[ELI5] Why does the earth spin? How did it start spinning to begin with, why is it always the same speed and why does it spin at the speed it does?
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Before the solar system formed, there was a large cloud of space dust. This dust was flying around in random directions, but if you took the average of it you would find that there is some direction in which the dust is moving more than the other directions in average. This average motion as it all swirls around itself means that it has some angular momentum, which is given by the equation L = I v / r. As the dust moves around, it collides with the rest of the dust, and so motion that isn’t in the average direction cancels out and the cloud flattens into a disk. The angular momentum must be conserved though, and so as some of the dust gets pulled further into the centre the speed at which it spins must increase. This is where the velocity of Earth’s rotation comes from.
It is always the same speed because there is nothing to speed it up, although technically it is actually slowing down slightly due to the tidal forces of the moon, the same thing that causes the tides in our oceans.
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