Then as the star moves around the center of mass, you see that all these lines drift in unison.
随着恒星绕在，质量中心转动，你能看到所有这些线，一致得移动。
On our spherical Earth, gravity pulls "down" us toward the planet's center of mass. So on a flat surface, we naturally stand up straight.
在我们地球上，重力会把我们拉向星球的质量中心，因此在平坦的地表，我们自然站得很直。
The velocity of the center of mass has not changed as seen from your frame of reference, because the velocity of center of mass is always the same, remember, because momentum is conserved.
质量重心的速度，并没有发生改变，你在参照系中可以看出，因为质量重心的速度，总是保持不变，要记住，由于动量守恒。
And so going to the center-of-mass system, you can always immediately calculate what the maximum heat is that you can expect from a collision.
现在看质量重心体系，你可以很快计算出，最大的热量是多少，你可以从碰撞中得到的。
And that's exactly what you see comes out of these equations whether you go to the center of mass or whether you do it from 26.100.
这就是你们看到的，从这两个方程里得到的，无论是质量重心，还是在26。100做实验。
And this gives the center of mass some amazing properties that I will discuss with you now.
这使质量重心，有一些神奇特质，我一会儿跟你们讨论。
In this case, it will rotate about the center of mass, so it's not static equilibrium.
在这个例子里，它会绕着重心旋转，因此不存在静力平衡。
Hard to predict but the center of mass behaves decently.
很难预测，但质量中心稳定运动。
We want to know what the velocity is of the center of mass of the other one.
我们想知道，质量重心的速度是多少，对于另一个。
For example, take a disk center at the origin, well, the center of mass should be at the origin.
举个例子，取一个圆心为原点的圆盘，重心应该在原点。
But the basic idea is, then, that the center of mass will always in stable configuration line itself up below the point of suspension.
但是基本概念是，物体的重心将一直，处于稳定的结构线上，在悬挂点下面。
If you have a problem asking you to find the center of mass of something and you have no information about the density, assume it is uniform.
如果一个问题，需要知道某物的重心，而又不知道其密度，就可以假设它是个常数。
We did a demonstration there with two vibrating objects with a spring, but yet the center of mass was moving with a constant velocity.
我们做过演示的，弹簧连接两个振动物体，但是质量重心，却保持稳定速度。
What I can do now, I can suspend it like this, and I know that the center of mass must be below my finger. Did you notice?
我现在能做的，我可以这么悬挂它，我知道重心,肯定在我手指下，看到了吗？
If I throw up a hammer, then the center of mass and only the center of mass will just go along a perfect parabola as if it were just a tennis ball.
我若是抛铁锤，那么质量重心，也是唯一的质量重心，会沿完美抛物线运动，就像一颗网球。
I will then transfer to the center-of-mass frame, and I will show you, then, this quite amazing property.
我将它转化到重心体系，我会给你们展示，这是神奇的性质。
And ultraviolet light will interact with that little ball, the center of mass, and when I then make it dark, you will only see the motion of the center of mass.
这紫外线光可以，与小球互动，也就是质量重心，当周围暗下来，能看到质量重心运动。
I have here a triangle, and I have no clue where the center of mass is.
这里有个三角形，我不知道，重心在哪里。
The center of mass of a system is defined as follows: I have here some kind of an object not just a point mass, but it has a finite size.
系统的重心,是这样定义的：,我有一个物体,不是有棱角的物体，但它大小有限。
So to get into the center-of-mass frame, you must subtract the velocity of the center of mass from the initial velocities.
要进入重心体系，你就必须减去速度，重心的速度，从原始速度中减去。
But the center of mass is below the rope, and so there is really no danger.
但重心是低于绳子的，所以没有危险。
Now I'm going to do the same calculation in the center-of-mass frame.
我们会做同样的计算,在质量重心体系中。
The other way to think about it, if I had a flat object then the center of mass would basically be the point where I would need to hold it so it is perfectly balanced.
从另一个角度看，一个平面物体的重心就是,如果在重心处拿起物体，它就能完全地保持平衡。
And, now, I think I know just by looking at the picture where the center of mass of this circle will be, right?
这样的话靠观察这张图片,圆的中心在哪就可以知道？
This, by the way — - it's not so obvious — is also the velocity of the center of mass.
顺便提一下-，这并不明显-，它也是质量重心的速度。
And so we have here a squash racquet and we have here the center of mass.
这里有一只壁球拍，这里是质量重心。
Whether you stay in your reference frame or in the reference frame of the center of mass, it's immediately obvious that all the kinetic energy is lost.
无论你在对照系中，还是在质量重心体系，很显然，所有的动能都被损耗了。
If you have a car or if you have an object like this, then it is, of course, much harder to calculate the center of mass.
若是车或类似的东西，那么它当然会，很难计算质量中心。
In the absence of any net external forces - on a system as a whole — - as we discussed last time — the center of mass will always have the same velocity.
在没有外力，作用于整个系统时-，我们上次讨论过-，重心总是，有相同稳定的速度。
If you ever want to move yourself to the center of mass, you will have to know what the center-of-mass velocity is.
你若想要移动，在重心体系中，你就必须知道，重心速度是多少。
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