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Offline hamza

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Earth's Rotation
« on: 05/10/2007 16:28:55 »
I have a question here regarding Earth's spin.
  We all know that the earth is rotating around its axis since the its very beginning some 3.5 billion years ago (correct me if i am wrong at the age). My question is that why, in the presence of so many frictional forces, is it constantly rotating? I mean doesn't this defy the physical phenomena of friction. Nothing can keep moving constantly when a frictional force is applied. There is a dissipation of energy in the form of heat. So why hasn't it slowed down or even stopped rotating around its axis??


 

Online syhprum

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« Reply #1 on: 05/10/2007 17:09:37 »
There is some evidence that when it first formed 4.6 billion years ago it was spinning about three times as fast as it does now but it does not have sufficient time to stop spinning completely before the Sun swells up and engulfs it.
 

Offline daveshorts

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« Reply #2 on: 05/10/2007 17:48:04 »
The earth is in a vacuum so the only frictional losses that have an effect on the spin of the earth are tidal forces from the moon and the sun. These are tiny compared to the mass of the earth so it takes a long time to slow down. It is slowing down though, if you look at about 250 million year old corals they have growth rings for every day, and you can see that there were about 400 days in a year so the earth has slowed down since.
 

Offline Ophiolite

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« Reply #3 on: 06/10/2007 17:57:56 »
Further to the last post, the best estimate of the length of the day around the time of the moon's formation is around six hours. As noted earlier it is largely the tidal interaction with the moon (and to a lesser extent with the sun) that has slowed the Earth's rotation. This same interaction has locked the Moon with one face always pointing to the Earth and progressively moving outwards.
 

Offline DoctorBeaver

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« Reply #4 on: 06/10/2007 17:59:05 »
As the moon moves away, will it start rotating with respect the earth so that we don't always see the same face?
 

Offline Ben Aldhouse

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« Reply #5 on: 06/10/2007 18:24:37 »
As the moon moves away, does the slowing effect it has on the Earth lessen? Will the moon fly off into space eventually?
 

Offline Ophiolite

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« Reply #6 on: 06/10/2007 18:54:09 »
Yes and no. :)
Yes, the tidal influence on the Earth - and hence the slowing effect on Earth's rotation - grows less as the moon moves further away.
Eventually the Earth would slow so much that it would also be tidally locked with the moon. that is the same face of the Earth would always face the moon. At this point the moon would begin moving back towards the Earth. However, I recall that the time for this to occur is longer than the time at which the sun will enter its red giant phase and probably engulf and destroy the Earth and the moon.
In any event the moon will not fly off into space.
 

Offline Andrew K Fletcher

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« Reply #7 on: 06/10/2007 19:44:10 »
Maybe the Earth was not spinning faster and slowing down now. What if the Earth was smaller, this could also account for the shorter days?
 

Offline hamza

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« Reply #8 on: 07/10/2007 06:39:36 »
Daveshorts! you said that earth is in a vacuum. I doubt that because space is not a vacuum. and if it were a vacuum than why does a rocket's head turns red when it moves about in space.. Is'nt it due to the frictional forces? And besides that, there are alot of smaller particles too, like we have in our atmosphere, which aid frictional forces. So do explain your claim!!
« Last Edit: 07/10/2007 06:41:19 by hamza »
 

Offline DoctorBeaver

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« Reply #9 on: 07/10/2007 08:22:32 »
Hamza - you are confusing space with the Earth's atmosphere. I'm not sure exactly where the boundary is said to be, but it is above the height at which a rocket's nose would glow red on re-entry.

As for space being a vacuum - to all intents and purposes it is. True, it is teeming with particles; but the particles concerned interact so weakly that they can be largely disregarded except where other elementary particles are concerned.
 

Offline Ophiolite

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« Reply #10 on: 08/10/2007 08:11:55 »
Maybe the Earth was not spinning faster and slowing down now. What if the Earth was smaller, this could also account for the shorter days?
That is correct. It is dependent on the preservation of angular momentum. However, there is no significant evidence to support the notion of an expanding Earth. The idea was popular in the first half of the twentieth century as one means to explain orogenies (mnountain building events). Plate tectonic theory provides a much better, all encompassing explanation of a cornucopia of geological events and features.
 

Offline thebrain13

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« Reply #11 on: 08/10/2007 16:33:34 »
just because an object is in a vacuum, doesnt mean it cant be slowed down. electromagnetic forces act over a distance.
 

Offline hamza

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« Reply #12 on: 08/10/2007 16:38:15 »
Hamza - you are confusing space with the Earth's atmosphere.
Could someone than please explain the difference. because as far as i know, vacuums don't have matter particles. But space does have it in the forms of stars, planets and meteoroids which are all over. and above all there are interactins between these bodies too. eg. sun's gravitational pull which keeps all planets  in their respective orbits and the tides caused by the moon. Is space supposed to still be a vacuum after all this??
« Last Edit: 08/10/2007 16:40:02 by hamza »
 

lyner

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Earth's Rotation
« Reply #13 on: 08/10/2007 17:12:13 »
"Vacuum" is not an absolute term.
The average density of space is something like one proton per metre cube. In the space region around the Solar System there is much, much more mass per metre cube.
The atmospheric pressure on Earth halves about every 2500m you go up.
We say the 'space' starts about 100km up -an arbitrary height but roughly the distance where friction effects start to be very low and, of course, the sky is black, once you are that far away.
Beyond this, the atmospheric density keeps dropping exponentially.
In the region of the Solar System there is a lot of stuff (relatively)  emitted from the Sun.
A lot further out than Pluto, there is the Heliopause, where the effects of the Sun are vanishlingly small - these include particles, radiation and fields.
If you go a very long way, you could find yourself in the newly - found "big hole';  nothin in there at all!
http://www.space.com/scienceastronomy/070823_huge_hole.html
That's really really empty.  Of course, if you went there, it wouldn't be empty any more - doh!
But the Vacu Vin that I use to suck air from a half empty wine bottle claims to produce a vacuum in the space over the wine. It's all relative.

