Have you ever wondered how skaters find the strength to get themselves to spin so fast?  I know I have, but as a spectator at an ice skating competition you have probably seen a skater spin in a circle.  As the skater brings his or her arms closer to their body, they spin noticeably faster.  Another example that will hit closer to home is an exasperated teacher that is sitting in a swiveling computer chair.  His class exasperates him, and in a gesture of frustration, he throws his arms out and spins the chair.  As he brings his arms together, the chair turns faster than before!  How can anyone explain why this happens?  

    Good, old Isaac Newton found that objects in motion tend to remain in motion unless they are interrupted by another force.  Today, it’s called the law of conservation of momentum, which is defined by the equation:

Ðp = mvr

m = mass of object/mass of smaller object (if two objects are used)
v = velocity of the object/velocity of smaller object (if two objects are used)
r = radius/separation of the two objects

    In the example of the spinning skater, the variables would be allotted in this manner:

m = mass of the skater
v = velocity at which the skater is spinning
r = how far the skater’s arms are extended from his or her body

As the skater pulls his or her arms closer to the body, the radius would decrease.  To counteract that change and keep the angular momentum constant, the velocity would increase (since the skater would not have changed in mass).

    Concerning the example of the exasperated teacher, the variables are assigned these values:

m = mass of the teacher and the chair combined
v = velocity at which the teacher spins in the chair
r = how far the teacher’s arms are extended from the body

The teacher begins spinning at a certain velocity with his arms flung out.  As his exasperation decreases, he brings in his arms and decreases the radius.  His velocity suddenly increases!  The angular momentum of this system, and all the systems mentioned before, is conserved.

http://www.exploratorium.edu/snacks/momentum_machine.html