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地球自转

Turn of the Earth

[音乐]
[MUSIC PLAYING]
各位好!
Hi.
我是艾伦·约翰逊
I’m Aaron Johnson, a PhD student in the Department
一位来自麻省理工学院航空航天系的博士生
of Aeronautics and Astronautics here at MIT.
现在大约是凌晨两点
And it’s about 2 o’clock in the morning.
我们在图书馆里 但不是在做作业
We’re in the library, but we’re not working on homework
也不是在做项目
or not working on a project.
我们实际上在证明
We’re actually proving that the Earth is rotating around
地球在绕着地心轴自转
on its axis.
我们是如何做的呢
How are we doing that?
我知道 它不是轻松的任务
Not a light task, I know.
今晚 我们将用一个有160年历史的
We’re using a 160-year-old device
名叫傅科摆的装置
called a Foucault’s pendulum that we built here
它就在我身旁
this evening.
我们会介绍我们如何建造它 它又如何工作
So we’ll show you how we built it, how it works,
同时 还要像我说的 证明地球真的
and like I said, prove that the Earth really
是在自转
is spinning around.
就像傅科的原话说的
And so in the words of Foucault himself,
“邀请你来看看地球的自转”
"You are invited to come see the Earth turn."
现在我们已经准备好开始展示
[INAUDIBLE] is off getting ready for our demonstration.
但是它究竟是怎样工作的呢
But how exactly is this going to work?
我是说 傅科摆如何
I mean, how can a Foucault’s pendulum
展现出地球在绕轴自转呢
show that the Earth is rotating around on its axis?
请看图示
Let’s take a look.
摆由两部分组成
The pendulum is composed of two parts.
一根长长的绳子 我这里是一根线
There’s a long string, or wire in our case,
在绳子底端有一个很重的悬挂物
with a bob at the end, a heavy weight.
当你让它开始摇摆
When you start it swinging, a pendulum
摆就会前后摆动 直到有外力作用(改变它的轨迹)
swings back and forth until acted upon by an outside force.
这种现象叫做惯性
This is called inertia.
摆动长时间后
A Foucault’s pendulum, if left for a long time,
傅科摆会进动 或者说旋进
will appear to precess, or rotate.
所以 如果我们从顶部看傅科摆
So if we look at the pendulum from the top,
你会发现它是这样摆动的[图]
you would start swinging like this.
在北半球的话
And then in the Northern Hemisphere,
过一段时间 就会变成这样[图]
after time, it will appear like this, and after more time still
再过一段时间 就会成这样[图]
like this.
你会发现傅科摆出现了顺时钟的进动
It’s appearing to precess clockwise
顺时针是发生在北半球
in the Northern Hemisphere.
而在南半球则是相反的
It’s opposite in the Southern Hemisphere.
那如果在南极或者北极有一个傅科摆会出现什么情况呢?
So what if we have a pendulum at the North or South Pole?
在这些点上 地球在转动
On this spot, the Earth is twisting,
包括建有有傅科摆的房子
and that includes the pendulum building.
傅科摆实际上没有移动
But the pendulum doesn’t actually move.
在房屋里的观察者会发现
So while the pendulum appears to rotate
似乎是傅科摆在旋转
to an observer in the building, it’s
但实际上是房屋在转
actually the building turning while the pendulum stays
而摆停留在相同的位置
in the same position.
傅科摆的进动周期是24小时
The pendulum precession period is 24 hours.
在赤道的傅科摆又是怎样的情形呢
What if the pendulum is at the Equator?
赤道上的点不是原地打转
This spot on Earth isn’t twisting,
而是在地球表面向东移动
but it’s traveling eastward on Earth’s surface.
在这里的傅科摆根本不会进动
Here, the pendulum won’t precess at all.
这是两种极端情况
So these are the two limiting cases.
在两者之间又是什么样的呢
But what about in between?
在这里 既有原地打转
At this spot on Earth, there is twisting and traveling
又有向东的位移
eastward.
傅科摆会显示出这种扭动运动
The pendulum will show this twisting motion, but not
但不能显示这种运动的位移情况
the traveling component of the motion.
这里出现的结果是 进动周期
As a result, the precession period
大于24小时
will be greater than 24 hours.
有一个简单的公式用于
There’s a simple formula that lets us find the angular
计算傅科摆的角速度
velocity of our pendulum, represented by omega,
角速度由w表示 意思是一天旋转的角度
in degrees per day.
w等于360乘以纬度的正弦值
Omega is equal to 360 times the sine of the latitude.
波士顿的纬度是北纬42.36度
Here in Boston, our latitude is 42.36 degrees North.
由此可以计算出这里的角速度
So this gives us an angular velocity
是242.56°/24小时
of 242.56 degrees per day.
同时也说明我们这个摆
This tells us that our pendulum will precess
每小时进动10.1度
