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雨滴究竟是什么形状? – 译学馆
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雨滴究竟是什么形状?

What Do Raindrops Really Look Like?

感谢KiwiCo对PBS数码工作室的赞助
Thank you to KiwiCo for supporting PBS digital studios.
我刚刚玩了室内跳伞
I just went indoors skydiving,
所以我可以感受到成为一个雨滴会是什么样子
So I could feel what it’s like to be a raindrop.
因为当你想象一滴雨的样子你可能并没有做出正确的想象
Because imagine a raindrop, you’reprobably imagining it wrong.
那么 如果我要你画一滴雨
OK.So if I ask you to draw a raindrop,
你可能会这么画
most of you would probably draw this.
我们小的时候
That’s how
一直都是这么画的 对吗
we’ve been doing it since we were kids,right?
但这种画法是错的
But this is wrong.
下落的雨滴不长这样 这从物理角度是不可能的
A falling raindrop doesn’t look like this,it’s physically impossible.
当水从水龙头滴下来时
Now, when water drips
每一滴水的确都是泪珠状的
Say from a faucet – each drop does kind of take a teardrop shape,
后面还拖着尾巴……但一眨眼
as its tail drags behind it… but only for a split second.
很快 水滴就会变成团状
Pretty quickly, the drops become blob-shaped.
那是因为水珠有表面张力的缘故
And that’s because surface tension takesover.
表面张力存在是
Surface tension happens
由于水分子之间相互吸引的力
because water molecules are more attracted to each other
大于了周围空气给它的力
than to the air around them.
所以当它们从水龙头或是其它地方落下时
So once they split from the faucet or whatever they fall from,
它们自身形成了
they form the shape with the
对于一定的体积而言最小表体积的形状 也就是球体
smallest surface area for its volume, whichis a sphere.
所以雨水绝不可能是之前所说的的泪珠状
So raindrops can never form that old teardropshape.
但它们也不是具有完美的形状的球体
But they also aren’t perfect spheres.
因为雨滴在坠落时
Because raindrops are falling.
速度非常快
Fast.
这就意味着它们会受空气阻力限制
And this means they’re subject to air resistance.
空气是流体
Ar is a fluid.
它显然并不潮湿
It’s obviously not wet,
虽然我们通常用湿润与否来判断是否一个物体为流体
the way we typically think of a “ fluid ”.
但在物理学上 流体是用来描述可变形
But in physics, a fluid is just a substance that deforms,
或者可围绕另一物体流动的物质
or flows, around an object
当一个物体在流体中被推动时
when that object is pushing on another one.
如果你曾试过把手伸出车外
if you’ve ever held your hand outside the car,
你会感觉空气
you’ve felt the air deform, or flow,
在你手的周围变形 或流动
around your hand.
但是空气同时也在对你的手施力
But the air also exerts a force against your hand,
如果你想让你的手保持静止 你的肌肉
and to keep your hand still, your muscles
必须要施加反向且等大的力
have to exert an equal force in the oppositedirection.
这就是水滴在穿过大气层时会发生的现象
This is what happens when water’s fallingthrough the atmosphere!
不同的力同时作用于它
Several forces are acting on it at once.
重力牵引它下落
Gravity is pulling it down.
空气分子的碰撞中给它提供了反方向的力
Collisions with air molecules provide a force in the other direction.
水分子间的引力使雨滴聚集
And there’s attractive forces between the water molecules holding the drop together.
所有作用力加在一起
All of these combined,
把水滴压平成类似像汉堡包的形状
flatten out the spherical drop into a sort of hamburger-kind of shape.
当然 我们要如何确切知道
Of course, how do we know for sure
它们到底是什么形状呢
that’s what they look like?
我们总不能飞到天上
We can’t exactly go up in the sky
跟雨滴一起下落 用放大镜观察
with a magnifying glass and fall along with raindrops
才能知道它的形状吧……但我们可以利用一个超级大的风扇
to examine their shape… but we can do that down here on Earth.
在地面上观察
With a really big fan.
让这些小水珠悬浮在一个垂直的风洞里
These droplets are suspended in a vertical wind tunnel.
这和室内跳伞的原理相似
The same kind used for indoor skydiving.
小水珠浮在一个垂直的风洞里
Droplets suspended in a vertical wind tunnel
和下落的雨滴受到的净力是相同的
are experiencing the same net forces as falling raindrops.
只有让小水珠代替下落碰撞空气
