Shots to the head and center mass. On my mark. One…Two. Three!
射击头部和要害。听我命令。一…二。三！
Most astronomers believe that the large concentration of mass at the galaxy's center is a black hole whose gravity is causing the gas to whirl.
大多数天文学家认为，星系中心的大量物质是一个黑洞，它的引力导致气体旋转。
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。
Hard to predict but the center of mass behaves decently.
很难预测，但质量中心稳定运动。
物体的中心在这里。
And so you can calculate the center of mass.
你可以计算质量中心。
And this, now, is the position vector of the center of mass.
现在，这里是，物体的矢量中心。
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。
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。
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。
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 particular, we have seen the notion of center of mass.
特别地，我们学过质心的概念。
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。
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。
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。
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。
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。
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。
看到质量重心了吧？
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 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。
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。
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。
The center of mass must be somewhere here.
重心肯定是这里某处。
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。
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。
Center of mass below the suspension point.
重心在悬挂点下面。
应用推荐