Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Spinning demonstration with V-channel

Demonstration

Angular momentum is conserved when the radius of orbit is changed.

Apparatus and materials

Air table

V channel

Steel balls, 2

Gardening gloves, heavy duty

Health & Safety and Technical notes


Take care when handling dry ice. Wear eye protection and use thick gardening gloves to handle it.

Two pieces of V-channel should be cut and mounted on a base in a V arrangement.
The demonstration can also be performed using a solid CO2 system, mounting the channel on a glass plate and with a small piece of solid CO2 between it and the table so that there is a frictionless bearing for spinning.

 

Procedure


a Load the V with two steel balls to act as planets, holding them high up on each arm by a piece of light metal tubing placed horizontally between the balls as a spacer.

equipment for spinning demonstration with V channelb Set the device spinning and snatch the metal tube away. When the balls run down to the bottom of the V, much closer to the axis, the system spins much faster.

 

Teaching notes


1 When the radius of the rotating mass is changed the angular momentum is conserved. Newton gave a geometrical proof of Kepler’s Second Law, using changes of momentum as vectors, because the Law is simply a statement of the conservation of angular momentum around the Sun when the only force acting on the planet is a central one passing through the Sun.
 
2 A much simpler alternative: have a student, standing upright, spin on one heel for a short time while holding a heavy book in one hand. Moving the book towards or away from the body will show a change in angular velocity.
 
It helps to raise the toes of the pivot foot. With a little practice, it is possible to make two or more revolutions spinning on one heel without losing balance and having the other foot coming to the rescue.