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Anderson Video - Newton's First Law

Professor Anderson
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>> Hello class. Welcome back to another edition of the Looking Glass Physics Lectures. I'm Professor Anderson, as hopefully you know by now. I'd like to talk to you a little bit about the next subject, which is Newton's Laws. So Newton's Laws are very, very important in physics. Particularly in introductory physics. Because they describe the motion of the world around us. The particles that are in this universe. And it has to do with Newton's Laws at a very basic level. So let's think about the following problem for a second. If I take an object such as this ping-pong ball. And I hold it here and I drop it, what happens? What do you guys think? How would you describe the motion of this ping-pong ball? Anybody have any thoughts? Yes, Megan, what do you think? >> (student speaking) It's falling. >> It's falling. Okay, absolutely. It's falling. All right. It started here, and it fell towards the earth. Now, did this thing start at rest? Or did it start in motion? Ben, what do you think? >> (student speaking) It started at rest. >> It started at rest. It's in my hand. And, in fact, right now, this thing is still at rest. Right? It is not in motion. And this is Newton's First Law. Okay, he had three. And the first one is, objects at rest tend to stay at rest. Objects in motion, tend to stay in motion. All right, and this sounds rather intuitive now. But, when Newton came up with these laws, they were rather counterintuitive. So let's just think about this for a second. Objects at rest tend to stay at rest. Okay. Here's a pen. I put the pen on the table. You can see the cap there. It's staying at rest. If I throw the pen, it's in motion. It's tending to stay in motion. And, in fact, that object, when we were holding, it was at rest. And then it started moving, it started accelerating. As we're going to see in a second, that's really Newton's Second Law. Because dealing with acceleration. But if we think about maybe Olympic curling. I know that's all your favorite sport out there, right? Olympic curling, what does that look like? Well, here's the ice. Right? And you have this stone, which comes from a particular island in Scotland. Which is where they make these stones. There's only one place in the world where they make these stones. Which is kind of strange. But curling itself is kind of strange, so why not? If that thing is moving along at velocity V. And I think about this object at a later time, when it's over here. How fast is it moving along at, roughly? Meagan, what do you think? What speed is it moving at, later on? >> (student speaking) The same speed that it started moving at. >> Aha! >> (student speaking) No friction. >> Yeah. If this is nearly frictionless. Then it will be moving at the same speed. And that's Newton's First Law. Objects in motion, tend to stay in motion. And so really, what we should put is, this little caveat, we can put this here. No external forces. Okay. There's nothing that's trying to slow it down. There's nothing that's trying to speed it up. Obviously, in the example of the ping-pong ball dropping, we do have an external force. We have gravity that is trying to pull it down. So Newton's First Law is really no external forces. Objects in motion tend to stay at motion. If I put that stone on the ice, and I just leave it there at rest. It will stay there at rest. If I push it, and it gets velocity V, then it will keep velocity V for a long time. All right. So that's Newton's First Law. And one way to think about Newton's First Law, is this. The net force is equal to zero. And therefore, there is no change in the velocity. That velocity could be zero. And at a later time, it would still be zero. Or it could be some value. And at a later time, it would still be that value. But we know exactly what change in velocity is. Change in velocity is acceleration. Right? A is Delta B over Delta T. So this must mean that there is no acceleration. Okay. This is already a little perplexing to people. Because, let's say you're doing the following experiment. Let's say you're driving down the road, in your car. At a constant speed, in a straight line. Is there any acceleration for that vehicle? Stacey, what do you think? Is there any acceleration? If I'm driving my car at a constant speed in a straight line? >> (student speaking) No, because it's in constant speed. >> That's right. Because it's constant speed and it's a straight line. As we learned, if I'm driving at constant speed, but I'm driving in a circle. Then there is, of course, acceleration. Right? >> (student speaking) Going towards the middle. >> Exactly. It's a centripetal acceleration towards the center of the circle. But if I'm going in a straight line, then there is no acceleration. All right. But this is perplexing to people. Because if there's no acceleration, then we should be able to follow this back up. And say, oh, the net force is zero. But we know when I'm driving down the road, do I really have a net force of zero? Doesn't seem like it. I have to push on the gas. Right? If I take off the gas, what happens? You slow down. So how are we making sense of this fact that constant velocity means no force? And yet, from our experience, it seems like we have to apply a force? Okay. And this is wrapped up in Newton's Second Law.