Just like an object moving with some velocity or speed has a momentum, objects that spin also have momentum, angular momentum. The angular momentum of an object moving in a circle with radius 'r' is the product of the mass, velocity or speed of rotation, and the radius of the circle.
Newton's first law tells us that unless there is a net torque, or twisting force, on the body that is rotating, angular momentum will be conserved. To conserve angular momentum means that the total amount of angular momentum stays the same even if the things that make it up change in value. For example, if the mass decreases, the velocity or radius must increase to compensate for this in order to keep the value of angular momentum the same. This conservation of angular momentum is most easily seen in figure skaters. When a figure skater starts a spin, they begin with their arms and probably one of their legs out wide. This gives them the maxium r possible. They then bring in their arms and legs tightly around them, thereby decreasing this radius. Because angular momentum is conserved in this case, it must stay the same, so when the skater decreases his/her radius, the velocity of rotation must increase since they can't increase their mass, so the skater spins faster. So, in an astronony context, when a cloud which is spining starts to collapse, the spin must get faster.
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... Last modified 18Apr00