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Now im isolating Atmosphere

I was reading somewhere about a really cheap way of travelling: using balloons to get ourselves away from the surface of the earth. The idea held that because the earth rotates, we should be able to land in a different place after some time. As we all know, this doesn't happen.

Someone said that the reason why this doesn't happen is because the atmosphere (air, clouds etc.) also revolves around the earth (with the same angular velocity as the earth's rotation). Since we are also part of the atmosphere, our position doesn't change relatively.

Well, I'm not convinced with that answer. Why does the atmosphere rotate along with earth? Gravitational force is towards the centre of the earth, but I don't see how it's making the atmosphere rotate.

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While you have some (right) answers about what make the air follow the earth's rotation, the rotation itself influence the air movement, and create some permanent west-east winds, like the jet streams en.wikipedia.org/wiki/Jet_stream , which has been used as "really cheap way of travelling" for japanes bombs during WWII –  Frédéric Grosshans Nov 22 '10 at 13:48
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I agree with the sentiment of the question, however I would like to point out that winds blow in all directions and as such the atmosphere does not really rotate in the way you seem to assume. –  Sklivvz Mar 20 '11 at 22:11
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I don't agree that the atmosphere is rotating with the earth because of friction. Put a ball on top of the head of a tennis racket...move the racket to the left, the ball rolls in the opposite direction and stays relatively on the same spot of the gravitational pull. Or use a ball and a board for the experiment. –  user17462 Jan 4 '13 at 0:59

8 Answers 8

up vote 49 down vote accepted

The atmosphere rotates along with the Earth for the same reason you do.

Force isn't needed to make something go. That's a basic law of physics - that a thing that's moving will just keep moving if there's no force on it.

Force is needed either to make something change its speed, or to make its motion point in a new direction. A force can do both or just one of these. Most forces do both, but a force that pushes in the exactly the same direction you're already going only changes your speed, and does not change your direction. A force that pushes at a right angle to the direction you're already going only changes your direction, and does not add any speed. A force at "10 o'clock", for example, will change both your speed and your direction.

As you stand still on Earth, you continue going the same speed, but your direction changes; between day and night you move opposite directions. So the forces on you must be at a right angle to your direction of motion. Indeed, they are. Your motion is from west to east along the surface of the Earth, and the force of gravity pulls you down towards the center of the Earth - the force and your motion are at right angles. Similarly for the atmosphere. It is moving along with the Earth, and moving at a constant speed. It does not need anything to push it along with the Earth. Since only its direction of motion is changing, it only needs a force at a right angle to its motion, the same as you, and the force that does the job is again gravity.

That's not the whole picture, because the amount that your direction of motion changes depends on how strong the right-angle force is. It turns out gravity is much too strong for how much our direction of motion changes as the Earth spins. There must be some other force on us and on the atmosphere canceling out most of the gravity. There is. For me it's the force of the chair on my butt. For the atmosphere, it's the air pressure.

So gravity doesn't "make the air rotate". The air is already going, and gravity simply changes its direction to pull it in a circle.

You may be wondering why the air doesn't just sit there and have the Earth spin underneath it. One answer to that is that from our point of view that would mean incredibly strong wind all the time. That wind would run into stuff and eventually get slowed down to zero (that's from our point of view - the air would "speed up" to our speed of rotation from a point of view out in space watching everything happen). Even the air high up would eventually rotate with the Earth because although it can't slam into mountains or buildings and get stopped from blowing, it can essentially "slam into" the air beneath it due to friction in the air. (This is a little redundant with dmckee's answer; I was half way done when he beat me to the punch)

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About the air spin-up -- this would also work for perfectly spherical Earth due to air viscosity. –  mbq Nov 22 '10 at 8:48
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A mention of the atmospheric boundry layer would be relevent in the last paragraph... –  Killercam Nov 9 '12 at 10:03

All scientific experiments to measure the motion of the Earth, for example "Airy's Failure", comparing the aberration of starlight through air with starlight slowed down through water, have shown that the Earth IS NOT MOVING, instead the ether itself rotates and shifts around the Earth, which is the fixed pivot point.

Therefore the problem of the lack of a continual 1000 mile per hour wind around the equator does not exist! If the Earth is not rotating, then neither does the problem of a rotating atmosphere exist.

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This is just plain wrong. The aberration of starlight shows conclusively that the Earth does rotate. –  Chris White Jun 19 '13 at 15:09
    
Wow... there are actually people who STILL think this way? No wonder we're so screwed as a species. –  SplinterReality Apr 6 at 13:55
    
Are you proposing the heliocentric model is accurate, or do I misunderstand your answer? –  Brian Wheeler May 9 at 20:42

There are at least two reasons:

  • the air layer adjacent to the Earth surface is dragged with it (being at rest with it).
  • air viscosity -- it could be thought as a friction between different air layers. Upper layers are carried along by underlying layers.

If the air were to stop suddenly it would result in ~1500 km/h wind speed. For comparison Hurricane Katrina's winds were ~100 km/h. It tells us that the air viscosity is sufficiently large that other effects (e.g., weather related) do not dominate its motion relative to the Earth surface.

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All matter is attracted to the centre of the Earth due to the gravitational force.

Air is also matter, and it is pulled towards earth's centre, and due to this force it travels along with the Earth when it (Earth) rotates. The balloon, even when it is lifted up above the surface of the Earth, is still attracted towards the centre of Earth and moves with the Earth, subject to other forces applied on it like friction, etc.

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The atmosphere rotates along with the Earth due to the friction. Friction makes molecules that are in contact with the ground to move. The friction between the air molecules (air viscosity) causes the upper molecules to move.

Note: that's why, as we move to the north, the air angular velocity becomes smaller because the Earth angular velocity is smaller.

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Does your analogy really hold? The extend of the water around the boat is pretty large compared to the size of the boat. The atmosphere is a relatively thin layer, and there is no stagnant air surrounding it, right? –  Bernhard Apr 17 '12 at 14:07
    
you are right,my analogy is not exact but no because the boat is small relative to the water but because sea water has no end in contrast with atmosphere that has.The angular velocity of atmosphere is the same but the velocity increase with radius(if not we could travel using balloons)... –  a.krimizis Apr 17 '12 at 23:21

The atomosphere surrounding the earth is over 1000 times less dense than the earth. The earth and atomosphere react with each other. Mountains moves air but not vice versa. If the earth is rotating on its axis at 1000 mph at the equator there should be violent storms as a result. Gravity is not moving the air around the earth as asingle entity. Gravity may effect the atomosphere but differently than it effects earth. Since there are no 500 mph storms along the equator one must ask: Is the earth rotating on its axis?

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This is nonsense, are you doubting earths rotation? –  Bernhard Oct 17 '12 at 6:04
    
Gravity also doesn't "effect" anything. It does have a tendency to "affect" things sometimes. –  Brian Wheeler May 9 at 20:41

Because of friction. The friction between the earth's surface and the atmosphere.

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Friction AKA wind resistance.

You must have tried to stand in a strong wind or stuck you hand out the window of a traveling vehicle. From that you can feel the force that moving air exerts on objects in its way, and by Newton's law of reaction things in the way exert an equal force tending to move the air up to speed with the ground near it.

Even if the atmosphere started at rest, it would have long since been brought up to speed.

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