University of North Carolina at Chapel Hill
Newton's Laws Basics - Example 1


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welcome to our first example of video and Newton's laws. In this video, we'll be talking about Newton's first law, so this will be less of an example video and mawr of an assist to help you visualize what the first law is talking about. Remember that the first law is saying that on object in motion tends to stay in motion. Object at rest tends to stay at rest. And hopefully this is something you've observed before, at least with regards to a knob checked at rest boxes that air sitting on floors don't just randomly start moving. This hopefully has been your experience in life to this point. Um, that's something strange happens like an earthquake or someone kicks it or something like that, a box just sitting there with nothing else touching it. Besides, the floor is not be going to begin to start moving, however. What you probably have seen is that if you were to give it a kick and have it slide across the floor, that eventually it would come to a stop, which seems to violate the first law that an object in motion didn't tend to stay in motion. It tended to stop. So the reason for this is because of friction is you might have guessed that your floor as some roughness to it and that roughness causes the boxes, slides over it to slow down. So the box actually does have a force on it. If I were to draw a free body diagram of a box sliding across the floor, I would say Well experiences gravity, pushing it down and experiences the floor, pushing it up. Hopefully, those two things are equal. We'll talk about that in a little bit, and then it will experience a force in the opposite direction of what it's traveling. Notice that if I want to put in velocity, I don't put it on the free body diagram itself. It's not attached to the dot It's over here, so we have a velocity this way, which means we have a force due to friction. Call that F sub little left over here. Then we have a force due to the floor, and we have a force due to gravity. So these three forces combined to cause a net acceleration. Remember, it's not a net motion, it's a net acceleration, and one thing we know about these forces is once that force is no longer there. For example, if friction just suddenly disappeared, our new free body diagram would look like this. And acceleration would be entirely dependent on what these two forces are. In other words, if you are exerting a force on an object and then you stop exerting it, the acceleration responds immediately. It doesn't continue to accelerate due to your force for a while. Think about pushing ah box across the floor. So if you're pushing a box for a while and then you let it go, it was accelerating, accelerating, accelerating while you're pushing it. But then as soon as you stop pushing it, it had less acceleration. So I've got a smaller. I've got a smaller vector here, which so it has some big velocity. Then it immediately starts toe have, ah smaller velocity once you let go. And that's because the new acceleration is backwards, whereas previously you had an acceleration forwards, so you immediately begin to reduce your velocity. It's not that you immediately turn around. It's just that you have a negative acceleration and a positive philosophy. So But if we were able to create an an area that did not have friction. Think about an air hockey table or an ice skating rink, even though both of those do have some friction to them. We were able to create a space that literally had zero friction to it. Then we would be able to push the box, and it would continue to move that way until some other horizontal force came to cancel it out. Notice it still has vertical forces, which are canceling each other. But forces in the vertical will not affect acceleration in the X direction. Just like how before, if you had a velocity in the X direction, it didn't necessarily mean that you had a velocity in the Y direction. If you have a force in the X direction, it doesn't mean that you have a force in the Y direction. Okay, so there's a few different ways to think about Newton's first law. Again. It's a nigh idealize law that it doesn't think about things necessary, like air resistance or friction. Um, though we can incorporate those, and in that way it will not violate the first law

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