 Now we're going to look at center of gravity. Now I've got two terms here, center of gravity, which is abbreviated CG, and center of mass, which is abbreviated CM. We tend to use the capital letters just so there's no confusion with, say, centimeters for center of mass. For our purposes in general physics, the center of mass and center of gravity are two interchangeable terms. In some advanced classes there might be a subtle difference. Now we're going to use this in the context of rigid bodies. And for this, what we're looking at is that gravity acts as if all the mass is located at that one special point that we call the center of mass or center of gravity. And it's really important to be able to understand this for static equilibrium, where we have to define not just what forces act, but where they're located. Now let's look at an extended object in a bit more detail. I've got a large object here. I can describe its total mass, but it's really made up of a whole bunch of small little masses, even more than what I've gotten shown here. And each one of these little masses feels gravity acting downwards on it. But if I were to try and do the force calculations using all of these little masses, it's going to get way too complicated too quickly. Instead, there's a formula that we can use to sort of find an average so that we can say the force on average acts at this one position. Now we're not going to be spending a lot of details on our formula here, but this point, this average point, that's our center of gravity. Now most of the time we're going to be using some uniform density objects, like a long rod or a flat board or something like that. And the center of gravity for these type things is always at the geometric center. So if I know how long my board is, the center of gravity is right in the middle of it. Now this is true even if I've got something off at an angle. My center of mass is still going to be at the center of it. One thing you do have to be careful here is that gravity still pulls straight downwards. So whether you're talking about a horizontal board or a board and angle, the location is at the center, but the direction is downwards. And notice here again that center of mass and center of gravity, we're using them interchangeably. So you could see figures that have either one of those. Now if we've got a non-uniform object, like a hammer, well it's got more mass over here at the hammer head than it does in the handle. And what we're going to find is that the center of gravity shifts towards the heavier end. So for a hammer you might end up having a center of gravity that's way up close to the head here. If we've got non-uniform objects in this class, most of the time we're going to tell you exactly where that center of gravity position is. Outside center of gravity. If we've got some special objects, ones that have holes or are curved, like these examples, it's possible that the center of gravity can actually be at a spot where there is no mass. So for my square example here, my center of gravity is right in the center. So it's nice and symmetric on every side, but the mass is all out here. And my center of gravity is acting on this location, even though my object doesn't have any mass there. For my curved example, it's a similar sort of thing. Now it's not at the geometric center, it's weighted towards the edge where I've got more material here. But again, there's no actual mass at that point. So we have to be careful how we think of the center of gravity. And it's going to be a little bit harder to balance an object like this than it would be our normal board. So that's your brief introduction to center of gravity so that we can continue to use these as we study static equilibrium.