 So now we talk about forces causing circular motion. In previous videos, we've looked at circular motion and say that if I have an object moving in a circular path, that its speed may be changing or constant, but its direction is changing. So every point around the circle is going in a different direction. And that meant we have an acceleration, and we call it a centripetal acceleration. Now again, in our earlier videos, we said our centripetal acceleration was an inward-pointing acceleration, and we came up with an equation for exactly how much acceleration was involved there. But remember that accelerations happen when we have a net force. If I don't have a net force, I won't get that acceleration. And if I don't get that acceleration, it won't be moving in a circle. So that leads us to centripetal force. It's the force needed to keep an object moving in circular motion. And using my standard f equals ma, we worked out that the centripetal force has to be mv squared over r. But this only told us how much force was needed, and we know it's in an inward direction. What we also need to realize is that this force is caused by one or more other physical forces. So this equation here tells us how much force. So thinking about these physical forces, again, to iterate that the centripetal force is not a new physical force. It's not a type of force at all. It's the force I need to keep the object moving in a circle, and I need a physical force to make up that amount. So our centripetal force equation only tells us how much force is needed. We still have to go back and figure out exactly what physical forces it is that's providing that amount of force. So here's an example of a mass swinging around on a circle by a string. And tension is the physical force. Without tension, it doesn't stay in a circle. So as we're going around here, what we see is that if I don't have tension, I don't have circular motion. And the amount of tension I need is going to be given by how much centripetal force is needed. And that'll depend on the mass, how fast I'm trying to make it swing around, and what the radius of the circle is. So now here's another example. And I'm going to take you real quick to a little bit of a YouTube clip here. And this particular one is showing an example of a device which demonstrates it. And we're only going to watch a little bit of it here. And they've got some cups, and they can swing it around. And as they swing it around, the water stays in the glasses. Now for this example, we've got a combination of forces. And I've got the YouTube link here, and I'll have it linked on my channel playlist as well. But if you think about it, we've got several objects moving in circular motion. The board was moving in circular motion. The cups were moving in circular motion. And the water in the cups were moving in circular motion. And if I look at all those forces on those objects, I've got tensions, I've got normal forces, I've got gravity, I've got friction. And the combination of those forces ends up providing a net centripetal force so that the objects stay in circular motion. Now we're going to do a lot more examples so you can see the specific things here. But it's important to realize that these forces are causing circular motion, but there's got to be a physical force providing it.