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自行车里的牛顿三大定律 – 译学馆
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自行车里的牛顿三大定律

Newton's 3 Laws, with a bicycle - Joshua Manley

你是否觉得骑自行车时刚起步那一会儿
Have you ever noticed that it’s harder to start pedaling your bicycle
难度大于一段时间后的恒定速度骑行?
than it is to ride at a constant speed?
或者说你有没有想过自行车是怎么动起来的 ?
Or wondered what causes your bicycle to move?
再比如说去想一想自行车为什么能保持向前行而不会倒退或摔倒?
Or thought about why it goes forward instead of backwards or sideways?
当然,肯定不止你会这么想。
Perhaps not, and you wouldn’t be alone.
直到17世纪
It wasn’t until the 17th century
牛顿提出了运动的基本规律后
that Isaac Newton described the fundamental laws of motion
我们找到了问题的答案
and we understood the answer to these three questions.
牛顿认为物体有保持原运动状态的特性
What Newton recognized was that things tend to keep on doing
所以当自行车停下来以后
what they are already doing. So when your bicycle is stopped,
它能保持停止,而当它开始前进后
it stays stopped, and when it is going,
也能保持前行。
it stays going.
当物体在运动它趋向于保持运动
Objects in motion tend to stay in motion
当物体静止时它趋向保持静止
and objects at rest tend to stay at rest.
这被称为牛顿第一定律
That’s Newton’s First Law.
物理学家们称它为惯性定律,这是一种很好的叫法。
Physicists call it the Law of Inertia, which is a fancy way of saying
那些移动的物体不会自动地加速、减速或者转向
that moving objects don’t spontaneously speed up, slow down, or change direction.
正是这种惯性,使你能够克服阻力,让你的自行车动起来。
It is this inertia that you must overcome to get your bicycle moving.
现在你知道你应该克服惯性使你的自行车动起来。
Now you know that you have to overcome inertia to get your bicycle moving,
但是什么让你能够克服它呢?
but what is it that allows you to overcome it?
这个问题在牛顿第二定律中解释了
Well, the answer is explained by Newton’s Second Law.
用数学术语来解释,牛顿第二定律说的是
In mathematical terms, Newton’s Second Law says
加速度多次聚集产生力。
that force is the product of mass times acceleration.
为了让一个物体加速,
To cause an object to accelerate, or speed up,
必须有力的提供。
a force must be applied.
你提供的力越多,
The more force you apply,
你加速的就越快。如果你的自行车重些,
the quicker you accelerate. And the more mass your bicycle has,
你也重一些
and the more mass you have too,
在同样的比例下你需要用更多的力来加速。
the more force you have to use to accelerate at the same rate.
这就是为什么骑一辆一万磅的自行车真的是一件很难的事。
This is why it would be really difficult to pedal a 10,000 pound bicycle.
就是你的腿提供的这个力压在踏板上,
And it is this force, which is applied by your legs pushing down on the pedals,
让你克服了牛顿的惯性定律。
that allows you to overcome Newton’s Law of Inertia.
你踩踏板踩得越重,你使的力就越大
The harder you push down on the pedals, the bigger the force
加速的也越快。
and the quicker you accelerate.
现在提出最后一个问题:
Now on to the final question:
当你确实让你的自行车动了,
When you do get your bike moving,
为什么它会向前走呢?
why does it go forward?
根据牛顿第三定律,对每个动作来说,
According to Newton’s Third Law, for every action,
都会有一个相等且相反的反应。
there is an equal and opposite reaction.
为了理解这个现象,想想当你投出一个弹球时会发生什么。
To understand this, think about what happens when you drop a bouncy ball.
当弹球打到地板上的时候,
As the bouncy ball hits the floor,
它对地板有一个向下的力。
it causes a downward force on the floor.
这是一个动作
This is the action.
这个地板通过对弹球施加一个相同的力来做出反应,
The floor reacts by pushing on the ball with the same force,
但相反的,向上
but in the opposite direction, upward,
使它朝你的方向弹了回去
causing it to bounce back up to you.
地板和小球叫做作一对相互作用物体
Together, the floor and the ball form what’s called
但对自行车来说,这种关系会稍微复杂些。
the action/reaction pair. When it comes to your bicycle,
当车轮顺时针滚动时
it is a little more complicated. As your bicycle wheels spin
每个轮子的很多部分与地面摩擦
clockwise, the parts of each tire touching the ground
对地面产生向后的推力
push backwards against the Earth:
这就是作用力
the actions. The ground pushes forward with the same force
地面也会对每个车轮有个向前的推力,这就是反作用力
against each of your tires: the reactions.
因为车有两个轮子,每个轮子都与地面有一对相互作用力
Since you have two bicycle tires, each one forms an action/reaction pair
又因为地球比自行车真的大很多很多
with the ground. And since the Earth is really, really, really big
所以地球在单车车轮的作用力下
compared to your bicycle, it barely moves
基本不动。
from the force caused by your bicycle tires pushing backwards,
但单车却载着你前进了。
but you are propelled forward.

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