It's February already.

已經是二月份了。

I'm back from my hiatus.

我從我的間隙中回來了。

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

我當時做那些SAT題都快累死了。但現在

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

我'準備好了，我將開始做一些物理學。

So we had done a bunch of projectile motion, what

所以我們做了一堆彈丸運動，什麼？

happens you throw something in the air or

發生你扔東西在空氣中或

drop it from a cliff.

從懸崖上掉下來。

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

但現在我想給大家介紹的是，你到底是怎麼做的？

affect the acceleration of an object?

影響物體的加速度？

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

為了做到這一點，我將向你介紹牛頓的。

three laws.

三法。

To some degree what we were doing before was derivative of

在某種程度上，我們之前所做的事情是衍生出的。

what I'm going to do now.

我現在要做什麼。

But this is kind of the backbone of classical physics.

但這也算是經典物理學的主幹。

So Newton's three laws.

所以牛頓'三定律。

And you've probably heard of these before.

而你'可能已經聽說過這些。

Newtow's three laws.

紐託'的三大法則。

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

有時他們'叫牛頓運動定律。

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

我在網上看了一下，只是為了確保

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

並看看是否有什麼正確的寫法，但每一個。

website seems to have a different

網站似乎有不同

paraphrase of the laws.

對法律的解讀。

But hopefully, I can give you an intuitive sense

但希望能給你一個直觀的感受。

of what they are.

的是什麼。

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

所以第一條規律是靜止的物體。靜止的物體

tends to stay at rest. And an object in motion

往往會保持靜止狀態。而運動中的物體

tends to stay in motion.

往往會保持運動狀態。

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

這是我小時候學過的，現在看來

at Wikipedia and things, there are some paraphrases.

在維基百科和東西，有一些解說。

And we'll go over those paraphrases because I think

我們會去看那些解說詞，因為我覺得

they're instructive.

他們'是指導性的。

Stay in motion.

保持運動。

And you might say, Sal, this is obvious.

你可能會說，薩爾，這很明顯。

Why does Newton get so much credit

為什麼牛頓的功勞這麼大

for stating the obvious?

因為我說的很明顯？

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

很明顯，如果我看我的沙發，比如說，它是一個。

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

物體在靜止狀態下，如果我一直盯著它看，它就會傾向於

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

保持休息。同樣，當我看著一輛輛汽車橫穿一條條道路時，我也會覺得很奇怪。

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

十字路口 -- 那不是一個紅燈，那是穿越一個。

intersection, it's an object in motion.

交點，它是一個運動中的物體。

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

然後，我不知道... ... 10秒鐘後，它仍然是

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

當然，它也會一直運行下去。

unless you press the brakes or whatever.

除非你按下剎車什麼的。

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

所以你可能會說，好吧，薩爾，這是最。

obvious thing ever.

很明顯的事情，有史以來。

This doesn't even need to be written down.

這根本不需要寫下來。

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

但讓我們說你是牛頓，你來找我--這是在。

the 17th century.

17世紀。

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

你說，薩爾，我有這些新法律。

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

而第一個是靜止的物體往往會保持靜止狀態。

and an object in motion tends to stay in motion.

而運動中的物體往往會保持運動狀態。

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

而我想說牛頓，我已經可以推翻你的定律了。

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

讓我們'說我有一個蘋果和我'舉著它在讓我們'說。

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

我的... 我用手扶著它，所以它大概是我的。

shoulder level.

肩部水準。

So I'm holding an apple.

所以我'拿著一個蘋果。

This is an apple.

這是一個蘋果。

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

看起來像一顆心，但它'是一個蘋果。

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

所以我'用手拿著它，我'用手畫著。

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

我不知道這對你是否有意義，但我拿著。

it with my hand.

它與我的手。

And what happens when I let go of that apple?

當我放開那個蘋果的時候會發生什麼？

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

所以當我用手拿著它的時候，它是一個物體在

rest, right?

休息，對吧？

But then when I let go, what happens?

但當我放手的時候，會發生什麼？

It falls.

它落下了。

Falls to the ground.

倒在地上。

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

所以我'會說，牛頓，我剛剛推翻了你的第一定律。

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

因為這是一個靜止的物體。而我什麼都沒做

I just let go.

我只是放手。

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

我沒有'應用，我沒有'推它，我沒有'拉它。

I didn't throw it.

我沒有扔。

I didn't do anything.

