 Now we consider rigid body rotation. Now when we talk about rigid bodies, we have to classify the different types of masses. Most of what we've done this semester has dealt with what we call a point mass, meaning I'm assuming that all the mass is at a single location. An extended object is one that's going to have the mass spread out over a region. Now one special type of extended object is a rigid body. And rigid bodies are ones where the shape is fixed. Specifically that means it doesn't bend or stretch. So now we can talk about rigid body motion. It can have translational motion, moves in a straight line, or it can have rotational where it moves around some pivot point. And the pivot point doesn't have to be a physical set point in it. It's just that everything goes in a circle around some part of the object. And an object can have both of these motions at the same time. So it can be rotating around a pivot point and moving sideways straight line. For the rotational motion part here. Here's an object and I've noted a few different locations on there as well as put some cross lines in there so you can kind of see the center. As it starts to rotate, each point on the circle is moving with the same angular displacement, velocity, and acceleration. But the linear quantities are not the same. If I were to back this up and play this again, what I see is that some of the points closer to the center move slower, some of the points on the edge move faster, but they all go around once at the same amount of time. And so that's how I know it's got the same angular displacement. If it didn't, the object would get twisted up. So the linear quantities are not the same, but the angular quantities are the same. Not that they're the same as each other, but the angular displacement for every dot is the same and the angular velocity for each dot is the same as all the other angular velocities. So that gives you just a little bit of an introduction to rigid body motion.