Hard to predict but the center of mass behaves decently.
很难预测,但质量中心稳定运动。
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。
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。
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。
Okay, I'm going to do the same thing, and now look at that center of mass.
我会做同样的事,现在看质量重心。
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。
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 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?
I'm going to explain to you now what we physicists mean by center of mass of a system.
我现在给你们解释,物理学家说的系统重心。
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。
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。
So now we're going to turn off the lights and I want to show you the motion of the center of mass.
我们要关上灯,我想给你们展示,质量重心的运动。
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。
But the center of mass — - you see it when I balance it on my finger — is way below the suspension point.
而重心-,你们可以看到-,在悬挂点下方。
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?
And where the two lines intersect-- - could be in this opening-- that's where the center of mass is located.
两条线的交点-,可能在这个空间-,这就是重心所在处。
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。
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。
Make v2 zero, and you see exactly that you see here the same result, so this must be the velocity of the center of mass.
使v2为,你显然知道,在这里有同样的结果,它必然是重心速度。
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?
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。
Ten kilograms is ten meters below the rope, so clearly the center of mass of the system as a whole will be below the rope.
绳子下方10米10公斤,所以很明显,重心,将会在绳子下方。
And so the center of mass will just laugh at us and ignore all these motions and will travel at a constant speed very nicely.
重心会嘲笑我们,它忽略所有运动,它会匀速运动。
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。
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。
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。
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。
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