Today we're going to talk about doing collision problems, in particular looking at elastic versus inelastic versus perfectly inelastic collisions. Hopefully this will be a quick tutorial to kind of help you get those things straight. So when two things fly together, we want to be able to do some physics on that's the whole idea. To do that we need to know some of this. The great thing is in any collision, any collision at all, regardless of which one of these categories it fits in, we can say that the momentum is conserved. So when all of these cases momentum initial is equal to the momentum final. Okay, momentum initial, secret to the momentum final, momentum initial, secret momentum final. That's just true for any collision, basically as long as it's not let's say a collision with the earth, which of course is very big, and it's hard to measure the actual final loss. But other than that, anytime you have a collision, momentum is always going to be conserved. It's a fundamental of nature. Momentum is conserved. That's just how the world works. Elastic is a special case. This is a special case where things collide in a way where they actually conserve energy. And what it means when you have an elastic collision is that no energy has been lost to heat. Okay, so there's been no in the collision. There's been no heat loss. So the things haven't gotten warmer. You haven't created any sound. So a sound actually is a way of carrying away elastic energy or a way of carrying away energy. So things that make sound aren't technically elastic. And so, but in any case, when you have what's called an elastic collision, that just means that the kinetic energy at the beginning is equal to the kinetic energy at the end. And in most cases, if you're doing an actual physics problem, it'll tell you that energy is conserved for that it's an elastic collision. There'll be some special examples that we'll talk about later where you kind of know that it's that this is true. But if it doesn't tell you that kinetic energy is conserved or that it's been an elastic collision, chance starts not an elastic collision. Okay, well, let's go to perfectly an elastic. What does perfectly an elastic mean? Perfectly an elastic is a special collision where after the things collide, they stay together and keep moving at the same speed as each other. So if I have, let's say, these two things, if some, let's say, put tape in between them. If some reason afterwards, they both go at the same speed, then that's a perfectly an elastic collision. Again, the important thing is that the two things move at the same speed and are stuck together afterwards. Why is it useful? Well, if we look at our equations, so what does this break down if we actually write this? So M1v1 initial plus M2v2 initial is equal to M1v1 final plus M2v final, V2 final. This is just our normal momentum equation. The thing that's special about perfectly an elastic is if they're stuck together, then V1 final and V2 final are the same. So we can just call that, and we'll just give it to the one, and just call that V final. Then we can write this as M1v final plus M2v final. That can just be written as M1 plus M2 times V final. That's what's special about perfectly an elastic, because that you can write down this equation. That's the only thing, that's the only thing special about it. If you're in elastic, if you're in the middle of it, they don't stick together and kinetic energy isn't conserved, they're in elastic. That's just basically everything else. Okay, so how can we see this in the real world? In the real world? In this case, you can ask one question, are they stuck together? The answer is yes, it's perfectly an elastic. If the answer is no, it is not perfectly an elastic. That's perfectly an elastic. Okay, straightforward enough. For elastic, to the question, is it an elastic collision? Most of the time the answer is going to be no. So there are a couple of special cases when atomic particles, so atomic particles collide. So this is kind of a special case. My atomic particles collide, they often do an elastic collision. Other than that, basically nothing is elastic. That's the only kind of case where you don't lose stuff to end to thermo-energy. Now, there is a special case where things are almost elastic. This is mainly just pool balls, or billiard balls. Billiard balls collide. They are hard enough and they're elastic enough. They're kind of bouncy enough that they're pretty close to being elastic. They don't lose a ton of energy. Another thing would be like two golf balls running into each other, probably that way. And so in that case, you get that this is true. Everything else is in elastic. And so large of us going to be true, and then you're just going to have to use a momentum equation. Go from there. Thank you. Hope that was useful, and I'll see you in part.