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我为一道物理题租了架直升机!

I Rented A Helicopter To Settle A Physics Debate

In 2014, the qualifying exam for the US Physics Team
2014年 美国物理研究团队资格考试
had this as question 19:
第19个问题是这样的
A helicopter is flying horizontally at constant speed.
一架直升机正匀速水平飞行
A perfectly flexible uniform cable is suspended
一条完全灵活的制式电缆悬挂在
beneath the helicopter.
直升机下面
Air friction on the cable is not negligible.
电缆遇到的空气摩擦力不可忽略
So, which of the following diagrams
所以下列哪个图
best shows the shape of the cable
能最好的展示出 当直升机
as the helicopter flies through the air to the right?
在天空中向右飞行时电缆的形态?
Is it A. hanging straight down.
是A吗 垂直向下
B, hanging diagonally to the left?
B 电缆呈向左的对角线
C, this hook shape.
C 是个钩子的形状
D, the inverted hook shape,
D 是个反向钩子的形状
or E, a kind of S bend.
或者E 类似S型的曲线
Now, apparently there’s been a certain amount of controversy
现在 关于这道题的正确答案
about the correct answer to this question.
明显存在一定的争论
So, today we’re gonna go up in this helicopter,
所以 今天我们就要登上这架直升机
and put this question to rest once and for all.
彻底地解决这个问题
Let’s go.
出发
(intense music) (helicopter blades whirring)
[强劲的音乐] [直升机螺旋桨旋转的轰鸣]
A portion of this video was sponsored by SimpliSafe,
这条视频由SimpliSafe部分赞助
which allowed us to rent this helicopter.
他们的赞助让我们得以租用这架直升机
More about them at the end of the show.
视频结尾有更多关于他们的信息
I’ve got several thousand hours of carrying sling loads,
我花了好几个小时带吊索载荷
fire buckets, concrete buckets,
消防桶 混凝土吊罐
doing power line construction, actual power poles.
做电力线路建设 真正的电线杆
You name it, anything that needs to be moved
凡是你能说得出的
around the mountains, I’ve done.
山上任何需要被移动的东西 我都做了
Biggest concern that I have is
我最大的担忧
that the rope’s gonna interact with the rotor wash
就是绳索会和旋翼洗流相互作用
as its interaction with the ambient air around the aircraft.
就像它和飞行器周围空气的相互作用一样
And we’ll get a whipping action that falls down the rope
我们把绳子放下来时会出现鞭打动作
and flips around on the end.
并翻转过来
To a point that we worry if it will have a tendency
我们比较担心的点是 绳索有
to work its way back up towards the aircraft.
往回朝飞行器甩的趋势
Obviously, we don’t want it getting in the rotors
很明显 我们不想让绳子碰到转子
or the tail rotor.
或尾部螺旋桨
(helicopter blades whirring)
[直升机桨叶转动]
– [Derek] Alright, we’re all set.
Derek: 好的 我们准备好了
Movin’ up.
上机
(helicopter blades whirring)
[直升机桨叶转动]
(intense music)
[强烈的音乐]
Here I’m deploying a battle rope,
我用了一条格斗训练绳
like you’d see at the gym.
就是能在健身房看到的那种
This one is about 15 meters long and it weighs 20 kilograms.
这条绳子大概长15米重20千克
– [Craig] See, the whipping action’s begun.
Craig: 看 绳子开始出现鞭打动作了
– [Derek] I see it.
Derek: 我看到了
So, if we go really slow, will we avoid that?
那么 如果我们飞的非常慢 可以避免出现这种情况吗?
– We’ll see.
我们可以试试
We’ll see where the rotor wash
我们看看旋翼洗流在什么地方
stops interacting with the rope
停止和绳子
and the ambient wind.
以及环境风的相互作用
Now the setup looks pretty simple,
现在这个装置看上去非常简单
but few people agree on what the right answer should be.
但是很少有人能就正确答案达成共识
(peaceful music)
[平缓的音乐]
When I polled YouTube, the most common answer was C.
当我在油管上做民意测验时 最常见的答案是C
Well done on making that beautiful bell curve by the way.
顺便一提 那个漂亮的钟形曲线做的很好
We got in touch with the question’s author,
我们联系到问题的出题者
