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


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welcome to our introduction to Newton's laws in this video will consider questions like, What are the three laws from Newton? And then also, why are they important now? Most people, if you were to ask them what are Newton's three laws of motion? They might be able to give you one or two of them, so we're gonna look at them here right now. The first one is one that people often remember. It's on object in motion tends to stay in motion. Meanwhile, it also says in the second part of part that a lot of people forget that an object at rest tends to stay at rest. So what does this mean? It means that something that is moving will continue to move exactly that way unless a force acts on it. That is to say, there has to be some outside influence that causes it to change its motion. It won't naturally change its motion, whether that motion be some actual movement or holding. Still, Now, remember, being in movement or holding still is all about your relative perspective. An object that might look like it's holding still to you might look like it's moving to somebody else who has a speed relative to you. So really, whether or not you're moving is a relative question, but a change in motion that requires ah force to make it happen. Now the question is that how do we How do we explain that? Well, the second law is our mathematical law. It says that an acceleration is equal to the sum of all forces on an object divided by something that will call its mass or inertia. At the time that it was first idea hated. Um, now this symbol up here is the Sigma is a capital sigma. It means summation. We're going to take a some of all forces, so example. If there were two forces acting on an object, then this would mean that we need F one plus F two. If there were three forces acting on an object, then there would be F one plus F two plus F three. However, many forces there are acting on the object. We need to add them up in order to obtain this character here. Now, another name for and what will refer to in the future is called the Net Force. Net Force is the sum of all forces on an object. When we take that net force divided by Mass, we will come up with the acceleration of the object with mass m due to all of the forces on it. So this is Newton's second law. It's a very important one because it's the one that when we are given a question, we will have to write an equation, and we'll set up our variables in a relationship that is to say, that will correspond to this equation right here. The third law, then, is equal, and opposite force is is usually how people remember an equal and opposite forces specifically. Usually what it says is that for every action there is an equal and opposite reaction, or for every force there is an equal and opposite force. For example, you sitting on your chair right now requires that the chair pushback on you with the same force that you're pushing down on the chair. If that weren't true, then either you or the chair would start to move. Likewise, the chair, because you're pushing it down, will then push down on the ground, which will push back up on the chair now keeping track of. All of these were going to call these reaction forces because their forces that occur on Lee because we initiated them. So something like walking across the floor causes a reaction force with the floor sitting on something, pushing on something. All results in reaction forces, and these tend to be where the majority of conceptual misunderstandings occur for students. So when we get to talking about reactionary forces, you really want to pay attention. Toe what's happening? Because even though it may see, Simple seems simple in one application because can be quickly become very complicated in another application, we'll make sure to look at both the simple and the complex applications of Newton's third law so that you have a fair shot at understanding what's going on now. The way that we're going to represent forces, generally speaking, is with something called a free body diagram that this is a it's hard to constantly be saying, Well, we're going to push on a cow, And so then I have to draw a cow and we're going to say Okay, well, I push it and you might get worried. Well, what if I push it here or if I push it here or here. Or what if I'm pushing on the head and you get worried about what part you're actually pushing on? So rather than say, I push on a cow and then draw a cow, What I'm going to say is I push on a cow and then I draw a dot and I say the cow is the dot Cow is represented as a point particle. Okay, And since the cow is represented as a point particle, that means when I draw an arrow out of a vector out of it, representing a force will call this Force One. Then you can understand what that means, and there's no confusion about where you're pushing it or how you're pushing. It's the cow is a dot and we push on it. You may have heard some jokes about physicists in the pastor physics, where we assume things like spherical chickens. And this is entirely true. What we're doing here is we're assuming that every object is a point particle. Now, as we move on and we begin to talk about more advanced concepts will consider what are called extended free body diagrams, where We won't just think about an object that's as adopt, but well, think of it as an object that has length and width and height, but for now are free body diagrams again. So this is free. Body diagrams are very easy to draw there, simply a dot and then we draw all of the possible forces that could be coming out of them, and we give them labels. Sometimes we'll use numbers. Sometimes we'll use more descriptive labels as you'll see here later on in some of the videos, because sometimes forces from the ground. And so we want to call it F ground or a forces due to friction. So we call it F friction or forces do to rope. And so we call it F Sub rope. Some books prefer to give different names to stuff like this. For example, if a forces from a rope they might refer to it as a capital T force or they'll say force of T as in force of tension. I'll try to be very explicit when I use these different variables, and you should do the same when solving problems so that your professors can see what exactly it is that you're doing. If you don't define what your force is, then it's possible you may try toe grab some forces that aren't actually there. So now that we've gone over Newton's three laws, it's important to go back and talk about what is Newton's zeroth law. Okay, so actually, there's a lot of people who claim that there is a zero flaw Thio Newton's laws. It's actually a little bit arrogant because the whole principle behind Newton's laws was that he didn't get them from anywhere they were postulated, which means there was no fundamental underlying principle. Um, and the zeroth law is an attempt to say that there waas on underlying principle. Um, so you may have run it come across this in reading at some point before on there's different versions of it. The version that I prefer goes something like this. All objects are all material has a quality called inertia or, as we know it now mass that effects how it reacts. Two forces. Okay, if you remember, we had Newton's second law, which said that acceleration is equal to the sum of forces. The net force, divided by mass, noticed that a is a vector f is a vector. Um, then M here is what we would refer to as inertia. Now, whether or not your class decides to go over a zeroth law is really kind of random. It doesn't show up in every textbook. It won't show up in every class. But if they do, it will probably say something along the lines of this very conceptual idea that objects material things have this thing called mass and that mass helps dictate what don't accept what the acceleration of the object is in response to any particular force.

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