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Thread: Effect of gravity on suspended/hovering objects

  1. #1 Effect of gravity on suspended/hovering objects 
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    We know that the earth rotates around its own axis once in 24 hours. So hypothetically, if I had a jet-pack that can keep me hovering, kind of suspended in the same spot, just a few feet off the ground, for many hours at a time, I am inclined to think (and I know I am wrong) that....
    (a) The earth would continue to rotate below me, just like how the moon technically 'hovers' and rotates above the earth while the earth rotates... and,
    (b) After about 12 hours of hovering, I would find myself hovering above a country that is diametrically on the other side of the earth, from where I started...

    I know I am missing something here... could someone please explain what it is?


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  3. #2  
    Your Mama! GiantEvil's Avatar
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    There's Newton's first law, inertia. Once an object is set in motion it tends to maintain that motion. After you leave the Earths surface you would still be carrying the momentum imparted by it's spin. Then there's atmosphere, while the atmosphere would dissipate the momentum you carried from the surface the atmosphere tends to be dragged around with the Earths spin and has it's own momentum that it would impart to you.


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  4. #3  
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    Quote Originally Posted by GiantEvil
    There's Newton's first law, inertia. Once an object is set in motion it tends to maintain that motion. After you leave the Earths surface you would still be carrying the momentum imparted by it's spin. Then there's atmosphere, while the atmosphere would dissipate the momentum you carried from the surface the atmosphere tends to be dragged around with the Earths spin and has it's own momentum that it would impart to you.
    I understand the first part of your explanation that the rotational momentum provided by the earth's spin would keep me moving with the same speed and in the same direction as the earth's rotation, even though I feel I am only hovering in one spot.
    However, the 2nd part of your explanation where you mention that the atmosphere can impart its own momentum... wouldn't that counter the first part of your explanation? If there was a storm with 60mph winds (measured relative to earth's rotational speed), and in the same direction as earth's rotation, would I be hovering and moving along with the storm, faster than earth's spin?
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  5. #4 Re: Effect of gravity on suspended/hovering objects 
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    Quote Originally Posted by drunkenmonk
    We know that the earth rotates around its own axis once in 24 hours. So hypothetically, if I had a jet-pack that can keep me hovering, kind of suspended in the same spot, just a few feet off the ground, for many hours at a time, I am inclined to think (and I know I am wrong) that....
    (a) The earth would continue to rotate below me, just like how the moon technically 'hovers' and rotates above the earth while the earth rotates... and,
    (b) After about 12 hours of hovering, I would find myself hovering above a country that is diametrically on the other side of the earth, from where I started...

    I know I am missing something here... could someone please explain what it is?
    This is completely dependent on what you mean by "hoveering". With respect to what reference frame ?
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  6. #5  
    Your Mama! GiantEvil's Avatar
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    The following is just a guess, so somebody correct me if I'm wrong.
    Let's for a moment dispense with the atmosphere to simplify the problem. Whatever vector of motion imparted to you(hovering) will be tangential to the curve of the Earths surface. If you hover at the equator then you will continue to hover geosynchronously. If however your point of departure is not the equator then you will drift towards the equator, albeit slowly.
    http://en.wikipedia.org/wiki/Focault_pendulum

    Once we add atmosphere and atmospheric turbulence into the problem, the wind could blow you anywhere.

    Attempting to answer DrRocket's question concerning "reference frame" will certainly sharpen your science skill's.
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