professor Paul Stanley.
Paul Stanley 教授
There was some creative students who actually constructed
一些很有创意的学生真正构建出
their own homemade scenarios.
他们自创的场景
One had a fan to the side and another fan blowing downward
一个风扇从旁边吹 另一个风扇从上往下吹
so that they could mimic the motion of the helicopter
这样他们可以模拟出直升机的运动
and suspended a string and said, “Oh, it’s this design.”
同时悬挂一根细绳 说“就是这样设计的”
And the faculty members looked at it and said,
教学人员看着这个会说
“Oh, obviously I can prove that the answer is this answer.”
“噢 明显我证明了就是这个答案”
They just didn’t agree with each other.
他们彼此不认同对方
– If we approximated as a chain of rigid links
– 如果我们把这个近似的视为一个刚性连杆链
– I don’t think you can just do that.
– 我认为你不能近似这样认为
– What do you think?
– 你认为呢?
– I think you’re more likely to be C.
-我想你更倾向C
– I think it is D.
-我认为是D
– D.
-D
– My guess is B.
-我猜是B
– What do you think? – B.
-你认为呢? -B
– B?
-B?
– Imma go with A.
-我选A
– Interestingly, no one chose E.
-有趣的是没有人选E
Have you locked in your prediction?
你确定好你的答案了吗?
(helicopter blades whirring)
[直升机桨叶转动]
To make sure the rope doesn’t come up
为了确保绳索不会飞上来
into the rotors of the helicopter,
进入直升机的转子
pilot Craig wanted to keep the rope on our right side
飞行员克雷格企图将绳索保持在右侧
so he could keep an eye on it.
这样他能时刻留意到绳索
Looks pretty good.
看上去挺不错
– I’m trying to make it look good.
我正在试图让它看上去不错
I mean I’m really working on it.
我的意思是我真的很努力
– You’re working hard.
你确实挺努力的
So, we’re not going straight forward.
所以 我们并不是走的直线
We’re actually going diagonally forward and to the left.
我们实际上是对角向前 朝左边
(upbeat music) (helicopter blades whirring)
[欢快的音乐][直升机桨叶转动]
But you can clearly see the rope is hanging straight,
但你可以清楚的看见绳索是直线悬挂的
diagonally to the left.
对角向左
That’s a pretty good diagonal, man.
这是个很完美的对角线 兄弟
So, the correct answer to the question is B.
所以问题的正确答案是B
You wanna pull it in?
你想把绳子拉回来吗?
– Up it comes.
上来吧
– [Derek] We’re going to try a few more experiments,
Derek: 我们要多做几组实验
adding a weight to the end of the rope and then a parachute.
在绳索尾端加一些重量 再加一个降落伞
But first I want to discuss why the answer is B.
不过首先我想讨论一下为什么答案是B
There are two external forces acting on the rope,
这里有两个外力作用于绳索上
gravity pulling it down and air resistance to the left.
重力把绳索向下来 空气阻力向左推
When flying along at constant speed,
在直升机以匀速飞行时
these forces must be perfectly balanced
这些力必须和绳子的张力
by the tension in the rope.
达到完美平衡
Now, when I set out to do this experiment,
当我着手做这个实验时
I wondered if the rope would be affected
我在考虑是不是绳索是不是会被
by the air pushed down by the helicopters rotor,
直升机旋翼产生的向下的气流力所影响
but judging by our results.
但从结果来看
This was not the case.
并不是这样
– The rotor wash doesn’t extend down below the helicopter,
旋翼洗流并不会一直延伸到直升机下方
all that far. It dissipates pretty quickly.
那么远的地方 它很快就消散了
– So, you can consider the air resistance on the rope
所以说 你可以认为绳索受到的空气阻力
as entirely due to its motion through still air.
完全是由于穿过静止的空气所导致的
Imagine dividing the rope up into many short sections.
想象一下 把绳索分成很多很小的部分
Each section has the same weight
每部分的重量相同
and experiences the same amount of air resistance
受到的空气阻力也相同
because it has the same cross-sectional area.
因为每个小部分都有同样的横截面积
And it’s moving at the same speed.
并且以相同的速度移动
