At the recent Cornwall teacher workshop on at Truro School, we had a sharing session where each delegate could bring their best ideas or demonstrate a teaching tool. Ben Lloyd-King from Richard Lander brought along his new ‘Heath Robinson’ wave machine. Simply constructed from wooden dowel, sandwiched between two layers of duct tape, it provided the added flexibility of wooden clothes pegs. These pegs are easily clipped on to the dowel to provide extra “mass-per unit length”. This then reduces the speed of the mechanical wave, given by the formula:
v = Ö(T/µ)
where v = speed or velocity, T = tension of the duct tape, µ = mass per unit length
If the pegs are attached just to half the length of the wave machine, there is a change of speed. This can simulate the change of speed that occurs when seismic waves enter a region of different density, or acoustic waves travel from one medium (air) to another (water).
Observations that can easily be made by pupils:
- there is a partial transmission and partial reflection at the boundary between the two different speed sections;
- the phase change on reflection at the ends of the wave machine can be altered by:
- holding the end dowel rigid (reflection is anti-phase)
- holding the end of duct tape and allowing free movement of the last dowel (no phase change on reflection)
- Standing wave scan be set up by adjusting the frequency; the demonstration is particularly dramatic and would easily hold the attention of pupils (as it also did for the Physics teachers!).
