It will remind you of center of mass and all that.
它将来会让你想起质心等等。
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。
物体的中心在这里。
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 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。
The center of mass must now be somewhere here. I balance it here.
重心肯定,在这条线上某个位置，我让它平衡。
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。
看到质量重心了吧？
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。
And let the center of mass be, for instance, here.
让物体的中心，比方说，在这里。
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。
And so you can calculate the center of mass.
你可以计算质量中心。
The center of mass must be somewhere here.
重心肯定是这里某处。
And this, now, is the position vector of the center of mass.
现在，这里是，物体的矢量中心。
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。
In particular, we have seen the notion of center of mass.
特别地，我们学过质心的概念。
Center of mass below the suspension point.
重心在悬挂点下面。
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。
And now comes another unique property of the center of mass.
它还有一个特性，质量重心的特性。
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。
For the pendulum, center of mass is here.
对摆来说，重心在这里。
So the center of mass will always find itself below the point of suspension.
所以物体的重心，经常处于悬挂点之下。
And the center of mass of this eraser is somewhere in the middle.
这个橡皮重心在中间的某一点。
You're not moving, you're in the center of mass.
你没有移动，你处于质量重心。
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