I think the story is that Newton is supposed to have built one of these. I don't know if that's true, but it doesn't matter. There's a lot of good physics in here. What we have is a bunch of balls of identical mass made of hardened steel, the highly elastic material. Highly elastic material means that when objects made of these materials collide, not only is the momentum conserved as it normally is in collisions, if there's no external force, but also the kinetic energy is conserved. The fact that we have these two strict conditions applied to collisions means that this highly symmetric system has some very interesting behaviors. For example, if I take one of these balls, I'm going to leave all the rest of the balls stationary, but I'm just going to give this ball here a bit of velocity and see what happens when they collide. You'll see, one ball here, collared with the collision, one ball comes off the end here, and so on and so forth. Now I'm going to leave two balls, and I'll leave it to you to think about what you think will happen. Let's see if you're right. I'll leave two balls off, and I get two balls coming off the end. I guess by now you probably know what's going to happen if I lift three balls up here. Yeah, so you get three balls off at the end, no surprises there. But now let's see something a little strange. I'm going to lift four balls. I'm going to lift more balls than there are left stationary here. What do you think is going to happen there? I never think about it. Let's try it. Yes, four balls are moving after the collision. Now, if you wanted to do this the way that a professional physicist does it, in order to solve this system and work out and predict what's going to happen, what you do is you write down a bunch of equations for the momentum of each of the balls, and a bunch of equations for the kinetic energies of all of the balls, and then you'd have your initial conditions on one bunch of balls, let them go, and then after the collision you would have to impose the condition that all those kinetic energies added up to the same thing before and after the collision, and all those momentum would have to add up to the same thing. If you only have two balls, that means that you basically would have to solve a quadratic equation, where that stuff you'd land in mass class and irritated you, well you'd have to do that. However, I'm going to give you a different way of thinking about this that doesn't involve quadratic equations or any kind of algebra, but it can be used to explain all of the behavior that you've seen here. My little rule of thumb is this. When you have a collision between two highly elastic objects of identical mass, the velocities swap. We have initially a ball that has a velocity of zero and another ball with some initial velocity. You can see that after the collision they swap. The one that is stationary is now moving and the one that was moving is now stationary. It's swapping their velocities over and over and over again. But that only happens if you have highly elastic collisions with objects of identical mass. Now when we've got this complicated situation with a lot more balls, we can still use that little rule of thumb, even though it applies to two balls. Think of it this way. If I lift one ball up, it swings down, I get a collision between these two balls here. Immediately after the collision, this ball swaps their velocities. This one which is initially moving with a velocity of say V to zero is now stationary. Now the second ball has a velocity of V zero, but it immediately collides with the one next to it. It swaps its velocity with the next one. Now these are stationary and this is the moving one. This collides with the next one. Basically the motion of the original, the velocity of the original ball, propagates down from ball to ball until you get to the end. And this ball has got no one else to collide with so it flies off. Now here's a little exercise for the viewers at home. Using my little rule of thumb, see if you can explain why when I drop two balls, I get two balls off at the end. If you can manage that, then try it for three, four, five, etc. Well go on, what are you waiting for? Go and start thinking about it. Is it still here? Yeah!