University of North Carolina at Chapel Hill
Newton's Laws Basics - Example 2


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Video Transcript

welcome to our second video on Newton's laws and this video. We're going to address an issue that really needs to be talked about before we go any farther. And that is the issue of Mass versus wait, which are two different things. And now you may have thought when I was talking about inertia before and I said, Well, all matter has this you're thinking, Oh, well, it it's how big you are, how much you weigh. No, it's actually literally how much mass you have, which is different from wait. Uh, remember in Newton's second Law, we said some of forces over this thing M which I called Mass, is equal to acceleration. Or if I reorganize this, the net force, which is the same thing, is the sum of forces is equal to mass times acceleration Here. Now let's think about what this in place, for example, for an object in free fall on object in free fall has an acceleration due to gravity that is G. So then we'd say F net is equal to M G, which means the force at which the force with which gravity is pulling on everybody and everything is equal to the mass of everybody or everything multiplied by the acceleration due to gravity wherever you happen to be standing, which, if we're on the planet earth near the surface, it's approximately 9.8 m per second squared. So this is a force. This force is equal to mass times gravity. If you think about what happens when you stand on a scale, you are literally being pulled down onto the scale by the force of gravity of the earth. So if we draw on Earth here with the scale and you stand on the scale, what's happening is the Earth is attempting to pull you in. But because there's something underneath you, it stops you from going down, and a scale just happens to be a meter that can tell you exactly how hard you're being pulled down. So a scale reads. Wait, it does not read mass unless it has some special math inside. So we have F net equals M. Times C. This is a weight. In other words, a weight is a force, but it is not a mass. A weight is equal to a mass times acceleration due to gravity. This is a really important point that we're going to come across again and again if we think about this in terms of what it looks like in a free body diagram. We know, for example, if we have ah, box on the floor has we've been considering In the last couple videos we have a box on the floor and we have a force of gravity that's pulling it down. Generally speaking, when I draw in a picture like this, I'll show the force of gravity applied to the center of mass of the object or approximately the middle of the object. But then it's not accelerating down, which means that the net force in the Y direction must be zero must be equal to zero. Newtons are the units we use here. Notice that we have mass times acceleration which is going to be equal to R S I unit from mass which is kilogram times meters per second squared. No one wants to stay kilograms meters per second squared all the time. So we call that a Newton for which we have a capital n. So we have our units when we say that we have zero force zero net force in the Y direction, which means that there has to be a secondary force pushing it up here. This is the force due to the floor, which we will name in a little bit, a little more more precisely than force due to floor. But forced to the floor has to be equal to this forced down. Now this force down is not mass. This is Wait. So the force of the floor pushing back on you has to be equal to your weight. We're going to examine this a little more closely as we talk about Newton's third law. But I wanted to bring up here. And now that mass times gravity is your weight, while just mass is the variable em there different things. And we need to make sure that we're keeping track of that as we continue drawing are free body diagrams.

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