Lath with a load

Demonstration

A demonstration experiment that makes a time trace of the oscillation and illustrates damping.

Apparatus and Materials

• Either metre rule or long lath
• G-clamps, large, 2
• clean smooth paper (e.g. roll of wallpaper or computer print-out paper)
• Felt-tip pen or brush with ink
• Rubber bands, 2
• Masses, 0.5kg or 1 kg, 2

Health & Safety and Technical Notes

Read our standard health & safety guidance

Clamp the metre rule so that it oscillates in a horizontal plane. The diagram below shows one possible way of obtaining the time trace.

The lath is clamped firmly at one end to a pair of table legs, about 0.1 m above the floor. In order to slow its rate of oscillation, suitable masses can be attached to its free end with rubber bands or string.

As shown, the paper rests over several layers of soft paper on a wooden board running on steel rollers. This allows the pen to write smoothly despite bumpy movement of the moving board, and is needed if the decay of the oscillations is to be anything like exponential.

Procedure

1. To obtain a time trace, deflect the lath or metre rule slightly to one side and release, while at the same time slowly advancing the paper sheet placed underneath it. The pen on the end of the lath will write on the moving paper. To get a clear time trace, you will need to adjust both the oscillation period and also the rate at which the paper moves.
2. One way of moving the board and paper at a steady rate is to pull it along with a string wrapped round a shaft that is turned by a motor.

Teaching Notes

• The first thing to point out is the sinusoidal shape of the trace produced on the moving paper. The lath or metre rule has a constant period of oscillation. Changing the rate at which the paper moves has the same effect as changing the time-base on an oscilloscope when studying an electrical oscillation.
• If appropriate, ask students if they can recognize the (exponential) shape of the envelope of the oscillation curve from earlier work.

This experiment has yet to undergo a health and safety check.