Now, the tension in any section of the rope
现在 绳索上任意一个小部分的张力
must balance the sum of the air resistance
都必须与空气阻力的总和
and weight of all the sections beneath it.
以及下面所有部分的重量相平衡
So, the tension is zero at the bottom of the rope,
所以 绳索底端的张力是零
and it increases linearly up to a maximum at the top.
这个张力会线性增加 在绳索顶部达到最大
You can see the tension is small at the bottom of the rope,
可以看到 绳索底部的张力很小
and that’s why it wiggles around
所以绳索底部会摆动
while the top is much steadier.
而绳索上端要稳定的多
Now, although the magnitude of tension changes
现在 虽然张力的大小
throughout the rope, its direction doesn’t.
沿着绳索改变 但方向并没有
And that’s because the ratio of air resistance
这是因为空气阻力和重量的比率
to weight is the same at every point along the rope.
沿着绳索的每一个点都是一样的
That is why a uniform flexible cable hangs
这就是为什么均匀柔性电缆吊具
in a straight diagonal line when pulled at constant speed
在直升机以匀速飞行时
by a helicopter.
会是笔直的对角线
If the helicopter flies faster,
如果直升机飞的更快
the angle of the rope changes,
绳索的角度会变化
but it still makes a diagonal straight line
但仍然是笔直的对角线
because the ratio of air resistance to weight
因为空气阻力和重量的比率
is still constant along the entire length of the rope.
在绳索的整个长度上仍然恒定
But this got me wondering,
但这让我很好奇
“What would happen if we added a weight
“如果我们在绳的末端增重
to the end of the rope?”
会发生什么呢”
Here, I have a 20 pound,
这是20磅
that’s an eight or nine kilos kettlebell.
这是个8-9公斤的壶铃
And I want to ask you to make a prediction.
下面我想请你预测一下
What shape do you think the rope will make now?
你认为这时绳索是什么形状?
Will it still be that diagonal,
它会仍然是对角线吗?
or will it be one of the other five options?
或者会是其他5个选项中的一个吗?
One more question for you.
这是给你的又一个问题
If we chucked a weight at the bottom,
如果我们在绳索底部挂上重量
which shape would it be?
绳索会是什么形状?
– Oh. – Probably D.
-哦 -可能是D
– B, or possibly C.
-B或者可能C
– [Derek] Does that change it?
Derek: 有变化吗?
– Then I think it would be B.
-我认为会是B
(whimsical music)
[古怪的音乐]
– Easy to do on the ground.
在地面上比较容易操作
(helicopter blades whirring)
[直升机桨叶转动]
Alright, we’re dropping the rope over the side.
好啦 我们从一边把绳索放下去
This one has 20 pound kettlebell off the end of it.
这条绳索底部悬挂了一个20磅的壶铃
How’s that feel when it’s dangling?
它晃来晃去是什么感觉?
– No lateral displacement at all.
完全没有横向移位
– Alright.
-好的
– How’s your forearms doing?
你的小臂怎么样?
– I’m feeling good, you know.
-我感觉不错 你懂的
I feel like this is the easy way, obviously.
我感觉这显然很简单
Gonna feel different when I gotta pull it up.
当我把它拉起来的时候可能会感觉不一样
– Okay, is it down? – Yup.
-好的 放下了吗? -是的
Okay, let’s give it a shot.
好的 让我们试试看
(whimsical music) (helicopter blades whirring)
[古怪音乐][直升机桨叶旋转]
Here the helicopter is flying
直升机正在以
at nearly a hundred kilometers per hour,
将近每小时100千米的速度飞行
and the rope makes a different shape.
绳索改变形状了
You can clearly see,
你可以清楚的看到
it looks like an inverted J.
看起来像倒过来的J
This is option D.
这是选项D
In fact, this is how the question
事实上 这个问题
on the qualifying exam originated.
最初在资格考试中就是这样的
– So, I taught for a semester in Hong Kong,
我在香港教过一学期课
and we’d go hiking in the new territories.
我们会去新界远足
One of the times I was hiking,
有一次我在远足时
I saw a helicopter fly with a cable beneath it.
我看见有架悬挂着一条缆绳的直升机飞过
It was carrying something to one of the remote parts
它正往香港新界的边远地区
