The Three Dumbbells
conservation of angular momentum - circular motion - moment of inertia
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
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.
Comments:
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. Rating ***
1
We use either two 4kg steel rods (17cmx5cm diameter) or commercially available
dumbbells. The commercial choices are 10 lb and 15 lb dumbbells.
2
Cenco #74790 turntable