【英语时差8,16】影响球体着地因素（在线收听）

Pop quiz: you have a flat table and two billiard balls. You and your strong-armed friend each roll one of the balls off the table so they pass the edge at the same time, only his ball is traveling much faster than yours. Which one hits the ground first?

It’s one of the perennial stumpers of physics class that, provided they both pass over the edge simultaneously, both balls will land at exactly the same time. In fact, no matter HOW fast the “fast” ball is moving, it will still fall at the same speed as the ball that just meanders over the edge. Yes, even if the “fast” ball is fired out of a cannon, this will still hold true. The reason is that horizontal motion in no way affects vertical motion: gravity acts on both the fast ball and the slow ball with the same force at all times. So while the fast ball lands much farther away, it doesn’t get to spend any more time in the air than the slow ball — or, for that matter, than any other ball moving at any speed in between.

In fact, the only thing that WILL make a difference in the landing times of the two balls is the curvature of the earth. If the fast ball moves SO fast that it’s able to get past the point where the ground starts to drop away significantly underneath it, it then will indeed spend a longer time falling, but only because it will now have a greater distance to fall. And if you can generate the kind of energy required to produce this effect, you have just succeeded in putting your fast ball into orbit.