Everyone in the world jumping in place at one time

[spizma]

I heard from a person that his physics teacher told him that if every person were to gather in one area on the Earth and jump at once the Earth would not get any extra motion. He said the reason for this was that the Earth is a closed system. Is this right? If it is could someone please explain what a closed system is exactly and why that means that the Earth wouldn't move.

 

[Normouse]

He probably meant that the Earth would not be effected in its orbit. Perhaps he meant vis-a-vis closed system that the center of mass of the Earth plus all the people that would be jumping is not acted on by an external force so that center of mass would not change, in the sense that it would continue going in the same direction at the same speed. That does not mean however that the Earth would not be moved from it's speed and direction temporarily until all the people landed again. Then it would continue as it was before they all jumped. Of course, part of the basic element of their being in a closed system also is gravity. That's what would pull the Earth and people back toward their center of mass.

 

[Nabeshin]

Like normouse said, the center of mass would experience no displacement. But when most people think of this (let's get everyone in the world on Manhattan and all jump!) I think they expect the Earth to fall away under them. We did a ballpark calculation in a physics class once, and it's nowhere close to even a meter of difference. Off by something like 10 orders of magnitude.

 

[Normouse]

Yep, the Earth is pretty heavy.

 

[phlegmy]

soo?
all the ppl are in manhattan
the center of mass in in location [A]
everyone jumps, time stops, the center of mass is still in location [A].?
time starts, everyone lands, the center of mass is still in location [A].?

am i with you so far??

so then after the big experiment everyone heads off home.
now the center of mass is still in location [A]?

i think i get it, like trying to fly by pulling up on you laces!

 

[Nabeshin]

i think i get it, like trying to fly by pulling up on you laces!

Exactly! That's a great analogy.