Untitled Document

Physics Demo Number: 122

Approximate

Run Time: 5 min

Conservation of Angular Momentum with Help of Medicine Ball

Demo Description

A medicine ball is thrown from a sitting position in a low frictional torque chair.

It is found that throwing one handed off axis produces a net spin of chair and demonstrator.

Scientific Principles

A linear momentum can also be an angular momentum when viewed about an axis that does not intersect the line of the linear momentum.

Equipment

Dentist Chair
Medicine Ball

Equipment Location

The chair lives to the left of the door from the prep room into 4327.
The medicine ball is in Kit (122) on [F-3-2].

Instructions

The photo shows the dentist chair, with the usual bike wheel and hand weights, and with the medicine ball from Kit (122) on [F-3-2].

One may sit in the chair and throw the medicine ball straight forward, keeping it centered along a horizontal radial line intersecting the chair's axis of rotation, to show no visible effect on the state of motion of chair plus demonstrator.

However one may hold the medicine ball in one hand at the end of an outstretched arm and throw it briskly along a horizontal path to show a very nice rotational final state of chair plus demonstrator.

This final state of rotation may be concisely explained by realizing that the linear momentum of the ball in the last case represents an angular momentum about the chair's vertical rotation axis.

Since there were no external torques about this rotation axis to explain this new angular momentum of chair plus thrower, one can invoke conservation of angular momentum of chair plus ball to explain the effect.

One may of course also view this as an example of the Newton's Third Law reaction force of the ball on the arm causing the demonstrator plus chair to rotate.

One needs to throw the ball carefully to avoid damage to other inhabitants of room.