Tonight I was watching part of the 20.16 US Figure Skating Championship on NBC (an American television network, for foreign readers of this blog). Ever wonder why a figure skater can do what s/he does, particularly concerning when the skater crosses his/her arms and thereupon spins at a much faster speed? Sometimes more elaborate moves can cause similar effects.
Some basic physics principles will do here. First, I shall discuss friction. Friction keeps things still when they are juxtaposed against one another. There is a little bit of friction involved with contact of the skates on icy surfaces, which helps allow for quick motion (as inertia keeps things moving unless a force opposes it, in this case, friction applied to stop the skater). Ice provides just enough friction for starting and stopping the skater, but otherwise it is smooth.
At the very core of figure skating, however, is angular momentum, or the momentum of rotation. Momentum, when in a straight line, is simply mass times velocity. In situations of rotation, however, it is the angular velocity (speed of rotation) times the “moment of inertia.” Without going into details, the latter quantity takes into account shapes of different objects. Since momentum, by its very nature, must be conserved, a change in the distribution of mass (and hence the moment of inertia) into a more condensed form will cause an acceleration of the skater when s/he spins!
Finally, Newton’s ever-famous third law (i.e., that of equal and opposite reaction) allows the skater to glide forward (or even leap up!), as the force directs down and back. The exact backward force determines the exact forward force, depending on the details of each.
Like classical music, classical mechanics can sure be beautiful!