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nitinshetty
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Why does the Earth rotate?
Andre said:Then why does Venus not (barely) rotate?
Here's a good question:, since angular momentum is conserved, how do you 'create' angular momentum from an original system containing zero angular momentum? (ie: you can't). So where does angular momentum come from?Russ_watters said:Since our solar system came from a collapsed cloud of gas and dust, the tiniest asymetry in the structure of the cloud would cause the entire system to rotate about its center. The rotation caused some of the matter to end up in orbit around the sun - no rotation and all of that matter would have fallen into the sun. Similarly, the condensing of the Earth in an asymetric way caused the Earth to rotate.
Any isolated system containing 0 angular momentum.dextercioby said:What do you mean "original system containing zero angular momentum??
Which system??The initial cloud of particles from which our Solar system evolved??
Astronuc said:There was probably some initial angular momentum. How much depends on the particular theory of the formation of the solar system.
Furthermore, with assymmetry in mass distribution and under the influence of gravity (not everything converging at an equal rate to the same center of mass), the consequence is non-zero angular momentum.
There was probably some initial angular momentum. How much depends on the particular theory of the formation of the solar system.
dextercioby said:The simplest analysis which i can imagine would start from the Boltzmann equation and would yield only in the simplest possible case (frition proportional to gradients of velocity field) the Navier-Stokes equations in which the gravity term would appear given by Newton's law.Essential would be turbulence (with vortices) and how would initial nonhomogeneities in density would evolve under the influence of gravity and strog rotational velocity fields.
Daniel.
Exactly. Since angular momentum cannot be created or destroyed the solar system still has the original angular momentum of the gas cloud from which the solar system formed (assuming no outside forces have acted on it in the last 4 billion or so years).marlon said:The best explanation has been given quite some times. It is possible for a system to have net zero momentum but due to internal inhomogeneous distributions of (for example) matter forces will start to act on these distributions because they are a deviation of the lowest and most stable energystate. This is what will lead to a generated local angular momentum that is nonzero. Due to the conservation of momentum the opposite value of this generated momentum needs to be present in the system and therefore something else will happen besides these disturbances in distribution of matter. Hence the rotation of the earth...
nitinshetty said:What would the Earth be like if it did not rotate?
If the Earth did not spin at all there would be more mountains because the Earth surface would contract.
The Earth radius is 6377 km at the equator which is about 22 km greater than at the poles. Work out the area of a sphere 6355 km in radius and then subtract that from the known area of the Earth surface. That is how much real estate you would lose if the Earth stopped rotating. Where would it go? The only place is up.FZ+ said:If the Earth did not spin, then it would have been shaped differently throughout its formation - in short it would be rounder. I don't see why there would be more mountains, though.
That mass would go to the poles, and the Earth would turn into a more perfect sphere. That's it.Andrew Mason said:The Earth radius is 6377 at the equator which is about 22 km greater than at the poles. Work out the area of a sphere 2355 km in radius and then subtract that from the known area of the Earth surface. That is how much real estate you would lose if the Earth stopped rotating. Where would it go? The only place is up.
Are you saying that the surface area would not be less?russ_watters said:That mass would go to the poles, and the Earth would turn into a more perfect sphere. That's it.
No, but I am saying the difference would be utterly insignificant. And besides which:Andrew Mason said:Are you saying that the surface area would not be less?
AM
We're not talking about a change, we're talking about what the Earth would be like if it hadn't been rotating to start with. Ie, before plate tectonics. (as FZ+ said)On second thought though, the mechanism of plate tectonics should be well capable of compensating for surface area change due to the shape change.
The plates would have to compensate. If the total surface are did not change and the plates could move quickly enough, then no problem, the plates just move around.Andre said:Most excellent hypothesis! Andrew.
On second thought though, the mechanism of plate tectonics should be well capable of compensating for surface area change due to the shape change.
I worked it out and the area of the equivalent sphere (equal volume) is actually greater than the area of the spheroid Earth by about 600,000 km^2.russ_watters said:No, but I am saying the difference would be utterly insignificant.
The question was "What would the Earth be like if it did not rotate?". That could be if the Earth never rotated in the first place or if it suddenly stopped rotating.And besides which: We're not talking about a change, we're talking about what the Earth would be like if it hadn't been rotating to start with.
