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Course: Physics library > Unit 3
Lesson 1: Newton's laws of motion- Newton's first law of motion introduction
- Newton's first law of motion
- Applying Newton's first law of motion
- What is Newton's first law?
- Newton's first lawNot started
- Newton's second law of motion
- More on Newton's second law
- What is Newton's second law?
- Newton's third law of motion
- More on Newton's third law
- What is Newton's third law?
- Newton's third law of motionNot started
- All of Newton's laws of motionNot started
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Newton's first law of motion
Current time:0:05Total duration:9:32
Newton's First Law (Galileo's Law of Inertia). Created by Sal Khan.
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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 into English. So the First Law,
+"Every body persists in a state of being at
+rest, or moving uniformly straightforward, 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 with a constant
+velocity, unless it is compelled to change
+its state by force. Unless it's 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, so completely at rest. So I have-- and
+this is something that we've seen before. Let's say that I have a rock. Let's say that I
+have a rock someplace 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's no force
+applied to that rock, that rock will just stay there. So the first part
+is pretty obvious. So, "Every body persists in
+a state of being at rest"-- I'm not going to do
+the second part-- "except insofar as there's some
+force being applied to it." So clearly a rock
+will be at rest, unless there's some force
+applied to it, unless someone here tries 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 a state of rest or moving
+uniformly straight forward, 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 aside here, because, this right
+here is Newton. And if this is
+Newton's first law, why do I have this huge
+picture of this guy over here? Well, the reason is is because
+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's why I made
+him bigger than here. But I was in the
+midst of a thought. So we understand if
+something is at rest, it's going to stay at
+rest, unless there's some force that acts on it. And in some
+definitions, you'll see unless there's 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. And the only way that it would
+move if there's a lot more force on one side than
+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's ice right here. And then, I have
+another block of ice sitting on top of that ice. So once again, we're
+familiar with the idea, if there's 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're pushing on
+the ice on that side with the same amount of force? The ice will still not move. So this right here, this
+would be 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 it more than compensates
+the force pushing it this way, then you're going to see
+the ice block start to move, start to really accelerate
+in that direction. But I think this
+part is obvious. This, you know,
+something that's at rest will stay at rest, unless
+it's being acted on by an unbalanced force. What's less obvious is
+the idea that something moving uniformly
+straightforward, which is another way of
+saying something having a constant velocity. What he's saying is,
+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's less intuitive. Because everything in
+our human experience-- even if I were to push
+this block of ice, eventually it'll 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'll 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 you keep, 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 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, at an atomic level-- so if you have the actual
+water molecules in a lattice structure in the ice
+cube, and then here are the water molecules in a
+lattice structure on the ice, on the actual kind of sea of
+ice that it's traveling on-- they do kind of bump and
+grind into each other. Although they're both smooth,
+there are imperfections here. They bump and grind. They generate a
+little bit of heat. And they'll, essentially, be
+working against the movement. So there's a force of friction
+that's being applied to here. And that's why it's stopping. Not only a 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's definitely going to
+keep it from going on forever. Same thing with the ball
+being tossed to the air. Obviously, at some
+point, it hits the ground because of gravity. So that's one
+force acting on it. But even once it
+hits the ground, it doesn't keep rolling
+forever, once again, because of the friction,
+especially if there's grass here. The grass is going to
+stop it from going. And even while it's in the
+air, it's going to slow down. It's 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 could, or at
+least he's probably theorized that, hey, maybe
+there's no air out there. And that maybe that's why
+these planets can just keep going round
+and round in orbit. And I should say their speed,
+because their direction is changing, but their
+speed never slows down, because there's
+nothing in the space to actually slow
+down those planets. So anyway, hopefully you found
+that as fascinating as I do. Because on some level,
+it's super-duper obvious. But on a whole other level,
+it's completely not obvious, especially this moving
+uniformly straightforward. And just to make
+the point clear, if gravity disappeared,
+and you had no air, and you threw a ball,
+that ball literally would keep going
+in that direction forever, unless some other
+unbalanced force acted to stop it. And another way to think
+about it-- and this is an example that you might
+see in everyday life-- is, if I'm in an airplane
+that's going at a completely constant velocity and
+there's absolutely no turbulence in the airplane. So if I'm sitting in the
+airplane right over here. And it's going at a constant
+velocity, completely smooth, no turbulence. There's really no way for me to
+tell whether that airplane is moving without looking
+out the window. Let's assume that there's
+no windows in that airplane. It's going at a
+constant velocity. And there's no turbulence. And let's say, I
+can't hear anything. So I can't even
+hear the engines. There's 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's another
+way to think about it. That it's actually
+very intuitive that they're similar states,
+moving at a constant velocity or being at rest. And you really can't
+tell whether you are one or the other.