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Physics, at its most basic, is just a description of the motion of the stuff in our universe.
物理本质上是对宇宙中物质运动的描述。
"This planet goes this way, that rocket goes that way" – except that some – in fact, many objects move without moving.
“这颗行星朝这个方向走，那枚火箭朝那个方向走”——除了一些——事实上，许多物体在动，却也没动。
Or, more precisely, they move without going anywhere.
或者更准确地说，它们在原地运动。
I'm talking objects that spin, revolve, rotate, pirouette, orbit, circle, gyrate, whirl, twirl, cartwheel, and so on.
我说的是那些自旋、旋转、转圈、圈圈绕、绕环、环回、回旋漩涡、涡流等等。
Like a planet around a star, an electron in an atom, or even our solar system going around the gravitational center of the milky way:
就像绕着恒星的行星，原子中的电子以及太阳系绕银河系的重心旋转：
from up close they're certainly moving, but in the grand scheme of things, that motion doesn't take them anywhere.
从近处看，它们在移动，但从宏观角度来看，它们的这种运动不会将其带到别处
We can still talk about it, though: just like "momentum" is a concept that describes
但我们仍可以谈论它，正如“动量”这个概念
how much oomph an object has when it moves in a straight line,
描述了物体在直线运动时具有的“冲劲”，
"angular momentum" is a way to account for how much oomph objects have when they're going in circles – figuratively, or literally.
“角动量”描述的是，象征或字面上说，物体在绕圈时具有的“冲劲”。
And angular momentum is simple, in theory: pick a point, any point.
理论上，角动量很简单：选择一个点，任意一个点。
Pretend your object is moving in a circle around that point. Figure out how fast the object is moving along the circle
假装你的物体在此点周围绕圈。先找出物体沿圈运动的速率
(never mind that it probably isn't moving exactly along the circle, and that the circle might have to change size over time to follow the object),
（先别管它可能并没有沿着圈移动，以及这个圈可能需要随着时间改变大小来跟随对象），
but anyway, then multiply that speed times the size of the circle and the object's mass, and there you have it: angular momentum.
然后将这个速率乘以圈的半径以及物体质量，于是你成功得到了角动量。
For example, a 2 kilogram 60 cm-diameter bicycle wheel going 20 km per hour
例如，一个重2公斤，直径60厘米的自行车轮以20km/h速度运动
would have an angular momentum of about 7 kilogram meters squared per second. Is that useful to know?
其角动量约为7千克平方米/秒。这有什么用吗？
The reason we care about angular momentum is that if you take a bunch of objects
我们关注角动量的原因是，如果弄一堆
that are interacting electromagnetically or gravitationally or whatever, and add up all of their angular momenta into one number,
互相发生电磁或引力作用的物体，将所有的角动量加到一起，
then that total value won't change over time (unless some other objects from outside come in and mess things up).
这个总和将不会随时间变化（除非有外部物体进来搅局）。
So as another example, the earth, which is 150 million kilometers from the sun, orbits at 30 km/s and has a mass of 6 times 10·24 kilograms,
举另一个例子，距离太阳1.5亿公里的地球，绕行速率为30千米/秒，其质量为6乘以10的24次方千克，
has an angular momentum of 2.7 times 10·40 kilogram meters squared per second.
其角动量为2.7乘以10的40次方千克·平方米每秒。
That's four thousand quintillion quintillion bicycle wheels!
这相当于4千万亿亿亿亿个自行车轮！
And this angular momentum stays roughly constant over the course of the earth's orbit year in and year out.
这个角动量在地球环绕的过程中几乎保持不变，年复一年、年年如此。
But what's amazing is that even if the sun and the rest of the solar system were to suddenly disappear,
但令人惊奇的是，即使太阳和太阳系的其他部分突然消失，
the earth would STILL have that same angular momentum about the point where the sun WAS.
地球仍然会有和太阳相同的角动量。
Without the sun's gravity, the earth would of course now move in a straight line,
没有了太阳的重力，地球当然会沿直线运动，
requiring an ever-larger imaginary circle as it got farther from the point where the sun used to be.
在其离太阳原先位置越来越远时，它所在的圆也越来越大。
But as the earth continued through space, its 30km/s velocity would also point less and less along the circle,
但随着地球在太空中继续运行，其30公里/秒的速度也会越来越慢，
so when you calculated the angular momentum, the decrease in velocity would exactly cancel out the increase in the size of the circle,
所以当你在计算角动量时，速度的下降正好抵消了圆的大小的增加，
and you'd always get the same answer: 2.7 times 10·40 kilogram meters squared per second.
你总会得到相同的答案：2.7乘以10的40次方千克平方米每秒。
So even when nothing is rotating at all, angular momentum is still conserved.
所以即使没有旋转，角动量仍然守恒。
And that's the beauty of a law of physics – it works even when you try to break it!
这就是物理学定律的魅力所在——即使你试图打破它，它也能起作用！
