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  • Good morning.

  • It's February already.

  • I'm back from my hiatus.

  • I was so burned out doing all those SAT problems. But now

  • I'm ready and I will start doing some physics.

  • So we had done a bunch of projectile motion, what

  • happens you throw something in the air or

  • drop it from a cliff.

  • But now I want to introduce you to is how do you actually

  • affect the acceleration of an object?

  • And to do that I'm going to introduce you to Newton's

  • three laws.

  • To some degree what we were doing before was derivative of

  • what I'm going to do now.

  • But this is kind of the backbone of classical physics.

  • So Newton's three laws.

  • And you've probably heard of these before.

  • Newtow's three laws.

  • Sometimes they're called Newton's Laws of Motion.

  • I've actually looked this up on the web just to make sure

  • and see if there's any correct way of writing it, but every

  • website seems to have a different

  • paraphrase of the laws.

  • But hopefully, I can give you an intuitive sense

  • of what they are.

  • So the first law is an object at rest. An object at rest

  • tends to stay at rest. And an object in motion

  • tends to stay in motion.

  • This is what I learned when I was a kid and now when I look

  • at Wikipedia and things, there are some paraphrases.

  • And we'll go over those paraphrases because I think

  • they're instructive.

  • Stay in motion.

  • And you might say, Sal, this is obvious.

  • Why does Newton get so much credit

  • for stating the obvious?

  • Obviously, if I look at my sofa for example, it is an

  • object at rest and if I keep staring at it, it tends to

  • stay at rest. Likewise, when I look at a car crossing an

  • intersection-- that's not a red light, that's crossing an

  • intersection, it's an object in motion.

  • And then, I don't know-- 10 seconds later, it's still

  • staying in motion and of course, it will stay in motion

  • unless you press the brakes or whatever.

  • So you might say, well Sal, this is the most

  • obvious thing ever.

  • This doesn't even need to be written down.

  • But let's say you were Newton and you came to me-- it was in

  • the 17th century.

  • And you said, Sal, I have these new laws.

  • And the first is an object at rest tends to stay at rest,

  • and an object in motion tends to stay in motion.

  • And I would say Newton, I can already disprove your law.

  • Let's say I have an apple and I'm holding it up at let's say

  • my-- I'm holding it up with my arm, so it's roughly my

  • shoulder level.

  • So I'm holding an apple.

  • This is an apple.

  • Looks like a heart, but it's an apple.

  • So I'm holding it with my hand, I'm drawing my hand.

  • I don't know if that makes sense to you, but I'm holding

  • it with my hand.

  • And what happens when I let go of that apple?

  • So while I'm holding it with my hand it's an object at

  • rest, right?

  • But then when I let go, what happens?

  • It falls.

  • Falls to the ground.

  • So I'll say, Newton, I just disproved your first law.

  • Because this was an object at rest. And I did nothing to it.

  • I just let go.

  • I didn't apply, I didn't push it, I didn't pull it.

  • I didn't throw it.

  • I didn't do anything.

  • And when I let go it just fell to the ground.

  • It started moving without me doing anything, even though it

  • was an object at rest.

  • And then Newton will say, oh, well that's because there's a

  • thing called gravity.

  • And it's the force of gravity.

  • And I would say, Newton, you need to start to learn to not

  • make up things.

  • Just because you're law doesn't make sense, you don't

  • need to make up artificial forces in the universe.

  • But anyway, he would end up being right.

  • And the way to think about this, if I did this exact same

  • experiment while I was in space and let's just say-- I

  • was going to say orbit because it would look like that, but

  • even orbit is kind of a-- you're still kind of falling

  • towards the earth, it's just you're moving-- well, I won't

  • go into that.

  • I'll go into orbit at another time.

  • But let's say we were just in deep space and me and the

  • apple were just floating around in space.

  • Maybe we're stationary.

  • It's hard to say.

  • We're floating with respect to what?

  • And then, if we're in space and I let go of this apple,

  • what happens to the apple?

  • Nothing.

  • It's not going to fall anywhere.

  • It's not going to move.

  • And so whenever you think about Newton's laws-- and

  • that's why this is so amazing.

  • He didn't know about space.

  • He's living in this planet that everything tends to fall

  • and things start moving for no reason because of whatever,

  • gravity, and the wind and whatever else.

  • And he actually theorized that there could be a place where

  • there's no forces acting on objects where if I were to let

  • go of this apple, it would just stay where it is.

  • And similarly, the object in motion

  • tends to stay in motion.

  • And there again I would've told Newton, well, that

  • doesn't make sense.

  • If I were to-- I don't know.

