Hello. I'm Lou Bloomfield and welcome to How Things Work at the University of Virginia. Today's topic, [SOUND] falling balls. Falling balls are everywhere. They're in most sports, many toys, and they're even in Times Square on New Year's eve. Whenever a ball isn't touching anything, whether you dropped it or tossed it or kicked it, it's a falling ball, and it moves according to physics of falling objects. A falling ball is experiencing only one force, it's weight. That is, the force exerted on the ball by the Earth's gravity. We've talked about forces in general up to this point, but we have never identified a specific one. So, our first specific force is weight, the balls weight. And it's going to play a leading role in this episode. If falling balls don't catch your fancy, then you can think of anything else that is experiencing only its weight. Whether that's a coffee mug, or a handful of coins, or a coconut falling from a tree. Any time an object is completely on its own, except for gravity, it's a falling object. And it descends according to the rules of falling objects. The one notable exception, or at least one I have in mind, is a sheet of paper. It doesn't quite fall like everything else. That's because it's experiencing a second force that we can't ignore, air resistance. air resistance is another one of these nuisances that makes our lives more difficult. So, for this episode, we can ignore air resistance We'll save it for another, another episode down the road. For now, all these objects that we will drop, whether it's a basketball, a tennis ball, a golf ball, we're going to ignore air resistance and treat only the influence of their own weight on their motion. As you toss a ball around, you'll find that there's some recurring features to its motion. For example, if you drop the ball from rest, it moves downward faster and faster with each passing second. Now, your head might not be sure that it's really picking up speed, but your gut is. And I'll show you, that you know it, too. For that, I need help from my assistant, Katrina here. Hi, Katrina. Hi. So, what I'm going to have Katrina do is put her hand right here on the table, okay? Mm-hm. And now, I'm going to drop the ball on Katrina's hand. First, from this height. [SOUND] It doesn't hurt. Okay. Not, not too bad, and we're not, we're not going off to in an ambulance yet. But now, let's try it again. [SOUND] [LAUGH] Yeah. Thanks, Katrina. As, as you can tell, so, like Katrina knew that the ball [SOUND] will have picked up a lot of speed on its route to her hand because it will have had a lot of time to fall. And it does, indeed, go faster and faster with each passing second. So, you know, out from under it. She doesn't want to be there when the ball arrives. Now, as the ball is going down faster and faster, that means it's descending ever more distance with each passing second. So, its height is going down by bigger and bigger increments as time goes by. Showing you that is going to require a little work, and we'll save that for later in this episode. As a second example, if you toss a ball straight up, [SOUND] it rises quickly at first but then more and more slowly as it reaches greater height. And you can see that just by looking at the ball hit my hand. It's going fast at first, but less fast and less fast still. And at very top, it is momentarily motionless. The ball rises to a peak at which point it's not moving at all, for an instant, and then it drops as though from rest. It's as though at that moment when it was motionless, someone had been holding it, and let go. And down it comes, faster and faster, and faster. So, in this episode, we'll look at the dropping from rest. We'll look at the rise to peak height, and then drop from, essentially from rest. And, we'll also look what happens when you don't throw the ball straight up, but instead throw it [SOUND] up at an angle. A tossed ball, like in most sports. As we explore the behavior of falling balls, we'll take a close look at the connection between weight and mass. Two fundamentally different physical quantities that are related to one another in interesting ways. And that work together to give us the motion of falling balls. Moreover, falling balls are a wonderful example of the laws of motion I introduced in the episode on skating. It's a simple, yet elegant example, and a good step along the path to the more complicated objects that we'll examine as we continue to look at how things work. At this point, I'm going to pose a question. I won't answer it yet, but I want it to be in your mind as you continue through this episode. If I take a ball and throw it upward, toss it straight up, during the time when it's above my hand, not touching my hand, and heading upward, is there a force pushing upward on the ball? You should neglect any effects during, due to the air in thinking about this question. So, it's just as the ball is on its way up, not touching my hand, is there an upward force acting on the ball? To help guide us through the science of falling balls, we'll pursue the following six how and why questions. Why does a dropped ball fall downward? How differently do different balls fall? How would a ball fall on the moon? How does a falling ball move after it is dropped? How can a ball move upward and still be falling? How does a ball's horizontal motion, affect it's fall? There's a video sequence for each of those 6 questions, and a summary video at the end. So now, on to the first question.