Quote
just because an object is in a vacuum, doesnt mean it cant be slowed down. electromagnetic forces act over a distance.
And gravity......

« Last Edit: 08/10/2007 17:14:13 by sophiecentaur »
 

Offline thebrain13

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« Reply #14 on: 08/10/2007 19:20:12 »
I was wondering, since if you are moving towards a light emitting source ie the sun, the light is blueshifted, thus it contains more energy. So wouldnt the side of the rotating earth encounter more blue light, thus slowing it down more than the other side of the earth which has a redshifted view?

 

Offline gfellow

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Earth's Rotation
« Reply #15 on: 17/12/2007 05:37:44 »
Maybe the Earth was not spinning faster and slowing down now. What if the Earth was smaller, this could also account for the shorter days?
Indeed - and your question opens a can of worms. Here a rough animation I made that looks at two Earth/time scenarios:
newbielink:http://goodfelloweb.com/nature/terra/terra.html [nonactive]
It is curious that the gross behavior of continents is to drift apart rather remain in equilibrium or clump together.
Frédéric Malmartel approaches this from another angle:
"When gravitation turns into a death trap - A panorama of a mass extinction" 
newbielink:http://frederic.malmartel.free.fr/Fin_des_dinosaures/eedinosaures3.htm [nonactive]
He's not the only one. A web search will bring up quite a few paleontologists who scratch their heads as they consider pteranodon's too fragile to fly, seismosaurs too cumbersome to move.

I mentioned a can of worms; if the Earth were to suddenly change its gravity without a corresponding quantity of mass, the edifice upon which our scientific understanding of how gravity works must be flawed, or at the very least may have missed something important.
 
 

lyner

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Earth's Rotation
« Reply #16 on: 18/12/2007 19:44:03 »
Quote
I mentioned a can of worms; if the Earth were to suddenly change its gravity without a corresponding quantity of mass, the edifice upon which our scientific understanding of how gravity works must be flawed, or at the very least may have missed something important.

That's a pretty big IF, though. I should look for evidence as to whether it is happening and then start worrying about cans of worms.

If we are trying to reconcile some basic physics with problems that paleontologists might have in estimating the  probable mass and strength of  creatures from their fossil remains, I should first look for the evidence which the fossils is actually providing. We have every reason to suspect that Physics is the same as it has always been and the same everywhere in the Universe. Looking at fossils is a lot more speculative; what if they  have all been distorted by  some natural process or had other structures which haven't been found yet? Perhaps the flying creatures didn't fly at all. Perhaps many of the creatures  wandered around in muddy water - didn't need to ba all that strong..

I am a Physicist and must declare an interest but I could repeat most of the experiments from which the current Physics arrived. You can't do those sorts of experiments on fossil remains.
 

another_someone

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Earth's Rotation
« Reply #17 on: 18/12/2007 20:41:33 »
Indeed - and your question opens a can of worms. Here a rough animation I made that looks at two Earth/time scenarios:
http://goodfelloweb.com/nature/terra/terra.html
It is curious that the gross behavior of continents is to drift apart rather remain in equilibrium or clump together.

The site refers only to the behaviour of the Earthy for the last 200 million years of its 4.5 billion year history - there is rather a large gap prior to that which is conveniently ignored.

http://en.wikipedia.org/wiki/Supercontinent_cycle
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The supercontinent cycle describes the quasi-periodic aggregration and dispersal of Earth's continental crust. There are varying opinions as to whether Earth's budget of continental crust is increasing, decreasing, or remaining about constant, but it is agreed that this inventory is constantly being reconfigured. Continental collision makes fewer and larger continents while rifting makes more and smaller continents. The last supercontinent, Pangaea, formed about 300 million years ago. The previous supercontinent, Pannotia or Greater Gondwanaland, formed about 600 million years ago, and its dispersal formed the fragments that ultimately collided to form Pangaea. But beyond this the time span between supercontinents becomes more irregular. For example, the supercontinent before Gondwanaland, Rodinia, existed ~1.1 billion to ~750 million years ago - a mere 150 million years before Gondwanaland. The supercontinent before this was Columbia: ~1.8 to 1.5 billion years ago. And before this was Kenorland: ~2.7 to ~2.1 billion years ago. Ur existed ~3 billion years ago and Vaalbara ~3.6 to ~2.8 billion years ago. One complete Supercontinent cycle is said to take 300 to 500 million years to occur.

The hypothetical supercontinent cycle is, in some ways, the complement to the Wilson cycle. The latter is named after plate tectonics pioneer J. Tuzo Wilson and describes the periodic opening and closing of ocean basins. Because the oldest seafloor is only 170 million years old, whereas the oldest bit of continental crust goes back to 4 billion years or more, it makes sense to emphasize the much longer record of the planetary pulse that is recorded in the continents.


I mentioned a can of worms; if the Earth were to suddenly change its gravity without a corresponding quantity of mass, the edifice upon which our scientific understanding of how gravity works must be flawed, or at the very least may have missed something important.

Are you suggesting that this only happened on Earth, and the rest of the universe retained the correlation between mass and gravity, or are you suggesting that the Sun also has changed its gravity over the same period of time.

If you are looking for ways the Earth could expand, it would be much easier (although still not without controversy) to look for changes in internal temperature within the Earth, or maybe changes in the chemical constitution of the Earth (although the latter will probably effect the crust more than the deeper levels).
« Last Edit: 18/12/2007 20:49:58 by another_someone »
 

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