Newton's First Law of Motion Newton's First Law (Galileo's Law of Inertia).
Newton's First Law of Motion
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- 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
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