10.1 degrees every hour.
这被称为科里奥利效应
This is called the Coriolis effect.
从顶端向下看 北半球的傅科摆
Looking from the top down, pendulums in the Northern
顺时针进动
Hemisphere precess clockwise, while pendulums in the Southern
而南半球的傅科摆是逆时针进动
Hemisphere precess counterclockwise.
这种现象我们每天都能看见
This has effects on Earth that we can see every day.
就拿飓风和台风作为例子
Take hurricanes and typhoons, for example.
如果在北半球 局部一些小股台风
If we have little bits of wind in the Northern Hemisphere
是顺时针旋转 而在整体上
rotating clockwise, we’re going to get an overall rotation
是逆时针旋转
that’s counterclockwise.
总体说来北半球的台风和飓风
So hurricanes and typhoons in the Northern Hemisphere
是逆时针旋转的
rotate counterclockwise.
而在南半球情况相反
In the Southern Hemisphere, the opposite is true.
这里整体上台风和飓风是顺时针旋转的
Hurricanes and typhoons rotate clockwise.
很多人认为科里奥利效应
Many people think that the Coriolis effect also causes
也会导致北半球马桶里的水逆时针旋转
the water in your toilet bowl to rotate counterclockwise
南半球的马桶里的水
in the Northern Hemisphere and clockwise in the Southern
顺时针旋转
Hemisphere.
这并不正确
It’s not really true.
科里奥利效应是存在 但是很小
The Coriolis effects are there, but they’re so small
以至于会被其它因素掩盖
that they’re really overwhelmed by other factors,
例如马桶的喷嘴指向哪个方向
such as which way the jets in the toilet are pointing
以及马桶是如何充满水的
and how the bowl was filled.
所以很不幸 这种想法不正确
So it’s, unfortunately, not true.
[音乐]
[MUSIC PLAYING]
开始傅科摆实验时
When we start our Foucault’s pendulum,
一定要确保不能出现偏离轨迹的运动
we need to make sure not to put any sideways motion on it.
那样的话我们只能看它在摆动
Otherwise, we’re going to see that motion, and not
而没办法看到地球自转带来的影响
the rotation of the Earth.
所以我们用一种传统的方法让傅科摆摆动
So we’re going to start our pendulum the traditional way.
[音乐]
[MUSIC PLAYING]
我们让傅科摆一直摆动
We got the pendulum swinging, but it’s
但需要很长的时间才能出现可观测的进动
going to take a lot for there to be any visible precession.
我们快进一个小时
So let’s skip ahead one hour.
一个小时 这个摆大概进动了10度
In one hour, the pendulum has precessed about 10 degrees.
同时我们也能看出来
But we can also see that its amplitude
摆的振幅比之前小了
is smaller than before.
这是因为空气阻力
This is due to damping.
我们用了一个很长的绳子和重锤
We used a heavy bob and a long wire
这样可以减少空气阻力的影响
to reduce the effects of damping,
但不可能完全消除空气阻力
but you can never completely eliminate them.
你在科学馆里看到的傅科摆
Foucault’s pendulums that you see in science museums have
会用电磁铁 在每次摆动时给绳子施加一个力
an electromagnet that gives the wire a kick each swing
保持摆一直摆动
and keeps the pendulum swinging.
[音乐]
[MUSIC PLAYING]
傅科摆也出现细微的横摆
There’s also a bit of sideways oscillation in the pendulum.
可以看出它的轨迹是一个窄椭圆形
You can see it traces out a narrow ellipse.
这主要是因为绳子是由
This is likely because the wire is made of many smaller
许多细线缠绕撮合成的 当绳子受外力时
wires twisted together, and then as [INAUDIBLE] the tension,
会出现反撮合方向的力
it tends to untwist a bit.
这会出现一个横向扭矩
This introduces a sideways torque,
进而导致了摆的椭圆形运动
leading [? through ?] the elliptical motion.
但是 我们依然可以
However, we can still clearly see
清晰的看到摆的进动
the precession of the pendulum, and thus
从而看出地球的自转
the rotation of the Earth.

Ah.
我想说 通过这个实验
Well, I’d say with that experiment
我们证明了地球确实在绕轴自转
we proved that the Earth really is
这也是我们想要发现的
rotating around its axis, which is what we hoped to discover.
我想这是很不错的
So I guess that’s good.
对得起一晚上的工作
Not bad for a night’s work.
现在我认为我应该出去
But I really think I’m going to head out and get
好好睡一觉了
some sleep now.
[音乐]
[MUSIC PLAYING]

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视频概述

我们都知道地球在自转,可是你亲眼见过吗?怎样可以看见地球在自转呢?看完本视频,你就知道了哦~

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收集自网络

翻译译者

成长吹来的-风

审核员

LD

视频来源

https://www.youtube.com/watch?v=_pECtfYa2Us

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