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Q & A: billion-person jump

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Most recent answer: 12/22/2009
Q:
I heard once, that if the more than one billion Chinese people jumped at the exact same time down from a table, they would create a force strong enough for the earth to be thrown out of her orbit. Is that true? Or just an exaggeration?
- Robert Haas
New Zealand
A:
The earth would continue on the same path because momentum is conserved. That means that the center of mass of the earth +jumpers system would continue to move just the same way as it would have moved. (Here I'm neglecting some very tiny changes in tidal forces due to the efective temporary change of shape of the earth.) 

(published on 10/22/2007)

Follow-Up #1: gravitational philosophy

Q:
I've heard that gravity occurs only if there is more than one odject exsisting. For example, if there was a ball siting in a room that has nothing or no other "mass" in it, then there would be no gravity. I've also heard that if there's gravity occuring, than both odjects react with the gravity. We could feel gravity on our daily lives, like jumping. That proves that we have mass, right? Because we have mass, then is it true that I am pulling the earth upwards also? (i no it's a little confusing) thanks, bowen
- bowen (age 13)
Altanta, georgia usa
A:
Let me answer your last question first.  You do indeed have mass. Definitely you pull on the earth just as it pulls on you.

As for whether gravity exists with just one object, let's ask what an object is. Maybe for this purpose a ball shouldn't be viewed as a single object. The insides of the ball pull the outsides in a bit via gravity. If the ball is as big as the earth, that's important!

Mike W.

(published on 12/22/2009)

Follow-up on this answer.