ABSTRACT
Imagine an ice skater starting to spin while standing upright at a particular spot of the ice rink (this is known in gure skating as the scratch spin or upright spin). Initially the arms are extended. To spin faster, the skater brings the arms closer to the body. A body’s inertia to rotation is smaller if its mass is distributed closer to the axis around which the body spins. us, when the skater brings the arms closer to the body, his or her inertia to rotation decreases. We dene the angular momentum as the product of the inertia to rotation and the spin velocity. e spin velocity is proportional to how many times in a minute the skater goes completely around. Note that this is analogous to the denition of momentum, which equals the mass (which measures inertia) times the linear velocity. In the situation described, the skater’s angular momentum is constant: as the skater brings the arms closer to the body, the decrease in the inertia is compensated by an increase in spin velocity.1 is is exactly what we observe.
