 We're going to be looking at several demonstrations that have to do with center of mass and how the position of the center of mass of an object is related to the object stability. Let's begin with a parlor trick that you've probably seen before. Take a fork and a spoon, interlock them, slip a match in between the tines of the fork and balance it on a glass or in this case a beaker. And you can make the whole combination balance there. The reason that works is because the center of mass of the system of the fork and the spoon is actually right at the lip of the beaker where it's balanced. So the point of support is directly below the center of mass. The reason the center of mass is at that particular point it's not actually within the fork or the spoon, it's somewhere here below it. Now by symmetry since the fork and the spoon are about the same shape the center of mass is going to be along a line approximately here but because the spoon extends way out and the fork extends way out this will displace the center of mass downward. And so the center of mass of an object does not actually have to be physically within the object. Let's look at another example. Here we'll be using our two-dimensional NCSSM student we'll call this 2D. We'll put some weight on 2D's hands and balance 2D right there on the top of the head. It's very easy to balance. Now like the fork and the spoon we've got a lot of weight on the outside extending out away from the point of support. Because of the symmetry of the object the center, we can know the center of mass will fall on a vertical line right here below my finger. And it's going to be quite a ways down because most of the weight is here on the clamps. Now this is a very stable object. So I actually want to talk about why it is that this is stable and what the position of the center of mass has to do with that. I'll begin by looking at a different object where it might be a little bit easier to see what's going on. I'll just use this this is a support for another piece of equipment. Has a wooden base and just a metal rod. Sitting like this it's very stable. It's very hard to make this thing tip over. But if I put it like this it's essentially impossible to keep it from tipping over. That has to do with what happens to the center of mass when the object tips. The center of mass of this object is inside the piece of wood. In order to tip the object notice that the center of mass has to rise first. It's going to rise before it finally starts to fall over again. It has to rise quite a large distance. On the other hand if the object is like this the center of mass is now up here. And as the object starts to tip the center of mass immediately begins going downward. So the difference is that if the center of mass has to be lifted then the object is going to be more stable than if it just falls immediately. And the farther it has to be lifted then the more stable the object will be. Now let's take a look at 2D one more time. The center of mass of 2D is way down here because that's where most of the weight is. So I would actually have to pull this to the side like so in order to raise the center of mass and in fact I couldn't even raise the center of mass far enough in order to make the object tip over completely. On the other hand if I put most of the mass above like this and now the center of mass of 2D will be up here it will be above my finger so as soon as I let go the center of mass will fall immediately. And it would be impossible for me to balance without moving my finger and even then it's very difficult. Now there are a number of toys that work on this principle. Here's one of them. This horseman has the horseman is plastic but there's a fairly heavy weight here on the end. So the center of mass is going to be down in here. Actually the center of mass might be somewhere within the weight here on the end or at least very near to it. We have no trouble balancing this on its platform. The center of mass is way down here below it. In order to make this thing tip over we'd have to raise the center of mass above this point and of course that would be impossible without practically pushing it off of its stand. Here's another toy that works similarly. It's a bald headed eagle and this one can be balanced on its beak. Now if you want to try this yourself and don't have toys like this at home it's very simple. All you need is a pencil with an eraser and a paper clip. Open up the paper clip stick one end in the pencil eraser and you can balance the other end on your finger. So the center of mass will fall on a straight line directly below my finger.