 Today we're going to look at some demonstrations involving the concept of rotational inertia. Now first of all let's review the concept of inertia. Inertia is a property that we associate with an object that deals with a resistance to a change in its state of motion. That particular property is the mass of the object. Now rotational inertia is a property that deals with the resistance of an object to a change in its state of rotational motion. That does depend upon the mass of the object, but it depends upon something else as well. And we're going to find out by looking at some demonstrations. Now the first one is one that you probably played around with before and that's balancing a pole on your fingers. It's actually fairly easy to do this. You just make small corrections with your hand and you can keep the pole balanced. Now it stays balanced because it's something else that we learned in a previous demonstration and that is that by moving my fingers, I'm keeping the center mass of the pole directly over the point of support. As long as I do that, it's not going to fall. All right, now let's make it even easier. I'm going to do that by putting a clamp near one end of the pole. This will increase the mass of the pole quite a bit, but it will do something else that we're going to see. Now here's a question. Will it be easier to balance this at the weighted end or close to the weighted end or with the weighted end far from the point of support? Well, let's try it both ways. With the weight close to the point of support or the axis of rotation, I can balance it, but I have to make very big movements with my hand in order to do that. That's because the pole is moving very quickly and so I have to respond very quickly. It's a lot easier if I do it this way. I don't have the emotions that I make with my hand to keep it balanced or much slower and that's because the pole has a tendency to rotate much slower. Why is that? It's because the rotational inertia of the pole is greater when I balance it like this than when I balance it like this. Now the mass hasn't changed. What has changed is where the mass is. As it turns out, the further the mass is distributed from the axis of rotation, the greater the rotational inertia that it has and therefore the greater the resistance to a change in a state of rotational motion. Let's look at another example of that. This one is sort of similar to what you may have seen some circus performers do, and that is balancing stacks of plates on a long pole. Now I'm not going to try that, but I'm going to balance something on this platform. First of all, you can see that it's fairly easy to balance even as it is, but it should be even easier if I put extra mass far from the point where I'm supporting it. So I'm going to put these sawed off soda pop bottles filled with water on the top and balance it on my fingers. And I can just stand here and this is fairly easy to do. Now my cameraman is pretty nervous right now because he's wondering if I'm going to drop this on his camera. But actually I'm just exaggerating. Now I can do pretty much with this what I want to do. I just have to be careful when I bring it back down again. Because that's the point where I'm most likely to tip the thing over and drop it. So that's because I had moved the point of support from far from the mass to close to the mass where it's more difficult to keep it balanced. Some other examples of this that you may have seen or one example of this you may have seen is another circus performer, the tightrope walker. The tightrope walker uses a long pole which extends way out. Now what this does is actually a couple of things. It increases his rotational inertia because there's a lot of mass far from him and so when he makes in order to correct his any slight motions one way or the other it's fairly easy to do because with a greater rotational inertia he tends not to rotate very quickly. The other thing it does relates to something that we've studied before and that is the position of the center of mass. You probably notice that these poles are very long and they drew way down on the ends. As a result they pull the center of mass of the man and the pole downward and you know from what we learned before that by bringing down the center of mass that increases the stability of the system.