英语听力文摘 English Digest 706 地球的引力改变过么？（在线收听）

Has Earth’s Gravitational Pull Been Changing?

Don, why are you slouching? Don’t you know it’s bad for you?

Did you know that about 200 tons of debris strike the Earth every day? So I was thinking! Although most of this stuff burns up in the atmosphere, the resulting dust still settles and adds to the Earth’s mass. That’s about half a million tons per year!

All right, but what does all this have to do with your posture?

Sit Up Straight

So if the earth keeps accumulating mass, doesn’t that mean that the Earth’s gravitational pull has been increasing? After all, gravity is directly proportional to mass.

Are you saying you can’t sit up straight because gravity is stronger now than it used to be?

Admit it, Ya?l–it’s a good one.

Earth’s Mass

Earth’s mass in metric tons is approximately six followed by twenty-one zeroes. So assuming that your estimate is correct, and the Earth accumulates a half a million tons of debris every year, this means that in the sixty million years since dinosaurs existed the Earth would have accumulated approximately thirty trillion tons of meteoric debris. That’s thirteen zeroes.

The point is that a number with thirteen zeroes is minuscule relative to a number with twenty-four zeroes. While thirty trillion tons sounds impressive, it is nothing compared to the Earth’s total mass. So it’ s definitely not all that extra gravity that’s making you slouch