我什麼也沒做。

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

我一鬆手，它就掉在了地上。

It started moving without me doing anything, even though it

我什麼都沒做，它就開始動了，儘管它... ...

was an object at rest.

是一個靜止的物體。

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

然後牛頓會說，哦，那是因為有一個。

thing called gravity.

叫做重力的東西。

And it's the force of gravity.

而這'就是重力的力量。

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

我想說的是，牛頓，你需要開始學會不... ...

make up things.

編造事情。

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

就因為你的法律沒有意義，你就不'。

need to make up artificial forces in the universe.

需要在宇宙中編造人工力量。

But anyway, he would end up being right.

但無論如何，他最終會是對的。

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

如果我做了同樣的事情，我就會想到這個問題

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

實驗，而我在太空中，讓我們'只是說 - 我

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

本來想說軌道，因為它看起來像，但

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

即使是軌道是一種 - 你'仍然是一種下降。

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

go into that.

進而。

I'll go into orbit at another time.

我'將在另一個時間進入軌道。

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

但是，讓我們說我們只是在深空，而我和...

apple were just floating around in space.

蘋果只是漂浮在太空中。

Maybe we're stationary.

也許我們'是靜止的。

It's hard to say.

這'很難說。

We're floating with respect to what?

我們在漂浮著與什麼有關？

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

然後，如果我們在太空中，我放開這個蘋果。

what happens to the apple?

蘋果會怎麼樣？

Nothing.

什麼都沒有。

It's not going to fall anywhere.

它'不會掉到哪裡去。

It's not going to move.

它'不會動。

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

所以每當你想到牛頓定律時...

that's why this is so amazing.

這就是為什麼這個是如此驚人。

He didn't know about space.

他不知道空間。

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

他生活在這個星球上，所有的東西都會掉下來。

and things start moving for no reason because of whatever,

和事情開始無緣無故的移動，因為什麼。

gravity, and the wind and whatever else.

地心引力，還有風和其他什麼東西。

And he actually theorized that there could be a place where

而且他實際上理論上認為，可能有一個地方，在那裡

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

沒有任何力量作用在物體上，如果我讓

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

去這個蘋果，它就會留在原地。

And similarly, the object in motion

同樣，運動中的物體

tends to stay in motion.

往往會保持運動狀態。

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

在那裡，我又會告訴牛頓，好吧，那個

doesn't make sense.

不'說不通。

If I were to-- I don't know.

如果我... 我不知道。

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

如果我推一個 - 好吧，我不知道如果他們有保齡球

balls back then.

當時的球。

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

但如果我用保齡球滾下去... ...

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

讓我們說，在山上 - 在某些時候，保齡球的

going to slow down.

要慢下來了。

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

如果我把它滾上山，在某些時候，它只是'。

going to slow down.

要慢下來了。

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

如果我做對了，也許它就會停在頂部，如果。

I did it perfectly.

我做得很完美。

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

我可以說，看，這是一個運動中的物體。

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

在某些時候，它停了下來，或者它實際上又轉了回來。

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

或者說，即使我這樣滾，在某些時候，它'的。

just going to stop.

只是要停止。

Right The bowling ball's going to stop.

右邊的保齡球要停了。

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

如果我盡我所能地去推動一些東西，也許它... ...

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

旅行了幾英尺，但隨後它'要停止。

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

他說，哦，好吧，你知道，有這些力量。

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

你沒有意識到有一種力量。

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

有'保齡球例子中的風阻。

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

在我剛才的例子中，有摩擦力。

pushed something.

推的東西。

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

我會說，好吧，牛頓，你只是在瞎編。

these forces again.

這些力量又。

And this is why this is so not intuitive.

這也是為什麼這一切都不直觀的原因。

Because he had to essentially realize that there were all of

因為他必須從本質上意識到，有所有的

these forces acting on something when to someone at

這些作用力在對某人時

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

那時候，你不會意識到這一點，你不會

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

竟然能夠想象到有一個地方

called space, for example, where these

稱為空間，例如，其中這些

things wouldn't happen.

事情就不會發生。

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

如果我在空間裡推一個東西，它就會繼續前進。

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

它將是一個運動中的物體，它將保持這種。

velocity until some other force acts on it.

速度，直到有其他力作用於它。

So it wasn't that intuitive.

所以並不是那麼直觀。

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

所以，更現代的寫法是這樣說的

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

有參照系，就有參照系。

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

參考 -- 我會解釋什麼是參考框架。

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

但存在一個參照系，在這個參照系中，這是真實的。

That could be the new way of saying Newton's

這可能是牛頓'的新說法。

first law of motion.

