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.
在皮亚杰之前，没有人注意到守恒。
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.
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.
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.
它讲的是能量的守恒。
下一个，动量守恒。
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.
但是动量守恒，因为这里的，摩擦力都是内部的。
But momentum is conserved, because those frictional forces are all internal.
我们也可以运用，机械能守恒，因为弹力，是恒力。
We were able to apply the conservation of mechanical energy because spring forces are conservative forces.
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.
动量守恒,可以写下2,这是完全-,非弹性碰撞。
Momentum is conserved, so I can m1*v1+ write down that m1 times v1 plus m2 - it is a completely inelastic collision-- *v times v prime.
不知怎么的，这一定律和,角动量守恒定律有所相似。
Somehow, it has the smell of some conservation of angular momentum.
我们知道，机械能守恒定律-，动能和，势能的总和。
Then we have the conservation - of mechanical energy — the sum of kinetic energy and potential energy.
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.
所以你也可以用，功能定理，或者是机械能，守恒定律。
And so you could have used the work-energy theorem or you can use the conservation of mechanical energy.
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.
我们援引了摩尔分数的道尔顿定律，要服从质量守恒定律。
We invoke Dalton's Law of molar proportions, we write it subject to conservation of mass.
那么墙壁也定有动量-，这是毫无疑问的，因为动量是守恒的。
That momentum must be in the wall — it's nonnegotiable, because momentum must be conserved.
若没有外力，作用于系统整体，在碰撞过程中，动量就是守恒的。
And if there is no net external force on the system as a whole during the collisions, then momentum is conserved.
之前我们讨论过，角动量守恒-,它们有很强的磁场-,这物体。
The neutron stars rotate around, - we discussed that earlier-- - conservation of angular momentum-- and they have strong magnetic fields.
动量是守恒的，因为它不受外力，因此动能也是守恒的。
Momentum is conserved, because there was no net external force, so now kinetic energy is also conserved.
现在，我很相信，机械能守恒，我愿意把我的生命，放到这条线上。
Now, I am such a strong believer of the conservation of mechanical energy that I am willing to put my life on the line.
其结果，其中一些方程非常眼熟，比如动量守恒定律和牛顿第二定律。
Turns out, some of these equations were very familiar: the law of conservation of momentum, and Newton's second law of motion.
守恒是指物体某方面的特征,不会因为其他方面特征的改变,而有所改变
The notion of conservation is that there's ways to transform things such that some aspects of them change but others remain the same.
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