Everyone in the world jumping in place at one time

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Discussion Overview

The discussion revolves around the hypothetical scenario of everyone in the world jumping in place simultaneously and its effects on the Earth. Participants explore concepts related to closed systems, center of mass, and the implications of such an event on Earth's motion.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the Earth would experience any extra motion if everyone jumped at once, referencing a physics teacher's explanation about closed systems.
  • Another participant suggests that the Earth would not be affected in its orbit due to the center of mass of the Earth and the jumpers not being acted on by an external force, though temporary displacement might occur during the jump.
  • A participant mentions a previous calculation indicating that the expected displacement of the Earth would be negligible, off by about 10 orders of magnitude.
  • One participant seeks clarification on the concept of center of mass, proposing a thought experiment involving jumping in Manhattan and questioning whether the center of mass would remain unchanged post-jump.
  • A later reply affirms the analogy of trying to fly by pulling up on shoelaces, reinforcing the understanding of the center of mass concept.

Areas of Agreement / Disagreement

Participants generally agree on the concept that the center of mass would not experience displacement, but there are varying interpretations of the implications of jumping and the nature of a closed system. The discussion remains unresolved regarding the exact effects on Earth's motion.

Contextual Notes

Limitations include assumptions about the closed system and the effects of gravity, as well as the lack of detailed mathematical analysis regarding the displacement during the jump.

spizma
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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.
 
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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.
 
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.
 
Yep, the Earth is pretty heavy.
 
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!
 
phlegmy said:
i think i get it, like trying to fly by pulling up on you laces!

Exactly! That's a great analogy.
 

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