of the Hong Kong new territories.
运送物资
And I saw the shape of the cable and thought to myself,
我看到缆绳的形状 自己思考
“That looks a little counter intuitive and very neat.
这看上去有些违背直觉又很巧妙
This might make a good multiple choice question
这应该能作为学生的选拔考试中
for the selection exam for the students.”
一道不错的选择题
The cable that I saw had a weight hanging on the end,
我看到的缆绳底端悬挂了重物
which gave it a certain curved shape.
这使得绳索呈现这样的形状
When I shared this with the other coaches, another coach,
当我和其他教员分享这个时 另一个教员
Andrew Linn, looked at it and said,
Andrew Linn 看着它说
“I think the question might even be more fun
我觉得 如果绳索上没有重量
if there is no weight on the cable
这个问题会更有趣
because that is even harder to imagine.”
因为更难去想象
– [Derek] To understand why the rope makes this shape,
Derek: 想要理解绳索为什么会产生这种形状
we can use the same analysis as before.
我们可以用和之前一样的分析方法
But now we need to add a large weight
不过现在我们需要给绳子末端
to the end of the rope.
再增加较大的重量
And this means at the bottom,
这意味着在绳索底端
the tension needs to be almost vertical
张力需要近乎垂直
to support the weight of the kettlebell,
以支撑壶铃的重量
which has a lot of weight but not much air resistance.
壶铃有很大的重量 但空气阻力很少
As you go up the rope, the ratio of total air resistance
顺着绳索往上 空气阻力和绳索及悬挂物整体重量的比率
to weight for everything beneath increases.
将会增大
So, the rope turns more horizontal in order for the tension
所以 绳索在张力作用下变得更加水平
to balance out that increasing air resistance.
以平衡增加的空气阻力
– You’re more than halfway. You got it.
你已经完成一多半了 你可以的
(woman laughing)
[女人的笑声]
– [Derek] For the final test,
Derek: 最后的测试
I wanted to add something to the bottom of the rope
我想在绳索底部增加一些东西
that added almost no weight, but significant air resistance.
增加一些几乎没什么重量 但会产生显著的空气阻力的东西
So, I picked a Veritasium flag, of course.
所以 当然 我选择了《Ve元素》的旗帜
So, what do you think about hanging this
你们觉得把这个挂在
at the end of the rope?
绳子底部怎么样
– Well, it’s all science experiment for me
这对我来说都是科学实验
’cause these are all the things you’d never do
因为这些是你绝对不会
with a helicopter in a row.
用直升机做的一系列事情
– What would it be the real risks,
真正的风险是什么呢
like tail rotor or this rotor, or both?
是尾部转子还是这个转子 或者都有?
– Either.
都有可能
Any kind of this getting into the inner tail rotor
任何这类东西进入尾部转子
would be destruction to the aircraft.
都会摧毁飞行器
(helicopter blades whirring) (intense music)
[直升机桨叶旋转声][强烈的音乐]
And we’ll start, here we go.
我们马上开始了 走起!
– [Derek] Now the rope still seems to be pretty straight.
Derek: 现在绳索仍然看上去是笔直的
I think it’s because the flag
我想这是因为这面旗子
isn’t actually adding much drag.
并没有真正加入拖拽力
Still looks pretty crazy, interestingly.
看上去仍然很疯狂 很有趣
So, we decided to add a small parachute
所以我们决定在绳索尾端
to the end of the rope.
加上一个小降落伞
All right, let’s try it.
好啦 试试看吧
The concern here was that the parachute
比较让人担心的是
could get flipped up into the rotors during deployment.
降落伞在展开时由于转动过快而进入转子
So, we bundled the shoot up into a backpack.
所以我们把发射器捆在背包里
– [Craig] Very nicely done.
Craig: 完成的非常好
– [Derek] Does it deploy more than that?
Derek: 它会展开的更大吗
– [Craig] No, but once we start moving,
Craig: 不会的 但我们一旦开始移动
its gonna grab a lot of air.
它会攫取大量空气
– [Derek] Now, with the parachute at the end of the rope,
Derek: 现在 绳索底部系着降落伞
it makes a J shape, which is answer C.
