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WEIGHTLESS ARROW
无重力射箭
Q. Assuming a zero-gravity environment with an atmosphere identical to Earth's, how long would it take the friction of air to stop an arrow fired from a bow? Would it eventually come to a standstill and hover in midair?
Q．假设有一个地方的大气条件和地球上的完全一致，但那里没有重力，如果我在那儿射出一支箭，那么这支箭要多久才会由于空气阻力而停下来？到最后这支箭会完全停下来并悬浮在半空中吗？
——Mark Estano
——马克•埃斯塔诺
A. IT'S HAPPENED TO ALL of us. You're in the belly of a vast space station and you're trying to shoot someone with a bow and arrow.
A．我们每个人都有这样的经历。你现在正在一个巨大的空间站表面，准备用弓和箭朝某人射击。
Compared to a normal physics problem, this scenario is backward. Usually, you consider gravity and neglect air resistance, not the other way around.1
和普通的物理问题相比，这次我们的思路要反一反。通常来说你会考虑重力而忽略空气阻力，但这次我们要忽略重力而考虑空气阻力。1
As you'd expect, air resistance would slow down an arrow, and eventually it would stop . . . after flying very, very far. Fortunately, for most of that flight, it wouldn't be much of a danger to anyone.
就像你预期的那样，空气阻力会使箭支减速，最终在飞行了很远很远的距离后……箭支会停下来。还好在大部分飞行时间内，这支箭都不会对旁人造成太大的威胁。
Let's go over what would happen in more detail.
让我们仔细看一下整个过程。
Say you fire the arrow at 85 meters per second. That's about twice the speed of a major-league fastball, and a little below the 100 m/s speed of arrows from high-end compound bows.
假设你以每秒85米的速度射出一支箭，这个速度差不多是棒球大联盟飞得最快的快球速度的2倍，和高端复合弓射出的每秒100米的箭相比还差一点。
The arrow would slow down quickly. Air resistance is proportional to speed squared, which means that when it's going fast, the arrow would experience a lot of drag.
射出的箭的速度很快就会降低，因为空气阻力正比于速度的平方，这意味着箭支在高速飞行时会受到相当大的阻力。
After ten seconds of flight, the arrow would have traveled 400 meters, and its speed would have dropped from 85 m/s to 25 m/s; 25 m/s is about how fast a normal person could throw an arrow.
射出10秒后，箭支飞过了400米的距离，速度从起初的每秒85米降到了每秒25米，差不多相当于一个普通的人用手扔出的箭的速度。
At that speed, the arrow would be a lot less dangerous.
在这个速度下，飞行的箭支危险性大大下降。
We know from hunters that small differences in arrow speed make big differences in the size of the animal it can kill. A 25-gram arrow moving at 100 m/s could be used to hunt elk and black bears. At 70 m/s, it might be too slow to kill a deer. Or, in our case, a space deer.
从猎人那儿我们知道，箭支速度的微小差异会极大影响到被射杀的猎物的体型。一支以每秒100米速度飞行的25克重的箭能够猎杀马鹿或者黑熊，而速度每秒70米的箭支可能都无法杀死一只鹿，或者，在我们的场景里，太空鹿。
