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Traveling with the Earth

  1. Sep 25, 2009 #1


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    So after swim practice I was waiting for my ride and pondering the universe. I was thinking about the Earths orbit around the sun and rotation around its axis. I then came across a very interesting question...

    If I were to simply jump straight up into the air, why doesn't the Earth move right under my feet? Assuming that there is no wind and stuff like that, why do I land in the exact same place I jumped from? Wouldn't the Earth just move really fast under me until I hit the ground?

    Hope my question made sense :)
  2. jcsd
  3. Sep 25, 2009 #2


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    It's the same reason you can jump on an airplane and not hit the back wall. You share the Earth's velocity. At the equator, Earth rotates at about 1000 mph. If you stood on the equator, you also share this velocity, so when you jump, the Earth does move, but so do you, so you land in the same place.

    At higher latitudes, you actually don't land on the same spot. After jumping, while in the air, you have a great circle trajectory, while the ground does not. But the amount of deflection for a human jump is negligible. Missiles and other objects on ballistic trajectories need to take this into account.
  4. Sep 25, 2009 #3
    Let's change the experiment a little to make the understanding easier. Imagine you're on a train running straight ahead at an even speed. If you jump, why doesn't the train move forward under your feet?

  5. Sep 25, 2009 #4
  6. Sep 25, 2009 #5
    If you jump straight upward on a moving train that has a constant velocity, in your frame of reference you go straight up and down. In the frame of reference of someone standing on the ground and watching you, your path was a parabola.

    Your parabolic path has a constant horizontal component of velocity that is the same as the constant velocity of the train.
  7. Sep 26, 2009 #6
    The Coriolis force if you jump up is actually proportial to cos(latitude), greatest at the equator and 0 at the poles. [itex] 2 \Omega \times v [/itex] has a magnitude of [itex] 2 \pi v cos(latitude) / T [/itex]
  8. Sep 26, 2009 #7


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    Try and think of what would happen if just after you jumped the train locked up its breaks.

  9. Sep 26, 2009 #8


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    Well what would happen if the Earth were to suddenly decelerate or accelerate? Would we feel the effects immediately?
  10. Sep 26, 2009 #9

    Doc Al

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    Staff: Mentor

    If you were on a train that suddenly accelerated, would you feel it? Same thing.
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