Ice Skating

When a skater goes into a spin, they usually start it with their arms out wide, spinning at a slow pace. Then the skater pulls their arms in and the speed at which their rotating increases and finally as the spin comes to an end, their arms extend again and they slow down. Many people understand that physics is incorporated in skating, but they don't understand how much goes into a simple spin in terms of physics. Rotational momentum is defined by the objects moment of inertia multiplied by their angular velocity. An object's moment of inertia is defined by their mass multiplied by their radius squared, multiplied by a constant determined by the shape of the object. Therefore, as a skater pulls in their arms, their radius decreases, decreasing their moment of inertia. Since rotational momentum is conserved during the skaters spin, their rotational velocity increases as their moment of inertia decreases. It is astonishing how simple something so mesmerizing can be after the physics behind it is understood.