Three Dumbbells

Lecturer rotates on turntable whilst holding two dumbbells.

What it shows:

Angular momentum, the product of a body's moment of inertia and angular velocity, is always conserved. A reduction in moment of inertia will result in a proportional rise in angular velocity.

How it works:

A volunteer holds the other two dumbbells 1 in each hand and stands upon a rotating platform. 2 With arms outstretched and a little push they begin to rotate at a certain angular velocity. By pulling in their arms to their chest, the moment of inertia is reduced and the volunteer spins faster and faster...

figure 1. the three dumbbells in action
figure 1

Setting it up:

The turntable needs to be placed on a flat surface, clear of all obstacles (so your volunteer can tumble off with confidence), and leveled. If desired, an adjustable thigh support can be attached -- the support helps you keep your balance.



For techniques on how to be a good dumbbell, study how ice skaters hold their head when they spin. Always try to look at the same point, allow your body to rotate, then bring your head around fast back to look at the same point (dancers call this 'spotting'). A good opportunity for audience participation if your class contains skaters or dancers.

1 We use either two 4kg steel rods (17cm × 5cm diameter) or commercially available dumbbells. The commercial choices are 10 lb and 15 lb dumbbells.
2 Cenco #74790 turntable