Can you speed up Earth's rotation by spinning?

  • #1
Hey,
Got into a discussion with my friends over this:
If you spin in your chair clockwise, since the earth spins counterclockwise, as you start spinning by conservation of momentum the Earth would also start speeding up. Then, frictional dissipative forces from the air would slow you down to a stop, but since the air is fairly uncoupled to the Earth, this angular momentum probably wouldn't find it's way back to Earth (or at least not completely) Thus you could (technically) speed up the Earth by spinning. Similarly, if you started spinning counterclockwise, you could slow the Earth.

I know that in an ideal situation (i.e. no frictional forces, no air, just you and Earth), it's impossible for a situation to exist where you are not spinning anymore but the Earth is spinning faster/slower, but with friction, is the above example true?

Thanks,
Ari
 

Answers and Replies

  • #2
Nugatory
Mentor
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Hey,
but since the air is fairly uncoupled to the Earth, this angular momentum probably wouldn't find it's way back to Earth (or at least not completely)

On the contrary, the air is VERY coupled to the earth - and a good thing too, or easterly winds would make life rather exciting across most of the earth's surface. Sure, you can make infinitesimal changes in the earth's rotation by rotating objects on the earth's surface (massive disks at the poles would be easier to analyze than your chair), but the effect will disappear very rapidly if you stop applying torque to the rotating object.
 
  • #3
HallsofIvy
Science Advisor
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Hey,
Got into a discussion with my friends over this:
If you spin in your chair clockwise, since the earth spins counterclockwise, as you start spinning by conservation of momentum the Earth would also start speeding up.
You say "clockwise" and "counterclockwise" but are spinning on an axis perpendicular to the axis of rotation of the earth so those do not apply. I do not see how spinning on an axis perpendicular to the axis of rotation of the earth will change the rotational spin of the earth. Of course, if you run westward, you will increase the earths rotation and if you run eastward, you will slow it.

Then, frictional dissipative forces from the air would slow you down to a stop, but since the air is fairly uncoupled to the Earth, this angular momentum probably wouldn't find it's way back to Earth (or at least not completely) Thus you could (technically) speed up the Earth by spinning. Similarly, if you started spinning counterclockwise, you could slow the Earth.

I know that in an ideal situation (i.e. no frictional forces, no air, just you and Earth), it's impossible for a situation to exist where you are not spinning anymore but the Earth is spinning faster/slower, but with friction, is the above example true?

Thanks,
Ari
 
  • #4
@ HallsofIvy - they still can be related. Picture yourself slowly increasing the speed of your spinning chair by pushing off from the ground at the exact same spot once per revolution. That would be analogous to running I think.
 
Last edited:
  • #5
35,806
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You say "clockwise" and "counterclockwise" but are spinning on an axis perpendicular to the axis of rotation of the earth so those do not apply. I do not see how spinning on an axis perpendicular to the axis of rotation of the earth will change the rotational spin of the earth. Of course, if you run westward, you will increase the earths rotation and if you run eastward, you will slow it.
Unless you are at the equator (or tilt your chair in a strange way), spinning in a chair is not perpendicular to the axis of rotation of earth.

Yes you can change the speed of rotation by a tiny amount if you spin in your chair. You increase the angular momentum, as you get an angular momentum in the opposite direction and total angular momentum is conserved (neglecting moon, sun and so on).

Running westwards is way more effective (unless you are at the poles).

Related comic
 
  • #7
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If earth were perfectly rigid, then yes, in principle, you spinning on a chair would cause the earth to spin in the opposite direction to conserve momentum. But the earth is so huge compared to you that the change in spin the earth would experience is so small it would just get overpowered by other effects, like fluctuations in the solar wind. So you could never measure that spin difference.

In reality, the earth is not perfectly rigid. You spinning on your chair just causes a small, localized area of ground under you to spin slightly in the opposite direction. Because the surface of the earth is solid, but not perfectly rigid, this slight twist you exert on your little patch of ground quickly turns into tiny vibrations that spread out in all directions (like tiny seismic waves) and get weaker as they go. The ground vibrations from you twisting, jumping, exploding your garage, etc. die out and get swamped by natural vibrations long before they reach the other side of the world.
 
  • #8
A.T.
Science Advisor
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In reality, the earth is not perfectly rigid. You spinning on your chair just causes a small, localized area of ground under you to spin slightly in the opposite direction. Because the surface of the earth is solid, but not perfectly rigid, this slight twist you exert on your little patch of ground quickly turns into tiny vibrations that spread out in all directions (like tiny seismic waves) and get weaker as they go. The ground vibrations from you twisting, jumping, exploding your garage, etc. die out and get swamped by natural vibrations long before they reach the other side of the world.
Angular momentum doesn't die out. It is conserved.
 

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