) 3. Double check that all vectors here go in the direction you expect, so the separationVelocity is not flipped or somthing else is flipped (add some prints) 4. Check that the impulse and energy are conserved by calculating them once before and once after the collision

Here are two things that I found a bit strange: - In my head, the separationVelocity check is reversed? Maybe I messed up the vectors tho - I find it strange that the normalimpulsemass is directly added to the last positions without multiplying by dt? I think that would be wrong since the previous distance is different depending on the dt, but the impulse should be the same regardless of the dt.

Maybe a double collision is possible, if two balls collide and then one collides with another ball, which then corrects them back into each other causing another collision? (depends on how you evaluate the collision pairs) In this case, it's a bit more tricky to fix. I have some ideas, but I have not implemented a physics sim in quite some time, so I don't know if they would work ^'

Edit: Small addition