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Possible Duplicate:
Why does every thing spin?

So why would the Earth, or any planet for that matter, rotate along an axis? I know of no force which could come into play here, so i assume it just started off with an initial rotation. But where did this rotational energy come from? And are there any known planets which do not rotate?

Assuming no force accounts for earth's rotation, would it be true that the angular velocity of the earth would decrease(by a very very small amount), when bombarded by meteors(Due to an increase in the moment of inertia by the increase in mass of the earth i.e. the additional mass of the meteors)?

Community
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Likhit
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6 Answers6

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Current theories have it that the Earth formed from a rotating disc of gas and dust, hence it has simply maintained angular momentum.

Nic
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For a finite mass to rotate with exactly zero angular velocity about its centre of mass is one state out of an infinite number of angular velocities and therefore impossible. So it's far more likely to be in a state of rotation.

Physiks lover
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And yes, the angular velocity of the Earth does change with meteorites. Conversely, it also changes when spaceshuttles take off. All systems obey the Law of Conservation of Angular momentum. Defining the boundaries of the system can be difficult, but the Earth is no exception.

Changes in the angular velocity can also occur from shifts of weight on the planet. Earthquakes and volcanoes are large enough events that popular news media often reports how much the rotation has been effected (as either a lengthening or shortening of the day).

AdamRedwine
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The only way it could not rotate would be if the initial cloud from which the solar system formed were all moving at the same speed and in the same direction as the gravity well that became the sun, and it were all pulled radially into it, making no planets at all.

Mike Dunlavey
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Almost all astrophysical objects up to the size of galaxy clusters rotate. Their angular momentum just has to have some value and why should that be zero? Even when you start of with a big cloud of gas without any angular momentum (the universe on large scales cannot rotate), it may fragment into smaller clouds which torque each other and each obtain some angular momentum, such that their sum is still zero.

Another important aspect is that angular momentum cannot be simply radiated away, unlike energy. This means that an object cooling (by radiating photons) will loose energy but not angular momentum. Usually this implies it will shrink (unless the radiation losses are balanced by some other source of energy, as for the Sun which harnesses fusion energy) and thus obtain larger angular frequencies. The total angular momentum of the Solar system, for instance, is dominated by the orbital angular momentum of Jupiter even though its rotation frequency is 1/12year while the spin rate of the Sun (which contains less angular momentum) is about 1/12hours.

Walter
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"And are there any known planets which do not rotate?"

Not in our solar system. Our moon is tidally locked to Earth, which is why we always see the same side. We do suspect that quite a few extrasolar planets are tidally locked to their stars.

You could however argue that such planets do rotate, at a speed of one revolution per orbit.

MSalters
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