绳索呈现J的形状 也就是选项C
With extra air resistance at the end of the rope,
绳索底端有了额外的空气阻力
but not much weight,
但没有太多重量
the tension has to be virtually horizontal.
张力几乎水平
And then as you go up the rope,
然后顺着绳索往上
the ratio of total air resistance to weight
直升机下物品的总空气阻力和重量的比率
for everything beneath decreases.
都在降低
So, the rope becomes more vertical
所以 绳索变得更加垂直
to balance out the increasing weight.
以平衡增加的重量
So, depending on what’s on the end of the rope,
所以 取决于绳索底端挂了什么
you could get answers B, C or D.
答案B C或D都可能会是正确答案
Now, if you enjoyed this video,
如果你喜欢这个视频
I bet you would also like my bullet block experiment series.
我猜你也会喜欢我的子弹块系列试验
So, check it out.
所以 看过来
(retro beeping sound effect)
[复古哔哔声效果]
Now, hanging out the side of a helicopter
悬挂在直升机的一侧
was not my safest moment,
并不是最安全的时刻
but I do always feel safe here at home.
但现在在家我总是觉得很安全
Thanks to SimpliSafe, the sponsor of this portion of the video.
感谢SimpliSafe 这部分视频的赞助商
SimpliSafe is an easy to use,
SimpliSafe是使用方便
totally customizable home security system.
且可以完全实现定制化的家庭安防系统
They allow you to design a set up to fit your space,
他们可以让你设计适合你空间的组件
and they ship it directly to your doorstep.
还可以直接送货上门
Then their interactive monitoring service ensures
此外 他们的交互式监测服务可以确保
that your home is professionally monitored 24/7.
你的家里会拥有7乘24小时的专业监控
You know, I started thinking seriously about home security
你懂的 我已经开始认真思考家庭安全问题了
when there was a police helicopter circling overhead
有一天 当有警用直升机在头顶盘旋时
one day, and it was not dangling any ropes.
它没有悬挂任何绳索
It turned out the house across the street
原来是对面街上
had been broken into.
有一个房子被破门而入
Luckily, SimpliSafe makes it easy to protect your home.
幸运的是 SimpliSafe让保护家庭变得非常容易
They sent me a kit that took only about an hour to set up.
他们给了我一套设备 只用了一个小时就可以组装好
I currently use the simply safe base station,
我现在用这个简单安全的基站
the cameras and sensors to keep tabs
摄像机和传感器持续记录
on what’s going on around my house.
发生在我的房子周围的事情
I also installed their new wireless outdoor security camera,
我也安装了他们新款无线户外安全摄像头
which doesn’t require an outlet.
这个不需要电源插座
So, you can set it up literally anywhere.
所以 理论上说你可以把它装在任何地方
This camera has eight times zoom, color night vision,
这个摄影机有八倍放大 彩色夜视
and two-way audio.
以及双向音频
So, you can speak to someone on your property.
所以你可以和在你房子里的人对话
SimpliSafe’s interactive monitoring service
SimpliSafe的交互式监测服务
will call the police if it’s alerted to anything.
能够在警报被触动时报警
Their team of always on professionals are easily reachable
他们的团队很专业 且很容易联系到
and always keep lines of communication open
而且在安保服务方面
for the most protective service.
总是保持沟通渠道畅通
Visit simplisafe.com/veritasium to learn more
访问SimpliSafe.com或Ve元素以了解更多讯息
and get at least 30% off your SimpliSafe security system.
而且购买SimpliSafe的安保系统还能获得至少30%的折扣
I want to thank SimpliSafe for sponsoring this portion of the video.
我要感谢SimliSafe赞助这部分视频
And I want to thank you for watching.
感谢你们的收看

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视频概述

Ve元素团队竟然为了验证一道物理资格考试题租用了一架直升机!验证的结果是什么呢?他们答对了吗?

听录译者

收集自网络

翻译译者

早早

审核员

审核员XY

视频来源

https://www.youtube.com/watch?v=q-_7y0WUnW4

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