 Now let's look at elastic and inelastic, particularly as they relate to collisions. So as objects collide, different things can happen. And it depends a little bit on the nature of the objects. So some objects bounce when they collide. Some bend or even break. Some even explode. So let's look at this from an energy standpoint. Taking an interaction, part or even all of our kinetic energy can undergo some changes. It can be transferred from one object to another. It can also be transformed from one type of energy, kinetic energy, into a different type of energy. But remember that momentum is always conserved for these, their isolated collisions. So let's start with elastic first. In an elastic collision, the objects bounce off of each other. During the interactions, if I look at the energy, part of the kinetic energy goes into potential energy. But then that potential energy can be released and go straight back into kinetic energy. So I end up with the same kinetic energy that I started with comes back out at the end. So that means mechanical energy is not lost. In an inelastic collision, if I think about what's happening during the interaction, that part of the kinetic energy that's undergoing the changes, well, some of it goes into potential energy, but some of it also goes into other forms of energy. Now the potential energy can then be re-released as kinetic energy, but it's going to be a smaller part of the kinetic energy. So that means I am missing some mechanical energy in this case. But it doesn't go away completely, it just goes into other forms. For example, sound or heat or deformation. And by deformation, I mean that the object got bent. It might get really smushed up or just a little bent. It might even break, but it takes some energy to do that. Now perfectly inelastic is one of those inelastic collisions, but in this case, the objects stay together after the collision. In this case, during the interactions, you can think of it as the kinetic energy, part of it just goes into other forms of energy. There's no potential energy being stored and released during the process. Again, the missing mechanical energy is going to go into other forms of energy, like sound, heat, or deformation. Now our last case is a lot more rare, but sometimes we have these explosive interactions. In this case, if I look at exactly what's happening during the interaction, I've got some sort of stored potential energy and it's released into a larger kinetic energy. And that extra mechanical energy had to come from somewhere. And it was some previously stored energy. Maybe this is a object which has some sort of spring or elastic potential and it was compressed before the collision. And as soon as the collision happens, it suddenly releases that energy. Or there might be some sort of chemical potential energy and there's actually a chemical explosion during this interaction. But in the end, you end up with a larger kinetic energy. So summarizing, momentum is conserved for all of these cases. Inelastic, I've got no kinetic energy lost. Inelastic, I've got some kinetic energy lost. And in a perfectly inelastic collision, I lose more kinetic energy than in any other case, so we've got the most kinetic energy lost. In explosive, kinetic energy is actually gained. So depending on what happens to the kinetic energy, comparing the initial kinetic energy and the final kinetic energy, we can determine which one of these cases it is. So that wraps up our introduction to elastic and inelastic collisions.