Transverse waves on a spring

Class experiment

Students, working in pairs, observe and try to explain what happens to wave pulses on a long narrow spring. This could be just one station in a circus of wave experiments.

Apparatus and Materials

For each student pair:
• Long narrow spring (if not available, use a Slinky spring)
• Metre rule
optional
• String
• Curtain ring, large nylon
• Retort stand base, rod

Health & Safety and Technical Notes

Long narrow springs do not become entangled as easily as Slinky springs do. Because the spring is narrow and closely wound, the shape of pulses is easy to see.

Each pair of students needs a long narrow space to work in, such as a corridor. It is usually best to work on the floor rather than on a bench.

A complementary experiment involves sending waves and ripples along a long shallow trough of water. This requires about 2 metres of flat-bottomed guttering (resting directly on a bench), with both ends closed by end stops. Because waves of many sorts can be made, and they travel slowly, the device is good for observing how wave pulses superpose and pass through one another without being affected.

Read our standard health & safety guidance

Procedure

You will need to produce pulses in the spring. It is easier to see what is happening if you make single hump-like pulses, by giving the end of the spring a single sideways flick.

1. Observe the pulse as it travels, with a view to answering questions
such as:
• Does the speed depend on the shape of the pulse - its height or length?
• Does the speed depend on how rapidly you flick the end of the spring?
• Does the speed depend on the spring - how could the speed be made larger or smaller?
• Does friction make any difference to the speed of the pulse? to its shape?
• What decides the shape of a pulse?
• What happens when pulses, starting from opposite ends of the spring, meet?
• What happens when the pulse reaches the far end of the spring, and that end is not free to move?
2. If you have time, try the following:
• Attach a large nylon curtain ring to the end of the spring and slide the ring onto a retort stand rod. When the pulse reaches this end, the end is free to move. What happens to the pulse?
• Tie a piece of thick string or cord onto the end of the spring. What happens to the pulse as it moves from the spring to the string or vice versa? Can you explain this?

Teaching Notes

• You may need to revise some basic terms before students begin this investigation, through class questioning:
• Define the terms wavelength , frequency , and amplitude for a wave profile (e.g. the one above).
• What is meant by the equilibrium position and the displacement of a point such as P?
• What determines the frequency of the wave, and what unit is frequency measured in?
• How are wave speed, wavelength, and frequency related? How does P move as the wave travels along?
• After carrying out these investigations, a plenary might usefully consolidate the following conclusions:
• The shape of a pulse on a spring is determined by the nature of the flick creating it: a quick flick gives a short pulse, whereas a slow flick gives a long pulse.
• Friction makes a pulse grow smaller in amplitude as it travels – its energy spreads out to its surroundings.
• The speed of a pulse is not determined by its shape, nor on how you flick the spring to create it.
• The speed does depend on the spring - and on the tension with which it is held. The speed increases as the tension is increased.
• When pulses meet they superpose - the displacements that each pulse alone would cause on the spring add together; but when the pulses pass beyond each other they continue with their original shape - see below.

• When a series of pulses is reflected, the returning pulses form a stationary pattern as they superpose with those pulses still moving outward (see below).

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