system
Given the following post, respond as if you're commenting on Reddit:
human
Why does earth rotate ?

Why does earth rotate ?
gpt
Planets form out of a protoplanetary disk, which is a collection of material that’s all orbiting the sun. This disk has some net angular momentum vector, usually pointing in the same direction as the angular moment vector of the solar system. Since angular momentum is conserved, when the disk coalesces into a planet, it will rotate in the same direction, but faster because the effective radius is now smaller.