Question

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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)?

Answer 1

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.