I agree, if the Earth did not rotate to begin with. But not if a rotating Earth stopped rotating.In fact, with less dynamic forces on the earth, there would likely be less plate tectonics and smaller mountains as a result.
what if the Earth was hit by a large asteroid so that its spin slowed rapidly all of a sudden.
If it hit vertically, the Earth would acquire all of the asteroid's momentum in the time it takes for the asteroid to stop. Why would it be any different for angular momentum? In other words, where does the momentum "go" in that 90 minutes?Andre said:The big problem with that scenario is inertia and propagation speed of impulses. It would take 90 minutes for the other side of the Earth to react on any impulse and it would continue to spin with the original speed whereas the hit side would have slowed down
Conclusion: the Earth would break up.
Of course. But my point was: why does the Earth angular momentum not change as soon as the asteroid begins impacting the earth? Why does one side wait 90 minutes to begin spinning faster?Andre said:Okay the mssing step here is that the Astroid has to hit at a rather oblique angle to exert a momentary torque force to change the angular momentum of the Earth to change the spinning rate. Inertia would spoil that scenario. The vertical hitting asteroid would only change the lateral momentum but not the angular momentum. Consequently a spin change would only be minor if at all.
Okay. I see what you are saying. But I think what would happen is that there would be an enormous matter wave that would keep going around the Earth until it died out eventually.Andre said:Well the fastest way a mechanical impuls travels is with the speed of the P-waves, about 5 km/sec in granite. The average for the Earth is about 3,5 km/sec (source not available at the moment).
If a impuls would tend to slow the spinning it would be no good if it went the shortest way, though the middle of the Earth because it would not have a effective component that would change the spinning. So our impuls has to follow the surface of the Earth to change the speed over there. To the other side of the Earth, that's 20,000 km or 4000 seconds (5 km/sec) or 5714 seconds (3,5 km/sec), or 67 mins resp 95 minutes, okay?
If the moon is the result of the Earth being struck by a giant asteroid, where do you think it hit? ie. where is the largest expanse of low lying ground on earth? What shape (generally) is it? Just a thought.Andre said:Would be interesting to model that hypothetical event. Perhaps the Earth may have been shattered initially by that event due to the mentioned inertia but gravity may have rebuild the heap of big chunks into the present Earth - Moon system.
Ok, fair enough: I was trying to conform this question to physical reality. Ie, it doesn't break any laws of the universe to speculate on if the Earth hadn't been rotating when it formed, but to speculate on it spontaneously stopping does break the laws of the universe. I don't consider such speculation useful, since (for example), if the Earth suddenly stopped rotating, earthquakes wouldn't be your first worry: stopping yourself before hitting your wall at 1,000 mph would be...Andrew Mason said:The question was "What would the Earth be like if it did not rotate?". That could be if the Earth never rotated in the first place or if it suddenly stopped rotating.
I agree, if the Earth did not rotate to begin with. But not if a rotating Earth stopped rotating.
That momentum distributes itself through the Earth for 90 minutes. Consider a tennis ball hitting a wall - the point where the ball hits the wall stops immediately on hitting the wall, but the other side of the ball is still moving toward the wall and the ball compresses. Consider a nail: when you hit it with a hammer, the far end of the nail does not start moving instanly: the nail compresses and the far end starts moving when the pressure wave reaches it at the speed of sound.If it hit vertically, the Earth would acquire all of the asteroid's momentum in the time it takes for the asteroid to stop. Why would it be any different for angular momentum? In other words, where does the momentum "go" in that 90 minutes?
The fact that such a large chunk of the Earth out in space implies that it did disintegrate. Because of gravity, this disintegration was not necessarily permanent (as Andre said).I still don't see why the Earth would break apart. The moon is thought to have come from a large asteroid impact on the Earth about 4 billion years ago, sending up a chunk of the Earth into space. That is one theory, at least. Obviously, the Earth did not disintegrate then.
Any such impact would be far too big to leave a crater. The Earth would have essentially been pulverized and reformed spherically.If the moon is the result of the Earth being struck by a giant asteroid, where do you think it hit? ie. where is the largest expanse of low lying ground on earth? What shape (generally) is it? Just a thought.