物体的中心在这里。
看到质量重心了吧？
The center of mass must be somewhere here.
重心肯定是这里某处。
Where is the center of mass of this system?
这系统的重心何在？
And so you can calculate the center of mass.
你可以计算质量中心。
And let the center of mass be, for instance, here.
让物体的中心，比方说，在这里。
So this is the velocity of the center of mass.
这就是重心速度。
You're not moving, you're in the center of mass.
你没有移动，你处于质量重心。
Hard to predict but the center of mass behaves decently.
很难预测，但质量中心稳定运动。
How do we calculate the velocity of the center of mass?
我们怎样计算，重心速度呢？
And this, now, is the position vector of the center of mass.
现在，这里是，物体的矢量中心。
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。
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。
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。
So here is the center of mass of that ladder, right in the middle.
这里是梯子的重心，在正中央。
because it's a unit disk. And, the center of mass is the center of a disk.
因为它是一个单位圆盘，这部分的中心就是圆盘的中心。
And look at this. The center of mass is way below my suspension point.
你们看，重心处在，悬挂点的下方。
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 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。
So this is a disk, and we just discussed the rotation about the center of mass.
这是圆盘，我们刚讨论过,绕质心旋转。
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。
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。
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 then you can really see that the center of mass moves in a very civilized way.
你可以看到质量重心，非常稳定地运动。
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。
Can you see that the center of mass is moving very uniformly, or can you not see that?
你能看到物体重心吗，它稳定地运动，你们能看到吗？
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 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。
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。
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。
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