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.