 So an inelastic collision is one where the energy is changing from one form into a hidden form typically So you might have a ball of putty that falls on the surface and goes splat and takes some energy to deform the ball of putty And so the kinetic energy is not going to be conserved a Rubber ball however is going to deform and then Undeform again and so it's back in the state it was before which means there's no sort of hidden form energy there And therefore it's got to have all its kinetic energy back again after it bounces Another thing that's very good at bouncing is not just rubber balls, but actually steel balls This is a Newton's cradle if you have a steel ball and a couple of These hit each other then what you find is that they don't change shape afterwards They don't change shape. They don't store energy anyway And so therefore the energy that goes into the collision comes back out of the collision now if momentum were conserved There are many ways this could happen if I've got one ball coming in it could come to a stop And then four of them could go a quarter of the speed that would conserve momentum or if I had One ball coming in I could have two going off at half the speed So there's all sorts of different ways I could conserve momentum in that collision. However, if I'm going to try and conserve Kinetic energy then this mass times v squared Has got to be equal the mass times the v squared of the final thing So if I have four things going at a quarter of the velocity That's going to have a much lower in v squared because the v has to be squared And so I've got a quarter that'll turn into a sixteenth and so when you have just one It turns out the only way you can conserve kinetic energy and momentum at the same time Is if you have one bouncing up exactly one and similarly if you have two To bounce up all the way through to if you have four hitting ones Is it going to fly off a lot? No, in order to conserve momentum and kinetic energy We're going to have four going on. Alright, so there's a little bit of jiggle there These have a little bit of stick and so the collisions are not perfectly elastic But they're very elastic and so that's why you get the kinetic energy being mostly conserved