Hey, guys. So now that we 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 going to just skip ahead. We're going to come back to this bullet point in just a second here. I actually want to start with the example because it's something we know how to do. We've got a force or a box that's pushed to the right with 20 and then 20 to the left. So we know this box has a mass of 6 kilograms. I just want to draw that real quick. This box here, we've got 2 forces. This f equals 20 and this f equals 20. So I'm going to go ahead and label these. Let's call this one fa and this one fb. And what we want to do is to figure out the acceleration. So I want to stick to the steps. I know I'm going to have to write f = ma, 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 going to write f = ma here. We've got to expand all of our forces and when we're doing this, any forces along our direction of positive get a plus sign and anything against gets a negative sign. So that's going to be our fb. This is our ma. Now we just replace our values. This is positive 20 plus negative 20, right? That's because this points backward, and this equals 6a. So the 20 and the negative 20 cancel out to 0, which means that the acceleration is 0 over 6, and that's 0. 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 0, like just we had in our example here, our net forces were 0 because 2020 cancel each other out, then our acceleration is 0. And if you ever have an acceleration at 0, your velocity is constant. So basically, what inertia means is that objects resist changes to their velocity unless they're 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've got a box that's at rest. Right? It's velocity is 0 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 balance out just like we did in our example. In both of these situations, the net force is equal to 0. So therefore, our acceleration is 0. 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 meters per second, but there's no forces acting on it. Here, we have this box that's moving at 5 meters per second, but you do have some forces 55 that perfectly cancel each other out just like we had in our example. In both of these situations, just like we did on the left, the net force is 0. So that means that the acceleration is 0. And that just means that this object keeps moving at a constant velocity. So this box is just going to keep moving with v equals 5. That's going to be its velocity. Right? So what happens is moving objects in which their velocity is not equal to 0 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 and it's moving, eventually, it's going to stop. But that's because we have friction. So if matching this box was basically floating in space and you push it with 5, 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. Alright. I've got one last point to make here, which is 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 had these 2 blocks here. Right? They're both 2 kilograms and you pull 1 with 12 newtons. If we wanted to calculate the acceleration using f = ma, you would have the acceleration is 12 over 2. Right? Force divided by mass. And that would give you 6 meters per second squared. Now imagine that you had a 3 kilogram block instead of 2, you pulled it with the exact same 12. So now we want to calculate the acceleration and this is just going to be 12 over 3, right, because that's the mass, and you would get 4 meters per second squared. So really, for the same exact net forces, a heavier object, right, in which the mass is heavier is going to accelerate slower. You can actually just see this from f = ma. Right? If you have the same exact net force but your mass is higher, then that means your acceleration is going to be lower which means it's going to resist changes in velocity more. It's going to change velocity slower. Alright. So that's it for this one, guys. Let me know if you have any questions.
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6. Intro to Forces (Dynamics)
Newton's First & Second Laws
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Newton's First & Second Laws practice set
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