第一運動定律。

So what's a frame of reference?

那麼什麼是參照系呢？

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

所以物理學中的一切--如果我在動。

moving relative to what?

相對於什麼而動？

Moving relative to the observer?

相對於觀察者移動？

Moving relative to the earth?

相對於地球移動？

You don't know.

你不知道

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

所以，一個參考框架就是觀察者在做什麼？

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

所以，舉例來說：當我'在空間裡，我放開蘋果的時候，我

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

和蘋果是一種在這 - 我觀察到的

apple from what I call an inertial frame of reference.

蘋果從我所謂的慣性參照系。

So this is a frame of reference actually where

所以這是一個參考框架，其實在這裡

Newton's laws hold.

牛頓定律成立。

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

如果我拿著地上的蘋果，我一鬆手，它就掉下來了，那。

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

為什麼第一條定律不成立，是因為我不是

really in an inertial frame of reference.

真正在慣性參照系中。

Because me and the apple are both constantly being pulled

因為我和蘋果都在不斷地被拉扯。

on by this force called gravity.

在這個被稱為重力的力的作用下。

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

所以，雖然看起來什麼都沒有發生，但我和那個...

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

蘋果都在同一個------沒有什麼'的真正作用於我們。

There is.

有。

There's this force of acceleration.

有'這種加速的力量。

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

同樣的，如果我在車內，而那輛車是。

accelerating, right?

加速，對嗎？

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

所以，讓我們說車 -- 看起來更像一個皮卡，所以... ...

I'll go with the pickup truck.

我'會選擇皮卡。

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

比方說，你的後背上掛著一對骰子

view mirror.

視鏡。

This is the dice right here.

這是骰子就在這裡。

What happens when the car accelerates?

汽車加速時會發生什麼？

Well the dice move back, right?

骰子往後移，對嗎？

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

所以，當你坐在卡車本身，它看起來

like the dice are just moving back.

像骰子只是移動回來。

No one's really doing anything to it.

沒有人'真的對它做什麼。

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

讓我們'的說，汽車沒有窗戶，你會只是都

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

突然神祕的感覺 - 好吧，你'會覺得一個。

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

小擠壓你的胸部了，但你也會只是。

see these dice move back.

看到這些骰子移動回來。

And you'd say, hey.

而你'會說，嘿。

Newton's first law doesn't hold.

牛頓'第一定律不成立。

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

我想說的是，好吧，那是因為你'在一個

non-inertial frame of reference.

非慣性參照系。

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

對卡車外的人來說，他們會看到，哦。

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

好了，卡車是移動的， 卡車實際上是加速。

and that's why the dice move back.

這就是為什麼骰子會往後移動。

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

所以，在我猜測你可以說水準維度，和。

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

我可能只是把你搞糊塗了，但我想給你一個。

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

真的很直觀的感覺，為什麼這不是那麼直觀。

In the horizontal direction, because there are no forces of

在水準方向，因為沒有力的

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

在這個方向上的重力或其他什麼作用，如果我'm

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

卡車外，我可以... ... 我將在一個。

inertial frame of reference in at least

慣性參照系

the horizontal dimension.

的水準尺寸。

I mean we always have gravity pulling down on us.

我的意思是，我們總是有重力拉著我們。

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

但是從卡車外面，我可以觀察到

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

哦，你知道，牛頓定律成立，因為整個框架。

of reference, this truck is actually being accelerated.

的參考，這輛卡車實際上正在加速。

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

所以，我在這之外，我將在一個慣性框架中。

of reference.

的參考文獻。

Hopefully I haven't confused you too much.

希望我沒有給你帶來太多困惑。

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

思考的方式是，一個慣性框架的。

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

參照物只是一個參照物，在那裡沒有's

outside forces acting on the whole frame of reference.

外力作用在整個參照系上。

And a frame of reference is just what is

而參照系正是

the observer doing?

觀察者在做什麼？

What am I doing?

我在做什麼？

Am I moving with the object?

我是隨著物體移動的嗎？

Am I being accelerated with the object?

我是不是被物體加速了？

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

或者，我和對象都沒有被作用？

And that's the way to think about it.

而這'就是思考的方式。

Oh, I already ran out of time.

哦，我已經沒有時間了。

I only got one law done.

我只完成了一項法律。

So I'll see you in the next video.

所以，我'會在下一個視頻中看到你。