Wednesday, December 7, 2016
My friend the ice skater.
I recently went to watch my friends ice skating practice. The team is putting together a routine for the holidays that includes many spins and jumps. My friend is about 5'11'' and 140 lbs (1.803 meters and 63 kilograms) with a wingspan of about the same if we follow the standard human proportion. If she is spinning with her arms out her rotational inertia is greater than if her arms are in. Since rotational inertia is a resistance to spin, the greater the inertia the slower the spin. With her arms out, her rotational inertia would be 102.4 kg/m2 if she is modeled as a cylinder. With her arms in, her rotational inertia would be 11.69 kg/m2. If we follow this equation: I = 1/2 m r^2 Her resistance to spin, or rotational inertia, is much lower when she has her arms in and thus she spins faster.