Building and using a wave machine

Demonstration

A wave machine is an entertaining way of introducing some basic ideas about wave motion.

Apparatus and material

For the demonstration
Duct tape
Clamps and stands, G clamps
Barbecue skewers
Jelly babies (jelly beans)
Metre rule
Stopwatch or stopclock
A 5 m wave machine will require about 100 skewers and 200 jelly babies.
Commercially-manufactured wave machines are available, but the attraction of this home-made version is that its construction and mechanism are clear to students.

Health & Safety and Technical notes

Students should be advised to take care when handling pointed skewers. They should be instructed not to eat the jelly babies, which should be disposed of safely after use (the jelly babies, not the students

Procedure

You can make a wave machine in advance, ready for demonstrating to your class. Alternatively, you can make it on the spot, or students can make their own.

This film shows how to construct a wave machine and shows briefly how to use it to demonstrate aspects of wave behaviour.

Wave machine

a Use clamps and stands to construct rigid horizontal fixing points at either end of a long bench.

b Attach a length of duct tape between the fixing points. The tape should be taut and have its sticky side upwards.

c Position skewers at equal spacings (approx. 5 cm) along the tape.

d Add a second layer of tape on top, to hold the skewers in place.

e Impale a jelly baby on each end of each skewer. Adjust their positions so that the tape remains horizontal.
(Note: Omitting the second length of tape will allow you to adjust the positions of the skewers, but they are more likely to come loose.)

Teaching notes

1 As with the ripple tank, it helps if you start by demonstrating single pulses before going on to continuous waves. Deflect a section of the machine close to the end or you will get pulses running in opposite directions. Try to avoid setting up standing waves which can be confusing.
Show that a pulse/disturbance travels along the line. Point out that the jelly babies are displaced up and down (vertically) while the wave travels along horizontally.

2 You can demonstrate the following:

• The basic principle of a wave: a vibrating source sends a disturbance through a medium. The wave travels, transferring energy, but the medium doesn’t move.
• A wave reflects when it meets a fixed end.
• Make bigger or smaller pulses – relate this to amplitude, and to energy, which is being transferred by the wave. Greater amplitude = greater energy.
• Make a faster disturbance – this makes a shorter pulse, but the speed is unchanged.
• Show that continuous disturbance by the source results in continuous waves.
• Emphasise the meanings of amplitude, wavelength, frequency and wave speed.
• Frequency and amplitude depend on the source; speed and wavelength depend on the medium.

3 You can adapt the machine to show that the speed of a wave changes when it moves into a different medium by adding or removing mass. A stopwatch is adequate for timing a wave to deduce its speed, but note that waves travel very quickly along a machine which has no jelly babies on its skewers.

4 Students could use the machine to investigate the relationship between wave speed, frequency and wavelength (speed = frequency ´ wavelength). They could also change various factors and find out how wave speed changes: separation of the skewers, mass/number of jelly babies, position of jelly babies on skewers, width of tape, thickness of tape, etc.
5 You could photograph the machine from the side to examine how the displacements of adjacent elements vary along the wave.

Related guidance

A wave is a disturbance that propagates through a medium, transferring energy. Water waves are familiar, but in science we extend the idea to other types of wave in other media.
A wave can only travel if the elements of the medium are connected in some way; in this case, each rod affects the next by twisting the tape that connects them.

Waves are characterised by their amplitude and frequency (determined by the source) and their speed (determined by the medium). The wavelength depends on the frequency and the speed.

Related experiments

A discussion of the importance of experimenting with waves: http://www.practicalphysics.org/why-experiment-waves.html
Some basic wave experiments: http://www.practicalphysics.org/variety-waves.html
A sequence of experiments on mechanical waves: http://tap.iop.org/vibration/progressive/page_39783.html