Jearl Walker

The mechanics of rotation are more mysterious than those of linear motion, largely because of our lack of experience with rotation and the seemingly counter-intuitive behavior of objects undergoing rotation. I introduce the relati...

Bill Payne

Rotation

The mechanics of rotation are more mysterious than those of linear motion, largely because of our lack of experience with rotation and the seemingly counter-intuitive behavior of objects undergoing rotation. I introduce the relationship between lever arm, torque and the force causing the torque. For these demonstrations I am stationed at a playground's merry-go-round. My torque on the merry-go-round causes an angular acceleration, but the size of that acceleration depends on the distribution of mass on the merry-go-round. The mass and its distribution is called the moment of inertia. Still at the playground I discuss how the angular momentum of a rotating system must remain unchanged when there is no external torque on the system. The angular momentum is constant even if the system has an internal redistribution of mass and a consequent change in its moment of inertia. When the angular momentum is constant and the moment of inertia is altered, the angular speed of the object changes. The change is surprising, even in common examples such as a spinning ice skater. Such subtle changes in the angular speed of a rotating object are responsible for much of the grace of ballet and somersaulting dives. For example, a ballet performer can alter her spin on her toes by drawing in or extending outward her arms. A diver from a high platform can increase or decrease his spin rate by extending his arms and legs or by curling up into a ball. Angular motion can also help in the kitchen. In the show I demonstrate how you can distinguish a fresh egg from a hard-boiled one. First, spin the egg. One kind of egg spins smoothly while the other spins erratically. As a second test, touch briefly each spinning egg, just enough to stop the rotation. One type of egg will lose its angular momentum completely due to the torque applied during the touch. The other type will spin again after the touch. If you are still uncertain of the egg, give it a large spin. One kind of egg will stand up on one end, the other is too erratic in its spin to follow. Special types of tops will also stand up or invert themselves. With them or the hard-boiled egg the inversion is due to friction from the table. The friction creates a torque that rolls the egg or top over to its strange orientation.

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Rotation

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