6. Intro to Forces (Dynamics)

Newton's First & Second Laws

# Newton's 1st Law

Patrick Ford

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Hey, guys. So now that we've talked about Newton's second law, we want to make sure we go back and cover Newton's first law. So let's go ahead and talk about that in this video. I'm gonna just going to skip ahead. We're gonna come back to this bullet point in just a second here. I actually wanna start with the example because it's something we know how to do. We've gotta force or a box is pushed to the right with 20 and then 20 to the left. So we know this box is a massive 6 kg, so I just want to draw that real quick. This box here, we've got two forces. This F equals 20 and this F equals 20. So I'm gonna go ahead and label these. Let's call this one f A. This one FB and we want to do is want to figure out the acceleration, so I'm gonna have to stick to the steps. I know we have to write f equals m A. But first I want to choose the direction of positive. So remember that usually that's to the right like this. And so now we're gonna write f equals m A so f equals m a. Here, we've got to expand all of our forces. And we're doing this. Any forces a longer direction of positive our A plus get a plus sign and anything against gets a negative sign. So that's gonna be our f b c r m A. Now we just replace our values. This is positive. 20 plus negative 20 right? That's because this points backwards and this equals six a. So the 20th negative 20. Cancel it to zero and you get zero equals six A. Which means that the acceleration is 0/6, and that's zero. So let's talk about Newton's first law. Newton's first law is sometimes referred to as the law of inertia. And basically what it says is that if your Net force is ever equal to zero, like just we just like we had in our example here are net forces was zero because 20 and cancel each other out in our acceleration is equal to zero. And if you ever have an acceleration of zero, your velocity is constant. So basically what inertia means is that objects resist changes to their velocity unless they are acted upon by a net force. The way you might have seen this written in your textbooks is that objects keep doing whatever it is that they're doing. Unless you have a net force, let me go ahead and show you some examples and situations here. So here we go two blocks that's at rest, right? It's velocity is equal to zero, and there's no forces acting on it. So there's no net force here. We have the same block at rest. But now we have these forces that perfectly balanced out, just like we did in our example. Here. In both of these situations, the net force is equal to zero. So therefore our acceleration is equal to zero. And if this box is at rest, then that means it's just going to stay at rest. So these objects just keep doing whatever it is that they were doing. So now let's talk about what happens when objects are moving. This is slightly less intuitive. Now you have this box that's moving at 5 m per second, but there's no forces acting on it. Here we have this box that's moving at 5 m per second, but you do have some forces five and five that perfectly cancel each other out, just like we had in our example and both of these situations, just like we did on the left. The net force is equal to zero, so that means that the acceleration is equal to zero. And that just means that this object keeps moving a constant velocity. So this happens. So this box is just going to keep moving with V equals five. That's going to be its velocity, right? So what happens is moving objects in which their velocity is not equal to zero actually don't require a force to keep moving. This is kind of something that is a little bit counterintuitive, right? If you push a box that's moving, eventually it's gonna stop. But that's because we have friction. So imagine this box was basically floating in space and you push it with five. It would keep on going forever unless something finally stopped it. So without net forces, these objects would just keep on moving forever. That's what Newton's Law basically tells us, right? I've got one last point to make here, which we talked about inertia as the resistance to changes in velocity and basically mass is kind of like a quantity or an amount of that resistance to change. What do I mean by that? Well, imagine we have these two blocks here, right? They're both 2 kg and you pull one with 12 Newtons. If you wanted to calculate the acceleration using F equals M A, you would have the acceleration is 12 over to write force divided by mass and I'll give you 6 m per second squared. Now imagine that you had a 3 kg block instead of to you pulled it with the exact same 12. So now we want to calculate the acceleration. And this is just gonna be 12/4 12/3. Right, Because that's the mass and you would get 4 m per second squared. So, really, for the same exact F nets, a heavier object right in which the masses heavier is going to accelerate slower. You can actually just see this from F equals M A right f equals M A. If you have the same exact net force. But your mass is higher than that means your acceleration is going to be lower, which means it's going to resist changes in velocity more. It's gonna change velocity slower. All right, so let's say for this one, guys, let me know if you have any questions.

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