Hello. I'm Lou Bloomfield and welcome to How Things Work at the University of Virginia. Today's topic: Seesaws. Seesaws are a simply toy that consists of a long board mounted on a central pivot. Two riders get on opposite ends of that board, and adjust their positions until the seesaw balances. At that point then, they can begin to make the seesaw rock back and forth. Either by leaning toward or away from the pivot, or by pushing on the ground with their feet. When I was a kid, seesaws were everywhere. Any playground worth its' salt had a couple of them. And either at recess or during a birthday party we'd clamber onto the seesaws, one at each end, maybe one big kid, one little kid. Maybe several at each end. And we'd rock back and forth furiously until our legs wore out, or one of us got hurt. Or we simply wanted to do something else. Nowadays, seesaws become rarer and rarer. It seems that either they're risky, or perhaps modern children don't enjoy that sort of activity as much. Whatever the reason, they're missing an opportunity to experiment with rotational motion, balance, levers, and mechanical advantage. Seesaws turn out to be a wonderful context in which to explore the physics of rotational motion. So I've stuck with them, even as they've somewhat abandoned me. If you have a seesaw nearby, I urge you to experiment with it. Although be safe, because there are ways in which you can get yourself injured playing with a seesaw. More on that later. If you don't have a see-saw, well, you can make one yourself. All you need is some object to serve as a central pivot and a board to balance on that pivot. You adjust the spacings just so and. Voila, a seesaw. You can have it rock back and forth, just like the real thing. Actually, that simplicity. The fact that you can make something like this so easily, explains why seesaws have been around so long. They're pretty simple to make. As I suggested earlier, the story of seesaws is also the story of rotational motion, balance, levers, and mechanical advantage. We'll study those concepts here in the context of seesaws and then use them repeatedly as we continue to look at how things work. Before continuing, however, I want to ask you a question to think about. Not to answer, but something you should have in mind as we work our way through the story of seesaws. It's a difficult question, one that doesn't have an obvious answer so it's a good prelude to the rest of the story here on seesaws. Suppose you and a child half your height lean out over a swimming pool at the same angle. So here's you, here's the child, and you're both leaning out over the swimming pool at the same angle. If you both let go at the same moment, so that you begin to rotate into the pool Which of the two of you reaches the water first? To help guide us through the science of see saws, we'll pursue six, how and why questions. How does a balanced seesaw move? Why does a seesaw need a pivot? Why does a lone seesaw rider plummet to the ground? Why do the riders' weights and positions affect the seesaw's motion? Why do the riders' distances from the pivot affect the seesaw's responsiveness? How do the seesaw's riders affect one another? There is one video sequence for each of those questions and a summary seqence at the end. And now onto the first question.