I bet most of you think it's:

我想你們大多會想起：

"What goes up,

“上了去的東西

must come down!" Is that right?

一定會跌下來！” 哈，對了

Well, sorta, but not really.

唔，差不多吧，但不完全如此

Technically, the law of gravity is an equation.

學術上，萬有引力是一條公式

It is: F = G x M1 x M2 / R^2,

如果Ｆ等於Ｇ乘Ｍ１乘Ｍ２除以Ｒ的平方

where G is the universal gravitational constant,

而Ｇ是萬有引力常數

M1 and M2 are the masses of the two objects,

Ｍ１和Ｍ２是兩個物體的質量

and R is the distance between them squared.

Ｒ是它倆的距離的平方

That was easy, right?

這個容易，對嗎？

Probably not. What does this actually mean?

也許不。但這公式是甚麼意思？

Well it means that - well, everything is attracted to everything else.

它的意思是：任何物體都會互相吸引

What I mean by that is if you have two objects, any two objects,

這句話的意思是如果你有兩個物體，任何兩物體

they are attracted to each other. OK.

它們會互相吸引。

Let's try and wrap our minds around this.

好，讓我們專心想想

What happens when you drop a rock off a cliff?

當你從懸崖掉下一塊石頭，會出現甚麼情況？

It falls to the earth. Right?

這石會跌落地面，對嗎？

Well, yes, but something else happens.

嗯，對的。但還有其他事情發生

You see, the law of gravity says that both objects,

你看，萬有引力定律說兩個物體

the rock and the earth, are attracted to each other.

即石頭和地球，會互相吸引

This means that the rock falls towards the earth,

意思是石頭會跌向地球

and the earth falls towards the rock.

而地球會跌向那石頭

Wait a second - you mean to tell me that if - the earth falls up to meet the rock?

等一等－－你意思是 地球會迎着石頭向上跌嗎？

Yes, that's exactly what I'm saying.

是的，我的意思正是這樣

And the reason you don't see the earth fall up to meet the rock

你看不到地球迎着石頭往上跌的原因

is because the objects move towards each other proportionate to their respective masses.

是因為兩個物體會按 它們質量的比例向對方移動

The earth is much much much more massive than the rock,

地球的質量比石頭多得很

so it moves a very very small distance,

所以它只移動了一個非常小的距離

and the rock is much less massive, so it moves farther

而石頭的質量小的多，因此它相對於地球而言

with respect to the earth.

移動得遠一點

Maybe a better way to understand gravity is to take two teenagers

可能更容易明白萬有引力的方法，是叫兩個小伙子

in spacesuits, and place them far out in space - away from all the planets and the stars.

穿上太空衣，然後放他們在外太空，遠離所有行星和恆星

It turns out they will be attracted to each other.

我們會發現他倆會互相吸引

I'm not talking about that kind of attraction.

我說的不是這種吸引力

See, they have mass, and since they have mass,

看，他們有質量，而由於他們有質量

they will move towards each other.

他們會移近對方

They are attracted to each other.

他們會被互相吸引

Maybe one more thing might help. Have you ever played with two magnets?

或許另一樣東西會有幫助 你試過玩兩塊磁石嗎？

You know, the magnets with the north and the south poles?

就是有南極和北極的磁石

When you take the magnets and put them closer to each other,

當你把兩塊磁石移近對方

they move together. They are attracted to each other.

它們會連接起來，它們會互相吸引

And the closer they are, the stronger the attraction.

它們的距離越近，吸引力越大

Think of the mass of the object like the strength of a magnet

試把物體的質量想為磁石的長度

and the distance between the objects like the distance between the two magnets.

而物體的距離就像兩塊磁石之間的距離

Now understand, I'm not saying that gravity and magnetism are the same,

要明白，我不是說萬有引力和磁力是一樣的東西

they just behave in a similar way.

只不過它們有類似的行為

Let's think of one other thing - astronauts.

讓我們想想另一樣東西－－太空人

You know, astronauts, they weigh less on the moon

你知道，太空人在月球上的體重

than on the earth. Why is that?

較在地球上較輕。為甚麼呢？

Well you see, the moon is less massive than the earth.

你看，月球的質量較地球小得多

Therefore it has a smaller gravitational pull on the astronaut.

因此它對太空人的引力便較小

It's like the moon is a weaker magnet.

就好像月球是一塊較弱的磁石

They aren't as attracted to each other.

它們之間的吸引力沒那麼大

Distance also plays a role. Think back to playing with a magnet.

距離也有影響。回想把玩磁石的時候

The pull of the magnets towards each other

磁石間的吸引力

are stronger when they are closer together. The same is true of gravity.

在它們較接近對方時較大 同樣情況也適用於萬有引力

For example, the sun is the most massive object near the earth.

例如，太陽是地球附近質量最大的物體

It dictates most of the gravitational forces in our solar system.

它操控了我們太陽系內的大部份萬有引力

It is very very massive. But it is relatively far away,

它的質量非常龐大。但由於它相對較遠

so even though the sun is a much stronger magnet, so to speak,

因此可以說太陽是一塊強得多的磁石

it is a long ways away.

但是它離我們很遠

Therefore the attraction isn't as strong.

因此它的吸引力不那麼強

So let's look back at that law of gravity.

讓我們回看萬有引力定律

The equation: F = G x M1 x M2 / R ^2.

這公式：Ｆ等於Ｇ乘Ｍ１，Ｍ２，除以Ｒ的平方

You see the force of gravity is equal to a number.

你看到萬有引力相等於一個數字

That's that universal gravitational constant G

那就是萬有引力常數Ｇ

times the mass of object one, times the mass of object two.

乘第一個物體的質量，再乘第二個物體的質量

Think of M1 being the mass of the sun

設想Ｍ１是太陽的質量

and M2 being the mass of the earth.

而Ｍ２是地球的質量

And then we divide by the distance between them squared.

然後我們除以它們之間距離的平方

This determines the force of attraction between the sun and earth.

便計算出太陽和地球之間的萬有引力

You could just as easily plug in your mass

你也可以代入你自己的質量

and the earth's mass and the distance between you and the center of the earth,

和地球的質量，和你和地心之間的距離

and find out how much you are attracted to the earth,

從而得出你被吸引往地球的力

and the earth attracted to you.

和地球被你吸引的力

So, what's gravity?

究竟萬有引力是甚麼？

Everything is attracted to everything else.

任何東西之間都會互相吸引

Everything. Oh, one last thing,

任何東西。噢，我忘了一件事

just to make you wonder. What causes gravity?

我想你想想。萬有引力的成因是甚麼？

Why are two objects with mass attracted to each other?

為甚麼兩個有質量的物體會互相吸引？

Well, the answer is -

嗯，答案是

We don't know.

我們不知道

The cause of gravity remains a mystery to scientists.

萬有引力的成因對科學家來說仍是個謎

We don't really know conclusively what causes gravity.

我們真的未能確實知道萬有引力的成因

It is one of the great mysteries of science.

這是科學界其中一個大奧秘