Physics,at its most basic,is just a description of the motion of the stuff in our universe."This planet goes this way,that rocket goes that way",except that some-in fact,may-objects move without moving.Or,more precisely,they move without going anywhere.
物理嘛 究其本质 就是描述宇宙内所有事物运动，这颗行星这边走 那个火箭那边跑，除了某些———其实是好多———物体在动 却也没动，更准确地说 它们在原地运动.
I'm talking objects that spin,revolve,ratate,pirouette,orbit,circle,gyrate,whirl,twirl,cartwheel,and so on.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:from up close they're certainly moving,but in the grand scheme of things,that motion doesn't take them anywhere.
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 就像绕着恒星 原子中的电子，以及太阳系绕银河系的重心旋转，近距离看 它们在移动 但从宏观角度而言，这种运动不会将其带到别处.
We can still talk about it,thought:just like"momentum"is a concept that describes how much comph an object has when it moves in a straight line,"angular momentum"is a way to account for how much oomph objects have when they're going in circles,figuratively or literally.
不过 这也有的可说 正如“动量”这个概念，描述了物体在直线运动时具有的“冲动” “角动量”描述的是，象征或字面上说 物体在绕圈时具有的“冲动” .
And angular momentum is simple,in theory:pick a point,any point.Pretend your object is moving in a circle around that point.Figure out how fast the object is moving along the circle(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)but anyway,then multiply that speed times the size of the circle and the object's mass,and there you have it:angular momentum.
理论上看 角动量很简单 选任意一个点，假装你是一个在此点周围绕圈的物体，先找出物体沿圈运动的速率（即圆切线方向速度），别在乎它有没有完全绕着圈运动，还有 圈的大小也会随着物体位置依时间而变，然后将这个速率乘以圈的半径以及物体质量，于是你成功获得 角动量.
For example,a 2 kilogram 60 cm-diameter bicycle wheel going 20 km per hour,would have an angular momentum of about 7 kilogram meters squared per second.Is that useful to know? The reason we care about angular momentum is that if you take a bunch of objects that are interacting electromagnetically or gravitationally or whatever,and add up all of their angular momenta into one number,then that total value won't change over time(unless some other objects from outside come in and mess things up).
比如 2公斤重 直径60厘米的自行车轮以20km/h速度运动，其角动量约为7千克.米`2/秒，这有啥用吗？我们在乎动量是因为 如果弄一堆互相发生电磁或引力作用的物体，将其有的角动量加到一起，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*10^24 kilograms,has an angular momentum of 2.7*10^40 kilogram meters squared per second.That's four thousand quintillion quintillion bicycle wheels!
再举一个例子 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*10`24千克其角动量为2.7*10`40千克.米`2秒 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.But what's amazing is that even if the sun and the rest of the solar system were to suddenly disappear,the earth would STILL have that some angular momentum about the point where the sun WAS.
这个角动量在地环球绕的过程中几乎保持不变，年复一年 年年如此 更惊人的是 即使太阳和太阳系其余部分，突然消失 地球相对太阳原先位置的角动量，仍然不变.
Without the sun's gravity,the earth would of course now move in a straight line,requiring an ever-larger imaginary circle as it got farther from the point where the sun used to be.But as the earth continued through space,its 30km/s velocity would also point less and less along the circle.
so when you calculated the angular momentum,the decrease in velocity would exactly cancel out the increase in the size of the circle,and you'd always get the same answer.2.7*10^40 kilogram meters squared per second.So even when nothing is rotating at all,angular momentum is still conserved.And that's the beauty of a law of physics:-it works even when you try to break it!
因此在计算角动量时，切向速率的减小会抵消圆半径增加的影响，你仍将得到同样的答案 2.7*10`40千克.米2/秒，即便什么都没在旋转 角动量仍守恒，这就是物理之美，一个死都不变的小婊砸。