  • If I were to push a-- well, I don't know if they had bowling

  • balls back then.

  • But if I were to roll a bowling ball down a-- well

  • let's say up a hill-- At some point that bowling ball's

  • going to slow down.

  • If I rolled it up a hill, at some point it's just

  • going to slow down.

  • And maybe if I got it right it would just stop at the top if

  • I did it perfectly.

  • And I could say, look, this was an object in motion.

  • At some point it stops or it actually turns back around.

  • Or even if I were to roll it this way, at some point it's

  • just going to stop.

  • Right The bowling ball's going to stop.

  • If I were to push something as hard as I could, maybe it

  • travels for a couple of feet, but then it's going to stop.

  • And he'll say, oh, well you know, there's these forces

  • that you're not realizing there's a force.

  • There's the wind resistance in the bowling ball example.

  • There's the force of friction in the example where I just

  • pushed something.

  • And I would've said, well Newton, you're just making up

  • these forces again.

  • And this is why this is so not intuitive.

  • Because he had to essentially realize that there were all of

  • these forces acting on something when to someone at

  • that time, you wouldn't have realized that and you wouldn't

  • have been able to even conceive that there's a place

  • called space, for example, where these

  • things wouldn't happen.

  • If I push something in space, it will keep going.

  • It would be an object in motion and it will keep that

  • velocity until some other force acts on it.

  • So it wasn't that intuitive.

  • And so a more modern way to write this is to say that

  • there is a frame of reference, there exists a frame of

  • reference-- and I'll explain what a frame of reference is.

  • But there exists a frame of reference where this is true.

  • That could be the new way of saying Newton's

  • first law of motion.

  • So what's a frame of reference?

  • So everything in physics-- if I'm moving,

  • moving relative to what?

  • Moving relative to the observer?

  • Moving relative to the earth?

  • You don't know.

  • So a frame of reference is what is the observer doing?

  • So example: when I'm in space and I let go of the apple, me

  • and the apple are kind of in this-- I am observing the

  • apple from what I call an inertial frame of reference.

  • So this is a frame of reference actually where

  • Newton's laws hold.

  • If I take the apple on earth and I let go and it drops, the

  • reason why this first law didn't hold is because I'm not

  • really in an inertial frame of reference.

  • Because me and the apple are both constantly being pulled

  • on by this force called gravity.

  • So although it looks like nothing's going on, me and the

  • apple are in the same-- nothing's really acting on us.

  • There is.

  • There's this force of acceleration.

  • Similarly, if I'm in a car and that car is

  • accelerating, right?

  • So let's say the car-- looks more like a pickup truck, so

  • I'll go with the pickup truck.

  • Let's say you have a pair of dice hanging from your rear

  • view mirror.

  • This is the dice right here.

  • What happens when the car accelerates?

  • Well the dice move back, right?

  • And so when you're sitting in the truck itself, it looks

  • like the dice are just moving back.

  • No one's really doing anything to it.

  • Let's say the car had no windows and you would just all

  • of a sudden mysteriously feel-- well, you'd feel a

  • little squeezing on your chest too, but you would also just

  • see these dice move back.

  • And you'd say, hey.

  • Newton's first law doesn't hold.

  • And what I would say is well that's because you're in a

  • non-inertial frame of reference.

  • To someone outside of the truck, they would see, oh

  • well, the truck is moving, the truck is actually accelerating

  • and that's why the dice move back.

  • So in I guess you could say the horizontal dimension, and

  • I'm probably just confusing you, but I want to give you a

  • really intuitive feel about why this isn't so intuitive.

  • In the horizontal direction, because there are no forces of

  • gravity or whatever acting in this direction, and if I'm

  • outside of the truck, I could then-- I would be in an

  • inertial frame of reference in at least

  • the horizontal dimension.

  • I mean we always have gravity pulling down on us.

  • But from the outside of the truck, I could observe that

  • oh, you know, Newton's law holds because the whole frame

  • of reference, this truck is actually being accelerated.

  • So me being outside of that, I would be in an inertial frame

  • of reference.

  • Hopefully I haven't confused you too much.

  • The way to think about it is that an inertial frame of

  • reference is just a frame of reference where there's no

  • outside forces acting on the whole frame of reference.

  • And a frame of reference is just what is

  • the observer doing?

  • What am I doing?

  • Am I moving with the object?

  • Am I being accelerated with the object?

  • Or, are neither me nor the object being acted upon?

  • And that's the way to think about it.

  • Oh, I already ran out of time.

  • I only got one law done.

  • So I'll see you in the next video.

Good morning.

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