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When we started with the entire class, I showed the children how to bind the book by hand.
这门课最开始的时候，我向孩子们展示了如何把书用绳子缝起来。
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I can do anything with string.
只要有绳子，我可以做任何事情。
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So, a massless body cannot have a net force on it, because the acceleration of the rope cannot be infinite.
所以没有质量的物体不会受到力,因为绳子的加速度不可能是无穷大
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Does that forces us to say, "Ah, so it's not ?" the same rope at the end as the beginning."
那我们是不是就必须说,“啊，这不是一开始那根绳子了“
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Point of absolute rest in a vibrating string we call a node.
一条绳子上绝对震动静止的点,我们称之为节点。
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He's got her on a string, right?
小汉斯想把妈妈拴在绳子上，不是吗？
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So you can imagine a bead with a wire going through it and the bead can only slide back and forth.
你可以想象一个穿在绳子上的珠子,小珠只能前后滑动
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Suppose I had a rope, that stretched from that into the room, all the way across-- to this end of the room.
假设我有一根绳子，从屋子的这一端，一直伸展到-,屋子的这一端。
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So, massless bodies will always have, like a massless rope, equal and opposite forces on the two ends.
所以无质量的物体,比如这个绳子,两边所受的力永远都相等
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Either a rope is pulling, a rope is pushing it, you are pushing it, you are pulling it.
无论绳子是在拉你还是推你,无论你是在推物体还是拉物体
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you're looking for that equal and opposite forces acting at two ends of the rope.
你就得求出作用在绳子两端的,大小相等方向相反的力
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What makes up a rope? Of course you know, ropes are basically bundles of fibers, very same fibers that have been woven together in a certain way.
绳子是由什么构成的？，你肯定知道，是由一束束纤维构成，是由一样的纤维按某种方式编织,在一起构成。
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Whenever you're asked on a problem, "What's the tension on the rope?"
不管什么题目问你,"绳子的张力是多少"
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In all these examples, this rope is assumed to be massless.
在所有这样的题目里都假设绳子没有质量
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I got 3 kg and I have a rope.
有一个 3 千克的物体和一根绳子
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Very same rope--at that end, at this end.
同样的一根绳子，从那边到这边。
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So, tension on the rope is 6 Newtons.
所以绳子的张力是 6 牛
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Now we really do have two ropes.
现在真的有2根绳子。
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You feel the thrill of having made it to the top and the wind bracingly striking your chin and you feel the satisfaction and you've got the memories of having almost died when the rope broke before.
你感受到登顶的兴奋,风吹袭你的脸颊,你获得满足感,你记得差点死去,当绳子断掉时。
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We know there is no thing called a massless rope, but most ropes have a mass, but maybe negligible compared to the two blocks you are pulling, so we'll take the idealized limit where the mass of the rope is 0.
我们都知道无质量的绳子是不存在的,是绳子就有质量,但是相比于这两个物体就可以忽略不计了,所以我们有了这个理想的条件,即绳子的质量都是 0
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kg is being pulled by the rope on the right with a force that I'm going to call T, which stands for tension.
千克的物体受到绳子向右的一个拉力,我称它为 T,也就是拉力的意思
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The rope is being pulled backwards by this guy, the T.
绳子也会被这个物体朝反向拽,拉力也是 T
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When you say the rope is under tension being pulled from both sides by a certain force, the tension is not 0 just because this T and that T cancel.
当绳子被绷紧,即两端被拉伸时,拉力并不为 0,因为这个 T 和那个 T 相互抵消了
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This is very important, because when you buy a rope, they will tell you how much tension it can take before it'll snap.
这非常重要,因为你去买绳子,他们会告诉你,绳子能承受的最大拉力是多少
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It won't snap, but something else will be a problem.
绳子不会断,不过会有其它问题
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That is called the tension on the rope.
那就是绳子的张力
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If your plan is to accelerate a 3 kg mass with an acceleration of 2 meters per second, you better have a rope that can furnish that force and it can take the tension of 6 Newtons.
如果你想用 2 米每秒平方的加速度,去拉一个 3 千克的物体,那么你就需要一根能提供拉力的绳子,并且它能够承受 6 牛的张力
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the ropes will be pulling the block with T in the other direction, then I got 10 Newtons here.
绳子也会对另一物体施加方向相反的拉力 T,于是我知道这里是 10 牛顿
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We'll give it a name, but by linking that T to that T, we can also figure out the same T must be exerted on this one by Newton's third law, because if this block is the only one that could be pulling this rope with T.
我们先给它命个名,把这个 T 和那个 T 联系起来,利用牛顿第三定律,我们可以知道这个 T 也一定作用在这个物体上,因为如果这个物体是唯一,受到绳子拉力 T 的物体
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