Would running change Earth's rotation?

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Sam7791
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Would running change Earth's rotation?

Postby Sam7791 » Fri Nov 12, 2010 2:15 am UTC

If everyone in the entire world ran in one directions (by cars or just running), could you change the speed the earth rotates? Because you think if you run on a big ball, the ball moves and spins faster. But I don't know how gravitational pulls would factor into that.

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Sir_Elderberry
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Re: Would running change Earth's rotation?

Postby Sir_Elderberry » Fri Nov 12, 2010 4:52 am UTC

Conservation of angular momentum says yes, I suppose. If you're running in one direction and you accelerate by pushing against the Earth, the Earth logically has to gain some momentum in the opposite direction. Obviously, it's not going to be notable.
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Re: Would running change Earth's rotation?

Postby ++$_ » Fri Nov 12, 2010 6:05 am UTC

When everyone stops running, the earth will go back to its previous speed, of course. So the impact would be limited in duration as well as in magnitude.

Sagekilla
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Re: Would running change Earth's rotation?

Postby Sagekilla » Fri Nov 12, 2010 6:13 am UTC

This is just like the whole "If everyone stood in the same place and jumped, could we move the earth?"
Well yes, by conservation of energy and momentum it would work. Is it practical? Hell no.

Here's a similar example: If I take a neutron source and pointed it at your face, with a couple billion
neutrons emitted every second, do you think I could push you away from me?

Hell no. Just as an order of magnitude calculation, the force exerted would be on the order of 10^-12 or so.
But it would probably give you a bad case of radiation poisoning.

Replace neutrons with humans, and your face with the earth. I think the situation there is even worse.
The ratio of mass of earth / mass of human is roughly in the same ballpark as mass of human / mass of neutron.

But with the neutron source, they're probably moving at some significant fraction of c. Humans don't.
So the change in any kind of momentum by humans is going to be stupidly small. And neutrons too.
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letterX
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Re: Would running change Earth's rotation?

Postby letterX » Fri Nov 12, 2010 4:24 pm UTC

Sagekilla wrote:This is just like the whole "If everyone stood in the same place and jumped, could we move the earth?"
Well yes, by conservation of energy and momentum it would work. Is it practical? Hell no.

Here's a similar example: If I take a neutron source and pointed it at your face, with a couple billion
neutrons emitted every second, do you think I could push you away from me?

Hell no. Just as an order of magnitude calculation, the force exerted would be on the order of 10^-12 or so.
But it would probably give you a bad case of radiation poisoning.

Replace neutrons with humans, and your face with the earth. I think the situation there is even worse.
The ratio of mass of earth / mass of human is roughly in the same ballpark as mass of human / mass of neutron.

But with the neutron source, they're probably moving at some significant fraction of c. Humans don't.
So the change in any kind of momentum by humans is going to be stupidly small. And neutrons too.


I love that the radiation poisoning part of this analogy still applies...

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Tass
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Re: Would running change Earth's rotation?

Postby Tass » Fri Nov 12, 2010 4:30 pm UTC

letterX wrote:I love that the radiation poisoning part of this analogy still applies...


Really? I hate it.

letterX
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Re: Would running change Earth's rotation?

Postby letterX » Fri Nov 12, 2010 4:33 pm UTC

Tass wrote:
letterX wrote:I love that the radiation poisoning part of this analogy still applies...


Really? I hate it.


Yes. That too.


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