Only instead of the droplets falling and hitting the air on their way down
空气上升
the air is rising
在它落下的途中碰撞
and hitting the drops on its way up.
作为一个开始下落的物体 由于受到重力 它会被加速
As an object begins to fall, due to gravity, it accelerates.
速度会越来越大
Its velocity increases.
直到与空气分子碰撞的力
Until the force of collisions
和向下的重力相平衡
with air molecules is equal to the force of gravity pulling it down.
在这个时候 它将停止加速
At this point, it stops accelerating.
速度趋于稳定
The velocity levels off.
这时候它达到了最终速度
This is terminal velocity.
不同的物质具有不同的最终速度这取决于其表面积
Different objects have different terminalvelocities, depending on their surface area,
其质量等等因素
their mass, things like that.
现在我的身体由于重力有落到地面的趋势
Now, my body wants to get to the ground because of gravity.
而风洞正在向上鼓风企图让我上升
And the tunnel is blasting air up, for me,
以大概每小时95到100英里的速度(标明95到100英里每小时 低于43到45米每秒)
at about 95 or 100 mph… ( label with 95-100 mph underneath 43 – 45 m/s)
对于职业的特级飞行者
For a professional stunt flyer,
风速可高达每小时150英里
the wind speed could be as high as 150 mph.
我显然不是专业的
I’m clearly not a professional.
重点是 我可以在风洞中漂浮 都是因为这一组对抗的力量
The point is, I can float in a wind tunnelbecause of those opposing forces: gravity
重力和与反方向空气分子的对抗
in one direction and the collisions of air molecules in the other.
一个在风洞中漂浮的物体
An object floating in a wind tunnel is
和以最终速度下落的物体
experiencing the same net forces as an object falling at
受到的净力相等
terminal velocity.
所以当我们使水滴悬浮于风洞中
So when we suspend a droplet of water in the wind tunnel,
我们正是在观察我们自己
we’re seeing exactly what we’d
如果在空气中下落会是什么样子
see if we were falling through the air,
差不多是一个以最终速度下落的雨滴
next to a raindrop, at terminal velocity!
而且我们观察到的绝不是
And what we see is definitely not the
我们孩童时期画的雨滴的形状
old shape we drew when we were kids.
真正的雨滴形状大致上可归为四种形状
Real raindrops actually come in four roughshapes.
我们可以称它们为 球形 汉堡包形 薄饼形 降落伞形
We’ll call them: spheres, burger buns, pancakes,and parachutes.
还记得我是怎么说球体的吗
Remember how I said spherical blobby shapes
由于表面张力 斑点形状能使表面积达到最小
minimize surface area thanks to surface tension?
最小的云滴就成型于球体
The smallest cloud droplets start out as thesespheres.
但是当它们一路直下时
But on the way down,
小水珠彼此碰撞 逐渐结合成了较大的水珠
small drops bump into each other and combine into bigger ones.
这些较大的雨滴具有更大的表面积 因此受到的空气阻力也越大
Those larger raindrops have more surface area for air to push on,
所以它们的形状变得更加扁平
so they flatten out even more.
一旦雨滴达到5到6毫米的大小
Once a drop reaches five-to-six millimeters
大概与家蝇一般大小
– that’s about the size of a housefly
它会从汉堡包形状转变为降落伞形状
– it’ll go from bun shaped, to parachuteshaped.
随着它越来越大 它会裂成两块
And as it gets bigger, it rips itself apart:
空气阻力比水分子间的分子引力更大
The force from air becomes more than the attraction between the water molecules,
导致它分散成一堆更小 更圆的雨滴
and it scatters into a bunch of smaller, rounder drops.
到底雨滴可以达到多大呢
So how big can a raindrop be?
这很难说
That’s hard to say –
但是根据研究显示 它们至多变大到7毫米就会分裂
but in tests they rarely ever hit 7mm across before they break apart.
所以
So.
物理学告诉我们雨滴更多是薄饼形和汉堡包形的 而不是眼泪形的
Physics tells us rain is more pancakes andhamburgers than teardrops.
希望我没有毁掉你的童年
Hopefully I didn’t ruin your childhood.
保持一颗好奇心吧
Stay curious.

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视频概述

在我们通常的认知中 雨滴是眼泪状的 但是真的是这样的吗?本视频将带你找一个答案。

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视频来源

https://www.youtube.com/watch?v=6G6omXLTAj0

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