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Newton's First Law (Galileo's Law of Inertia). Created by Sal Khan.
Video transcript
In this video, I want to talk a little bit about Newton's first law of motion, And this is a translation from Newton's principia from Latin to English. So, the first law: Every body persists in its state of being at rest. So, state of being at rest or of moving uniformly straight foward except insofar as it is compelled to change its state by force impressed. So, another way to rephrase what they're saying is that if there's something every body persists so everything will stay at rest or moving at a constant velocity unless it is compelled to change its state by force, unless it is acted on by a force especially an unbalanced force, and I'll explain that in a second. So, if I have something that's at rest, completely at rest So, I have a...a, this is something we have seen before. Let's say that I have a rock. I have a rock on some place and it's laying on a field of grass I can keep observing that rock and it is unlikely to move, assuming that nothing happens to it. If there is no force applied to that rock, that rock will just stay there. So the first part is pretty obvious: so every body persists in it's state of being at rest. So I'm not going to do the second part, except insofar there's some force applied to it. So, clearly a rock will be at rest unless there is some force applied to it. Unless someone here is trying to push it, or roll it, or do something to it. What's less intuitive about the first law, is the second part. Every body persists in, either being in the state at rest, or moving uniformly straight foward. Except insofar as it is compelled to change its state by force impressed. So this Newton's first law, and I think I should do a little side here Because this right here is Newton. And if this is Newton's first law then why do I have this huge picture of this guy over here? Well the reason is that Newton's first law is really just a restatement of this guy's law of inertia And this guy - another titan of civilization really - this is Galileo Galilei And he is the first person to formulate the law of inertia And Newton just rephrased it a little bit and packaged it with his other laws But he did many, many, many other things So you really have to give Galileo credit for Newton's first law So that is why I made him bigger in here But I was in the midst of a thought! So we understand that if something is at rest, it is going to stay at rest unless there is some force that acts on it. In some definitions you will see "unless there is some unbalanced force" And the reason why they say "unbalanced" is because you could have two forces that act on something and they might balance out For example: I could push on this side of the rock with a certain amount of force And if you push on this side of the rock with the exact same amount of force the rock won't move The only way that it would move is if there is a lot more force on one side than there is on the other side So if you have an unbalanced force So if you have a ton of - and maybe the rock is a bad analogy Let's take ice because ice is easier to move Or ice on ice So there is ice right here and then I have another block of ice sitting on top of that ice So once again we are familiar with the idea: if there is no force acting on it, that ice won't move But what happens if I'm pushing on the ice with a certain amount of force on that side And you are pushing on the ice on that side with the same amount of force? The ice will still not move So this right here will be a balanced force A balanced force So the only way for the ice to change its condition, to change its restful condition, is if the force is unbalanced So if we add a little bit of force on this side so that it more than compensates the force pushing it this way Then you are going to see the ice block start to move, start to really accelerate in that direction But I think this part is obvious: something that's at rest will stay at rest unless it is being acted on by an unbalanced force What is less obvious is the idea that something moving uniformly straight forward Which is another way of saying something having a constant velocity Constant velocity What he is saying is that something that has a constant velocity will continue to have that constant velocity indefinitely Unless it is acted on by an unbalanced force And that is less intuitive, because everything in our human experience Even if I were to push this block of ice, eventually it would stop It won't just keep going forever, even assuming that this ice field is infinitely long That ice will eventually stop. Or if I throw a tennis ball, that tennis ball will eventually stop It will eventually grind to a halt. Or if I roll a bowling ball, or if I - anything! We've never seen, at least in our human experience, it looks like everything will eventually stop So this is a very unintuitive thing to say: that something in motion will just keep going in motion indefinitely Everything in human intuition says if you want something to keep going in motion You have to keep putting more force, keep putting more energy into it for it to keep going Your car won't go forever unless the engine keeps burning fuel to drive and consuming energy So what are they talking about? Well, in all of these examples - and I think this is actually a pretty brilliant insight from all of these fellows - is that all of these things would have gone on forever The ball would keep going on forever, this ice block would be going on forever Except for the fact that there are unbalanced forces acting on them to stop them So in the case of ice, even though ice on ice doesn't have a lot of friction There is some friction between these two And so you have, in this situation, the force of friction is going to be Acting against the direction of the movement of the ice And friction really comes from an atomic level So if you have the actual water motecules in a lattice structure in the ice cube And then here are the water molecules in the lattice structure on the actual sea of ice that it is travelling on They do kind of bump and grind into each other Although they are both smooth, there are imperfections here They bump and grind, they generate a little bit of heat And they will essentially be working against the movement So there is a force of friction that is being applied to here, and that is why it is stopping And not only the force of friction, you also have some air resistance The ice block is going to be bumping into all sorts of air particles It might not be noticeable at first, but it is definitely going to keep it from going on forever Same thing with the ball being tossed in the air Obviously at some point it hits the ground because of gravity, so that is one force acting on it But even once it hits the ground, it doesn't keep rolling forever once again because of the friction You know, specially if there is grass here, the grass is going to stop it from going And even while it is in the air it is going to slow down, it is not going to have a constant velocity Because you have all of these air particles that are going to bump into it And exert force to slow it down So what was really brilliant about these guys is that they could imagine a reality Where you didn't have gravity, where you did not have air slowing things down And they could imagine that in that reality something would just keep persisting in its motion And the reason why Galileo frankly was probably good at thinking about that Is that he studied the orbits of planets And he probably theorized that, hey, maybe there is no air out there And that maybe that's why these planets can just keep going around and around in orbit And I should say their speed, because their direction is changing, Their speed never slows down, because there is nothing in space to actually slow down those planets So anyway, hopefully you found that as fascinating as I do Because on some level it is super, duper obvious, but on a whole other level it is completely not obvious Specially this "moving uniformly straight forward" And just to make the point clear, if gravity disappeared and you had no air And you were to throw a ball, that ball would literally keep going in that direction forever Unless some other unbalanced force acted to stop it Another way to think about it, and this is an example you might see in everyday life Is if I'm in an airplane, that is going at a completely constant velocity And there is absolutely no turbulence in the airplane So if I'm sitting in the airplane right over here and it is going at a constant velocity Completely smooth, no turbulence There is really no way for me to tell whether the airplane is moving without looking out the window Let's assume that there are no windows in that airplane, it is going at a constant velocity And there is no turbulence, and let's say that I can't hear anything So I can't even hear the engines There is no way for me to sense that the plane is moving Because from my frame of reference, it looks completely identical To if I was in that same plane that was resting on the ground And that is another way to think about it that is actually very intuitive that they are similar states Moving at a constant velocity or being at rest You really can't tell whether you are in one or in the other