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Earth's Rotation and Frames of Reference

  1. Jul 6, 2013 #1
    I have a question about Earth's rotation around its own axis. Earth rotates at around 1000 miles per hour. However, if I stand in an open field and let a balloon float beside me in mid-air, I wouldn't expect the balloon to fall away (or appear to fly away) at that speed when I let go of it. I understand that if I jump up or throw something vertically, I give myself or the object it an initial velocity with a vector that is in the same direction as the rotation of the Earth. But what happens in the case of the balloon, an object which appears stationary in my frame of reference.

    Looking forward to hearing your clarifications.
     
  2. jcsd
  3. Jul 6, 2013 #2

    D H

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    When you let go of the balloon it doesn't suddenly lose whatever momentum it had. From the perspective of an Earth-centered inertial frame, the balloon was moving at about 1000 mph prior to when you let it go, so it will continue to move at 1000 mph just after you let it go. An external force is needed to change that velocity.

    The external forces that act on the balloon are buoyancy and drag. Buoyancy makes the balloon go upwards. Drag tends to make the balloon move at horizontally with the air, which is more or less rotating with the Earth.
     
  4. Jul 6, 2013 #3

    mathman

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    The earth's rotation is in angles/day (approx 361°/day). Speed (mph) is latitude dependent.
     
  5. Jul 6, 2013 #4

    CWatters

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    Actually no. It had that that initial velocity before you threw it.
     
  6. Jul 7, 2013 #5
    What causes air to rotate with the Earth?
     
  7. Jul 7, 2013 #6

    CWatters

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    What would cause it not to rotate with the earth. Ditto the oceans.

    The earth (including the oceans and atmosphere) was spinning when/as it was formed and there is nothing really to stop it spinning.

    Having said that... The moon and the tides are causing it to slow down gradually, Wikipedia says a day is now 1.7 milliseconds longer than it was a century ago.
     
    Last edited: Jul 7, 2013
  8. Jul 7, 2013 #7

    A.T.

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    There is also friction with the surface, that accelerates/deaccelerates the air that moves North<->South, so it has to change it's tangential velocity.
     
  9. Jul 7, 2013 #8
    Gravity! causes the air to rotate with the Earth, it also attracts the oceans and everything upon it and above it.
     
  10. Jul 7, 2013 #9

    D H

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    Gravity does keep the atmosphere from escaping, but it does not cause it to more or less rotate with the Earth. Use your gravity model to explain the very different behavior of Venus' atmosphere.

    The simple answer to the question is friction, not gravity.
     
  11. Jul 7, 2013 #10

    CWatters

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    It's interesting that the jet streams rotate west to east. eg faster than the earth.

    Jetstreamconfig.jpg

    Presumably (on average) the atmosphere rotates at about the same speed as the planet so there must be a similar air mass going east to west around the equator? I assume that's what this is telling us..

    Earth_Global_Circulation.jpg
     
  12. Jul 7, 2013 #11

    D H

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    The Earth’s atmosphere rotates faster than the underlying planet by about 10 m/s on average. See http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/25447/1/96-0861.pdf [Broken]. That's why I said "more or less".

    With Venus, it's not more or less. It's mostly more. Portions of Venus' atmosphere superrotate by a huge amount, 35 times the planet's rotation rate.
     
    Last edited by a moderator: May 6, 2017
  13. Jul 7, 2013 #12
    So there is no friction on Venus?
    Gravity keeps the atmosphere from escaping on Earth and enables friction to act.
    If gravity was less on Earth would it still rotate with planet and retain it's atmosphere in the same way.
    Gravity on Venus is similar to Earth, but Venus is a lot warmer so what happens to friction does it disapear or does heat and pressure play a larger part than gravity and friction?
    There is no simple answer other than a combination of forces but as you mentioned there would be no atmosphere to rotate if it were not for gravity.
     
  14. Jul 8, 2013 #13

    A.T.

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    Sure there is, that's why the winds on the Venus' surface are not greater than here on Earth. It's only the upper Venus atmosphere that circulates very fast. That's mainly because the Venus spins very slowly so there is lots of time to build up a large temperature difference and a steady circulation between the day and night sides.

    There would be no Earth if it were not for gravity. So you can answer any question regarding the Earth with: "Because of gravity!"
     
    Last edited: Jul 8, 2013
  15. Jul 8, 2013 #14
    If that is true then at what altitude does friction stop having an effect and why.
     
  16. Jul 8, 2013 #15

    Bandersnatch

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    It never does "stop having an effect". It's a gradual decline. The farther away from the drag-inducing surface, the lesser the drag force. Every layer of the atmosphere above the ground exerts diminishing drag force on the layers above it.
    It's the same as stirring a pot. You can use a tiny spoon to stir only the upper volume of the soup, and the circular movement will be transferred to the bottom parts through drag forces, with the lowest parts having the largest difference in rotational velocities as compared to the velocities of the top volume.
     
  17. Jul 8, 2013 #16
    What if we consider other planets and varying the amount of gravity.
    So rather than removeing gravity completly, what would happen if gravity was more or less.
    For example are frictional forces the same on all the planets, or do they vary from planet to planet depending on the mass of each individual planet.
     
  18. Jul 9, 2013 #17
    There's friction on Venus like there is on earth, so there must be something supplying energy to keep the winds blowing. The sun supplies a lot of energy....

    On the outer planets there is less friction, however, due to the very cold temperatures. I understand that Neptune has winds of ~1000 mph despite little energy from the sun.

    Anyway, imagine a 1000 mph wind on the earth surface...and how much energy that would take. A hurricane is about an order of magnitude slower and look how much destruction it causes. The forces and destruction would only scale exponentially with speed. That's not something that can just keep on running unless you pump enormous energy into it.
     
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