Pulses and continuous waves with a Slinky spring

Demonstration

Use this experiment when introducing transverse and longitudinal waves. Later it might also be used when introducing standing waves or factors that affect wave speed through a medium.

Apparatus and Materials

For the demonstration, or for each pair of students

• Slinky spring
• Rubber tubing

Health & Safety and Technical Notes

Read our standard health & safety guidance

The Slinky spring should be at least 10 cm long when closed up. It is useful to tie a bright ribbon marker on one loop of the spring, so that students can watch how a single loop moves when a pulse passes it.

The length of rubber tubing should be at least 5 metres long, diameter at least 8 mm.

Procedure

Pulses

1. Hold the rubber tubing on the floor under slight tension. Give one end a sharp flick horizontally. This is most easily done by holding a hand against the ankle and then jerking the tubing sideways and back to the foot again.
2. Try different tensions and slower pulses. (Pulses of a different shape can be produced by having a stop such as a chair leg to limit the motion of the tube.)
3. Repeat the same demonstration with a Slinky.

Continuous waves

1. Lay the rubber tube or Slinky on the floor or on a bench.
2. Fix one end and make the other end oscillate transversely by hand, with a small amplitude and a frequency of about 5 cycles per second.
3. Impulses at regular time intervals will produce a continuous travelling wave. This is usually clearer with the rubber tubing.
4. With a Slinky it is also possible to show travelling longitudinal waves by oscillating the end of the Slinky backward and forward.

Teaching Notes

• If your main object with the Slinky is to see how pulses and continuous waves travel, avoid distractions such as producing standing waves or making the Slinky walk down stairs. Pulses of different shapes will be seen if you demonstrate both a rubber tube and a slinky spring.
• You can produce both longitudinal and transverse wave pulses on a slinky spring, to contrast and compare them.

• In transverse waves the particles oscillate at right angles to the direction of travel of the wave. In longitudinal waves the particles oscillate along the direction of travel of the wave.
• One difficulty can arise the graphical representation of a longitudinal wave. Often it is represented in the same way as a transverse wave, and this can be misleading. Remember that the particle displacement, at any particular point in the medium, is really in the direction of travel of the wave. The diagrams below explain what the displacement - distance graph means.

• It is interesting to double and triple the length of the springs and to note that the pulse takes the same time to travel down it. The increased tension needed to extend the spring has resulted in an increase in velocity. For example, double the tension by doubling the length of spring, which halves the mass per unit length. This means the tension/mass per unit length will be four times as great. The pulse must be travelling twice as fast if the time of travel has remained the same. Thus velocity2 = constant x tension / mass per unit length.

This experiment was safety-checked in February 2006