|Motion | Pages 26-27 ||
the friction to a single point of contact between you and the ball. As a result, it spins much longer and your friends walk away impressed. Jugglers also reduce friction in their routines by perching spinning objects at the ends of pointed sticks, or even on the tips of their noses.
Finally, imagine that the rotating thing touches only air. A tossed disk of pizza dough doesn't stop twirling in midair because the friction between the dough and molecules in the air is very small. In the same way, crisply struck golf balls or smartly thrown boomerangs maintain most of their spins until they fall back to Earth. If we go one step further and subtract the air itself, there's no limit to how long an object can spin. Physicists can do this in the laboratory by levitating objects with magnetic fields in vacuum chambers where almost all of the air is pumped out. In such settings, objects could rotate for years if anyone cared to watch.
But the best vacuum of all is in space. Once a body starts to spin there, it won't stop unless it collides with something or feels some source of drag, such as tidal pulls from nearby objects. The physical principle that governs such long-lasting motion is called the conservation of angular momentum. According to Newton's laws of motion, an object's straight-line momentum stays the same unless some outside force acts upon it. Angular momentum behaves the same way, with an added twist. The amount of momentum carried by a spinning object depends not only on its rotation speed but also on how its mass is distributed around its axis of rotation. That quantity, known as the "moment of inertia," dictates how much force it takes to spin up an object of a certain shape--and how much the object resists slowing down once it is spinning.
It's easy to see this concept in action. A familiar example is the dramatic spin used by figure skaters to end their performances. Watch as the skater spins slowly at first, then draws her arms and legs toward her body. That brings her distribution of mass closer to her axis of rotation, which lowers her moment of inertia. But because her total angular momentum must stay the same (ignoring the minor friction of the skates against the ice), her rotation speed must increase drastically to make up for it.