动量被认为是“守恒的”，也就是说，动量一旦被建立，就一定会转移到其他地方。
Momentum is said to be "conserved", that is, once you build it up, it has to go somewhere.
例如，无论一个舞者多么想要跳离地板，然后开始旋转，角动量守恒定律绝对阻止这样的动作。
For example, no matter how much a dancer wishes to leap off the floor and then start turning, the law of conservation of angular momentum absolutely prevents such a movement.
能量在其任何形式都是守恒的。
我们要再一次假定，动量是守恒的。
是违背守恒定律而凭空消失了吗？
仅仅利用机械能概念，它是守恒的。
We only used the concept of mechanical energy, which is conserved.
在皮亚杰之前，没有人注意到守恒。
系统作为一个整体，动量是守恒的。
It's just the system as a whole for which the momentum is then conserved.
我可以第一次应用，动量守恒。
And so now I can apply, for the first time, the conservation of momentum.
动量守恒，这就叫作动量守恒。
Momentum is conserved, and that's called the conservation of momentum.
这是角动量守恒的，立竿见影的效果。
That's immediate consequence of the conservation of angular momentum.
我们相信,机械能守恒定理。
它紧跟着，机械能，守恒定理而产生。
It follows immediately from the conservation of mechanical energy.
我要写下整体的，能量守恒，机械能。
I'm going to write down the conservation of total energy, mechanical energy.
机械能是守恒的，因为重力是守恒力。
Mechanical energy is conserved, because gravity is a conservative force.
现在我想回到，机械能守恒。
Now I want to return to the conservation of mechanical energy.
这是唯一的方法,自然使动量守恒之道。
它们的波长就是我们评估是否守恒的关键。
The wavelengths of that light are our key to assessing conservation.
动量守恒定律告诉你,碰撞后速度是多少。
So the conservation of momentum tells you what the speed is after the collision.
让我们试着用机械能，守恒定律解答一下。
因此这是，机械能守恒。
我相信，机械能守恒定理，我忽略了张力。
I believed in the conservation of mechanical energy and I overlooked the tension.
角动量，在点Q守恒，也只有在点Q守恒。
Angular momentum is conserved about this point Q, but only about that point Q.
总能量必须是守恒的。
它讲的是能量的守恒。
要服从质量守恒定律。
所以总机械能，守恒。
我们仍将看到违反CP守恒的现象。
下一个，动量守恒。
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