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I'm wondering about air travel and the rotation of the earth

  1. May 11, 2015 #1
    It would seem to me that if you fly from point a to point b following the rotation of the earth, you'd have to go faster than this rotation to move forward. For example, if the earth spins at 1,040/mph, then wouldn't a plane have to go faster than this just to escape the spin of the earth? I get that when the plane LEAVES the earth it moves at its speed PLUS the rotation of the earth, but surely the effect of the earth's momentum from its spin on the plane would decrease and go away very quickly and predictably. Also, it would seem to me that if you made a trip following the spin of the earth and it took 10 hours (random example) then the trip back, going against the spin of the earth should be a lot faster? When I say a lot faster, I mean 1,040/mph faster. Surely, going against the spin, the momentum of the earth's spin on the plane would be irrelevant to the plane, or perhaps even negative?

    Can any one help me understand this in simple english?
     
  2. jcsd
  3. May 11, 2015 #2

    Drakkith

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    No, momentum is conserved, so unless something took it away from the plane it still remains. One way to take it away is to apply a force against the direction of motion, which is exactly what happens when the plane flies the opposite direction of rotation.
     
  4. May 11, 2015 #3
    Ok, great, but I"m not sure I understand the implications of your answer. So if on the way back momentum is quite obviously not a factor, then the earth's spin should shorten the distance and the travelling time by a lot. But it doesn't. Why?
     
  5. May 11, 2015 #4

    russ_watters

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    Planes fly in air, so what matters is what the air is doing: the air moves with the earth. So the rotation doesn't matter.
     
  6. May 11, 2015 #5
    The air moves with the earth...? Really? What about when you're going against this movement of the air/earth, then wouldn't it be safe to assume that the movement of the earth and air would move counter to the air plane shortening it's distance back to point A?
     
  7. May 11, 2015 #6

    Drakkith

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    We're talking about relative velocities here. It takes just as much work to decrease your speed relative to something as it does to increase your speed by the same amount. The situation is a little muddled because we have a lot of air that moves in various currents around the earth, but the basic principle remains.

    The air does indeed move with the Earth. Otherwise you'd have thousand mph winds. But that doesn't affect the answer much. The only difference between this scenario with and without air is that with air the plane has to keep applying a force to avoid slowing down thanks to drag.

    The key idea here is that for all intents and purposes we can consider the Earth as stationary unless you want to talk about getting into orbit or about other astronomically relevant effects.
     
  8. May 11, 2015 #7
    Pardon me if my response seems stupid then, but what you're saying is that if on it's way back from point B my plane left our atmosphere, then the earth would spin 1000/mph underneath it and thereby decrease the distance needed to return to point A?
     
  9. May 11, 2015 #8

    Drakkith

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    No, that's not what I'm saying.
     
  10. May 11, 2015 #9
    Ok, but just to clarify, you said "The key idea here is that for all intents and purposes we can consider the Earth as stationary unless you want to talk about getting into orbit or about other astronomically relevant effects." I took this to mean that once you leave the earth's orbit, then you're not part of it's spinning anymore. Then you can stay in one place and watch the earth spin. So if a plane left the earth's orbit and just stayed put, wouldn't the earth's spin bring it back to point A? What am I missing here?
     
  11. May 11, 2015 #10

    jbriggs444

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    If a plane on the equator is flying westward at an air speed of 500 miles per hour, its velocity as measured against a non-rotating frame of reference would be 500 miles per hour eastward. Measured against that frame of reference, it is flying into a roughly 1000 mile per hour headwind and is losing ground.

    Fortunately, the earth is moving at about 1000 miles per hour eastward as measured against a non-rotating frame. So even though the plane is losing ground at 500 miles per hour, it is still gaining ground at 500 miles per hour as measured against the rotating earth.
     
  12. May 11, 2015 #11

    SteamKing

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    You are assuming that while aloft, the plane is somehow no longer held by gravity to the earth, and that the velocity of the plane is relative to some fixed point, not located on earth.

    Like what was pointed out in earlier posts, the atmosphere and everything in it (including birds and planes), is attracted to the earth by gravity. There is no relative motion between these objects and the earth.

    Even swallows, (African or European), are bound to the earth by gravity. I dare say you would not argue that swallows must fly 1000 mph to escape the spin of the earth. :wink:
    Why should airplanes be any different?
     
  13. May 11, 2015 #12

    Drakkith

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    No, you're skipping all the details of how the plane got into orbit in the first place, not to mention how it would remain stationary against gravity. But most of that's irrelevant to this conversation since planes don't go into orbit.

    Do you understand the concept of inertia? A moving object will continue to move unless acted upon by a force. We are already moving with the Earth's rotation, so even if you get into a balloon and float above the ground you will still be moving with the Earth's rotation because you were already doing so. The speed and momentum you have doesn't simply go away as soon as you leave the surface.
     
  14. May 11, 2015 #13

    russ_watters

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    Walk outside. Is there a 1000 mph wind in your face?
    No, nothing has changed. Since the air and earth move together, your airspeed is roughly equal to the ground speed in either direction.

    Caveat: the coreliolis effect causes winds to have a favored direction for different latitudes.
     
  15. May 11, 2015 #14

    russ_watters

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    BTW, I don't like invoking momentum or inertia here because when the plane leaves the ground, nothing changes, so there is nothing for inertia to do. The momentum conservation statement reduces to 1=1.
     
  16. May 11, 2015 #15
    As the earth rotates you rotate with it gravity pulls u with it even if your in a plane if u travel 560 mph at 38000 ft then you travel that distance in an hour minus drag and wind resistance ,I thought about this too as a kid it baffled me then it made sense
     
  17. Jul 30, 2016 #16

    A.T.

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    Gravity acts radially and doesn't make things rotate with it.
     
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