What on Earth is spin? - Brian Jones

地球轉動的原因 - 布萊恩瓊斯 (Brian Jones)

Translator: Andrea McDonough Reviewer: Jessica Ruby

譯者: Jephian Lin 審譯者: Allen Li

The next time you see a news report of a hurricane

下次你看到新聞報導有颶風、

or a tropical storm

或熱帶風暴

showing high winds battering trees and houses,

強風正吹襲樹木和房屋時

ask yourself, "How did the wind get going so fast?"

問問自己： 「風為什麼可以這麼快？」

Amazingly enough, this is a motion that started

令人驚奇的是，這個空氣的運動

more than five billion years ago.

源自於五十多億年前

But, to understand why, we need to understand spin.

然而，要了解原因 我們比須了解「轉動」

In physics, we talk about two types of motion.

在物理學中，我們會談論兩種運動

The first is straight-line motion.

第一種是直線運動

You push on something, and it moves forward.

你推某樣東西，它就向前移動

The second type, spin, involves an object rotating,

第二種是轉動，涉及了物體的旋轉

or turning on its axis in place.

或者說是沿著旋轉軸旋轉

An object in straight-line motion will move forever

直線運動的物體，不斷向前運動

unless something,

除非有像

like the friction of the ground beneath it,

地板上摩擦力的這類力量

causes it to slow down and stop.

讓它變慢，然後停止

The same thing happens when you get something spinning.

同樣的事也發生在轉動上

It will keep on spinning until something stops it.

旋轉會持續到有力量將它停下來

But the spin can speed up.

但旋轉也可以加速

If an ice skater is gliding across the ice

如果溜冰的選手沿著直線

in straight-line motion and she pulls her arms in,

滑過冰場，然後把手臂收回來

she keeps on gliding at the same speed.

她會以等速繼續滑行

But if she is spinning on the ice

但如果她是在冰上轉動

and she pulls her arms in,

然後把手臂收回來

you know what happens next.

你很清楚會發生什麼事

She spins faster.

──她會轉得更快

This is called the conservation of angular momentum.

這叫做「角動量守恆」

Mathematically, angular momentum is a product of two numbers,

數學上，角動量是兩個數的相乘

one that gives the spin rate

一個是轉動的速度、

and one that gives the distance of the mass from the axis.

一個是質量到旋轉軸的距離

If something is freely spinning,

所以當物體自由轉動時

as one number gets bigger,

一個數字變大

the other gets smaller.

另一個就變小

Arms closer, spin faster.

當手收回來，就轉得快

Arms farther, spin slower.

當手伸出去，就轉得慢

Spin causes other effects, too.

轉動還會造成其他效應

If you are riding on a spinning merry-go-round

如果你騎著旋轉木馬

and you toss a ball to a friend,

然後朝著朋友丟球

it will appear to follow a curving path.

看起來球的路徑是曲線

It doesn't actually curve, though.

但其實並不是

It really goes in a straight line.

它走的是直線

You were the one who was following a curving path,

你才是沿著曲線運動的人

but, from your point of view,

但從你的角度看

the ball appears to curve.

球變成了曲線運動

We call this the coriolis effect.

這種現象叫做「科氏力效應」

Oh, and you are riding on a speeding merry-go-round

你正坐在超高速的旋轉木馬上

right now at this very moment.

就在此時此刻

We call it the Earth.

它叫做地球

The Earth spins on its axis once each day.

地球沿著地軸，每天自轉一圈

But why does the Earth spin?

但為什麼地球會轉動？

Now, that's a story that starts billions of years ago.

這是幾十億年前的故事了

A cloud of dust and gas that form

有一團灰塵氣體雲

the Sun and the Earth and the planets

形成太陽、地球、行星、

and you and me

和你我

started to collapse as gravity pulled it all together.

重力把它們拉在一起，並開始坍塌

Before it started to collapse,

在坍塌前

this cloud had a very gentle spin.

這片雲緩慢的轉動著

And, as it collapsed,

在坍塌時

like that ice skater pulling her arms in,

就像溜冰選手把手收回一樣

the spin got faster and faster.

轉動變得越來越快

And everything that formed out of the cloud,

而所有這雲形成的星體：

the Sun

太陽、

and the planets around the Sun

圍繞太陽的行星、

and the moons around the planets,

圍繞行星的衛星

all inherited this spin.

都繼承了這股轉動力量

And this inherited spin is what gives us night and day.

就是這個轉動 讓我們有了白天與黑夜

And this day-night cycle is what drives our weather.

日夜循環讓我們有各種天氣

The Earth is warm on the daytime side,

地球上白天的那面較暖

cool on the nighttime side,

夜晚的那面較涼

and it's warmer at the equator than at the poles.

赤道也比南北極溫暖

The differences in temperature

這些溫度上的差異

make differences in air pressure,

造成氣壓不同

and the differences in air pressure

而氣壓的不同

make air move.

造成了空氣的流動

They make the wind blow.

造成了風

But, because the Earth spins,

但由於地球在旋轉

the moving air curves to the right

北半球的空氣運動

in the Northern Hemisphere

會向右偏成曲線

because of the coriolis effect.

這是科氏力的緣故

If there's a region of low pressure in the atmosphere,

如果大氣層中有一區低壓

air is pushed toward it,

空氣就會流進來

like water going down a drain.

像是水流進排水管

But the air curves to the right as it goes,

但空氣運動時會向右偏

and this gives it a spin.

這就造成了旋轉

With the dramatic low pressure in a storm,

由於風暴中心的超級低壓

the air gets pulled in tighter and tighter,

空氣吸進來的力量越來越強

so it gets going faster and faster,

風速就越來越快

and this is how we get the high winds of a hurricane.

這就是颶風的風速極快的原因

So, when you see a spinning storm on a weather report,

看到氣象報導上旋轉的風暴時

think about this:

想想看：

The spin ultimately came from the spin of the Earth,

風暴的轉動源自於地球的轉動

and the Earth's spin is a remnant,

地球的轉動是種遺跡、

a fossil relic,

化石級的遺跡

of the gentle spin of the cloud of dust and gas

源自於一團灰塵氣體雲

that collapsed to make the Earth

坍塌形成地球

some five billion years ago.

時間是五十億年前

You are watching something, the spin,

你看到的現象──轉動

that is older than dirt,

比塵埃還古老、

that's older than rocks,

比岩石還古老、

that's older than the Earth itself.

比地球本身還古老