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.
动量守恒，这就叫作动量守恒。
Momentum is conserved, and that's called the conservation of momentum.
我们相信,机械能守恒定理。
现在我想回到，机械能守恒。
Now I want to return to the conservation of mechanical energy.
我百分百相信，机械能守恒。
显然是质量守恒，而不是摩尔数守恒。
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.
因此这是，机械能守恒。
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。
我们仍将看到违反CP守恒的现象。
你看，这正是，验证动量守恒的方法。
So, you see, that's exactly how you can demonstrate the conservation of momentum.
下一个，动量守恒。
动能是守恒的。
但动量却守恒。
也就在反应期间,我有个合理假设，动量是守恒的。
It's just during the impact I claim that to a reasonable approximation, momentum will have to be conserved.
这是我们第一次，在这种系统里使用，角动量守恒定律。
Now we're going to apply for the first time in systems like this, the conservation of angular momentum.
这一原理，也即能量守恒，是物理学中最宝贵的定律之一。
This principle, called conservation of energy, is one of our most cherished laws of physics.
因此，我们不能把语句“能量是守恒的”做为一个绝对的陈述。
As a consequence, we cannot turn the statement "energy is conserved" into an absolute statement.
势能的改变量，将会转化成动能的改变量，这就是能量守恒定律。
The change in potential energy is going to be the change in kinetic energy You have conservation of energy principles.
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