Thus large translation of atoms mass center motion is realized.
从而可实现较大的原子质心动量交换。
It will remind you of center of mass and all that.
它将来会让你想起质心等等。
And so you can calculate the center of mass.
你可以计算质量中心。
In particular, we have seen the notion of center of mass.
特别地，我们学过质心的概念。
物体的中心在这里。
And this, now, is the position vector of the center of mass.
现在，这里是，物体的矢量中心。
If I look at the definition of the center of mass, it's the average value of x.
由质心的定义可知，它就是x的平均值。
The center of mass is basically a point that you would say is right in the middle of the object.
理论上，重心就是,恰好处在物体中心的点。
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。
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 now comes another unique property of the center of mass.
它还有一个特性，质量重心的特性。
And let the center of mass be, for instance, here.
让物体的中心，比方说，在这里。
看到质量重心了吧？
But one that you might have seen in physics, we care about maybe where is the center of mass of a given object?
但你们可能也在物理学中见过了，就是求一个给定物体的重心？
So the knife is now in this pencil, but now I have to get that center of mass under under the pencil.
现在刀在铅笔里面，但我必须要把，重心移到，铅笔下面。
The center of mass must be somewhere here.
重心肯定是这里某处。
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。
How do we calculate the velocity of the center of mass?
我们怎样计算，重心速度呢？
Where is the center of mass of this system?
这系统的重心何在？
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。
For the pendulum, center of mass is here.
对摆来说，重心在这里。
It will arrange itself so that the center of mass will fall below my finger, and it will be completely happy.
它会自我调节，然后重心会，低于我的手指，它会很高兴。
You're not moving, you're in the center of mass.
你没有移动，你处于质量重心。
The center of mass is lower, around half of the machine height.
该中心的质量较低，大约一半的机器的高度。
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。
So this is the velocity of the center of mass.
这就是重心速度。
For this to be effective, it is necessary to keep the center of mass in normal flight directly below the center of pressure.
对，这是有效的，有必要保持中心的大规模正常飞行直接低于中心的压力。
For this to be effective, it is necessary to keep the center of mass in normal flight directly below the center of pressure.
对，这是有效的，有必要保持中心的大规模正常飞行直接低于中心的压力。
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