Why do planets rotate ?
Why do the planets rotate? What force cause them to rotate? There is no force that causes the planets to rotate. Most of the rotation comes about from the conservation of angular momentum. Angular momentum is given by L=m*w*r2 where m is the mass, w is the angular velocity in radians per second, and r is the radius of the circular motion. Due to conservation of angular momentum, if the radius of the orbit decreases, then its angular velocity must increase (as the mass is constant). All planetary and stellar systems are born from the collapse of dense interstellar clouds. The clouds may originally be very large (even thousands of light years across). Consider a portion of the cloud the collapses from a size of a light year or so to the size of the solar system. That is a huge change in the size of the system. So, the very slight rotation that the cloud has in the beginning is increased dramatically when the collapse takes place. In fact, this is one of the barriers in star formation: th
