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Why do we rotate along with the earth's rotation?

  1. Sep 25, 2007 #1
    I've been having a huge debate about this, some people are saying "air resistance" and that just makes zero sense to me, because the moon has no atmosphere and Neil Armstrong rotated along with the moon with no problem whatsoever.

    I am thinking it has to have something to do with the properties of a gravity field around a spinning object. Does anyone know?
  2. jcsd
  3. Sep 25, 2007 #2

    D H

    Staff: Mentor

    The answer is friction. Neither the Earth nor the Moon is coated with teflon.
  4. Sep 25, 2007 #3
    But the whole of the earth's atmosphere rotates along with the earth, why?
  5. Sep 25, 2007 #4

    D H

    Staff: Mentor

    Friction again. Suppose the atmosphere was not rotating at all. The word hurricane doesn't begin the describe what the resulting 1000 mile/hour wind would do. The Earth would quickly spin the lower atmosphere up the Earth rotation rate. The lower atmosphere in turn would spin the upper atmosphere up the Earth rotation rate.
  6. Sep 25, 2007 #5
    Why would the earth spin the lower atmosphere? there is no friction between them.
  7. Sep 25, 2007 #6
    Gases have friction between them.
  8. Sep 25, 2007 #7
    is it really enough friction to consistently keep the whole of the atmosphere rotating at 1000mph? that sounds really odd.
  9. Sep 25, 2007 #8

    Doc Al

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    Staff: Mentor

    Once the atmosphere (or people!) is rotating along with the earth, friction is not needed to keep it moving with the earth. (You do need gravity, lest we fly off at a tangent.)

    When you accelerate in your car, what keeps you going with the car? The car exerts a force on you, of course. Once you have reached your cruising speed, no force is needed to maintain your speed and keep you moving with the car. (Assuming you maintain a constant direction as well as speed.)
  10. Sep 25, 2007 #9


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    In Arthur Clarke's "Fountain's of Paradise", he doesn't have the "space elevator" nearly long enough. He seems to think that you could put the upper end at a satellite in geo-syncronous orbit. In geosynchronous orbit, the centrifugal and gravitational forces of an object orbiting once a day are balanced.

    Of course, the gravitational forces on the lines below that are enormous and would pull the thing down. You have to go far enough above geosynchronous orbit so that the net upward forces would over come that.
  11. Sep 25, 2007 #10
    The Gravity of the Situation

    Yes, gravity holds you on. You can do an experiment by setting a cup of very expensive latte on a childs rotating ride at a park. Spin it around and see what happens when there is no gravity.
  12. Sep 25, 2007 #11
    I think it's the same thing that lets you toss a ball up and down on a moving vehicle, with out the ball streaking to the back. I don't know what that would be called, though.
  13. Sep 25, 2007 #12

    Doc Al

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    Call it inertia.
  14. Sep 25, 2007 #13
    Ok I think I've got it pretty much, it's all due to friction and inertia. That brings me to my next question, which is actually where this debate started.

    I'm sure many of you are aware that plane journeys from East to West take less time than the same journey from West to East. The classical explanation is that's just the way the wind blows, and of course that is a major factor, but I was wondering if there is actually some effect of breaking free of the earth's rotation. This would be due to the following:
    1. Obviously, not touching the ground anymore.
    2. Minimized air resistance due to aerodynamic build, so we can pretty much ignore the atmosphere for a qualitative question (i.e. is there an effect AT ALL?)
    3. Again obviously, accelerative force generated by the jet engines.

    The plane has inertia as soon as it takes off, but as it begins to push air back with its engines it gradually overcomes the inertia. Due to the massive air drag it actually experiences in the practical situation, it can only manage this to some extent. Does this make any sense?
  15. Sep 25, 2007 #14


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    By the way - when Neil Armstrong landed on the moon, he had to match the speed of the spacecraft to the rotation rate of the moon (very slow). But once on the moon, Newton's first law keeps you moving with it and the acceleration from the rotation is perpendicular to the rotation direction.

    No, the jet stream is entirely responsible for the difference in flight times from east to west.
  16. Sep 25, 2007 #15
    That's very interesting indeed, a good way to convincingly prove the point (however obvious it may be to some).

    Ok, but why? considering the things I mentioned above... I mean you sort of need to address the question of how the inertia is maintained if the friction is much lower and there IS a net force acting against it.

    Surely the fact that the atmosphere is much thinner at that altitude means something with regards to this issue!
  17. Sep 25, 2007 #16
    Wikipedia jet stream. Google coriolis force. Draw diagrams and figure out for yourself what would happen if we made the poles warmer than the equator.
  18. Sep 25, 2007 #17
    (Wait a sec, my diagram didn't work out.... ah..)

    Haven't you got that completely backwards?
  19. Sep 25, 2007 #18

    Are you implying i could have got it somewhat backwards? :P
    Last edited: Sep 25, 2007
  20. Sep 25, 2007 #19
    Ok never mind I worked it out with some help from someone. Basically since the airplane just pushes air back in order to propel itself forward, the inertia is always taken into account and never disappears, so anything that happens in the frame of reference of the atmosphere that actually depends on the atmosphere will maintain its inertia as normal.

    However, I would assume that a space shuttle would not, if it was using, say, its ion thruster within the earth's atmosphere. Not that it would, just theoretically speaking, since its propulsion would not depend on the atmosphere, it would escape its inertia.
  21. Sep 25, 2007 #20
    "Nope." No reasoning nor explanation? I've no patience for people who are just obstinately wrong.

    EDIT: Ah, I see you've edited your statement. "with help from someone" indeed.
    Alas, saying "escape its inertia" does not indicate that you have understood the previous posts here.
    Last edited: Sep 25, 2007
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