Physics Demo Number: 118

Approximate

Run Time: 5 min

Arm Waving Angular Momentum Conservation With Matchbook-String-Cup

Demo Description

A bottle and a match book are tied together by a string.

The match book is held in one hand, while the bottle is dangled over a horizontally- held finger of the other hand.

Following the directions given below, the match book is released and manages to wrap the string around the horizontal finger of the other hand rapidly enough to arrest the fall of the bottle.

Scientific Principles

• Conservation of Angular Momentum

• Exponential increase of friction between a cord and rod by wrapping cord around rod.

Equipment

• Match book

• Bottle

• String

Equipment Location

• Kit (118) on [F-2-5].

Instructions

The first photo shows the overall view of the demo with a clamp holding the matchbook in the orientation required for the start of the demo. The next photo shows a closeup of the bottle and string.

The last photo shows a closeup of the match book and string.

When the red clamp is replaced by one of demonstrator's hands, and the steel rod (over which the string passes at the bottle end of string) is replaced by a finger on the demonstrator's other hand , then the demo is good to go.

Release of the matchbook results in

1. the bottle falling vertically, while

2. the matchbook falls vertically and

3. the matchbook is simultaneously pulled to the right.

   To the extent that the matchbook acquires angular momentum about the axis of the horizontal supporting finger, the matchbook will start to spin rapidly in counterclockwise motion as the length of string between the finger and matchbook becomes small.

4. The net result will be the string rapidly wrapping around the finger and halting the downward motion of the bottle.

Writeup created by David A. Burba

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