Waves: Introduce With Trolleys, Not Slinkys

A number of interesting questions came up in the recent author Q&A run by @Chatphysics about my contribution to Cracking Key Concepts in Secondary Science: many thanks to Jinny Bell (@MissB0107) for hosting!

In this post, the question I want to address in more detail is: why do I recommend introducing waves using dynamics trolleys rather than a slinky spring or elastic rope?

I think I have a couple of persuasive reasons; but first a digression.

A thing doing a thing…

Robert Graves wrote his charming comedic poem Welsh Incident in 1929. It describes (at least as I read it) the visitation of disparate group of space aliens to a small town on the North Wales coast in the 1920s.

I will paraphrase one line where the Narrator says:

Then one of things did a thing that was finally recognisable as a thing.

The first example of a thing doing a thing…

Now don’t get me wrong: slinky springs and elastic ropes are brilliant examples of mechanical waves and I do use them (a lot!) — I just don’t use them as the first example.

The problem as I see it is that they could lead students to file wave mechanics in an entirely separate and independent category from mechanics.

I want students to see that wave behaviour isn’t distinct from forces and particles but rather is a direct (and fairly straightforward) consequence of a particular arrangement of particles with a specific pattern of forces between them.

Since the first example, is often the ‘loudest’ (metaphorically speaking), it’s not a bad idea to start with longitudinal waves.

I use standard wooden dynamics trolleys. Dowel rods or metal posts can be used to link the trolleys together. The system is more stable if a pair of springs is used at the front and back of each trolley. The springs used are the ones we typically use for the Hooke’s Law experiment.

A compression carrying energy along a line of trolleys linked by springs can be easily modelled:

Modelling energy transfer by compression of a longitudinal wave using a line of dynamics trolleys and springs

So can a rarefaction:

Using a line of dynamics trolley linked together with spring to demonstrate the transmission of a rarefaction pulse

Transverse waves can be modelled like this:

A transverse wave modelled using a line of dynamics trolleys linked with springs

Amongst the advantages of this approach are:

  • Students are introduced to an unknown thing (wave behaviour) by means of more familiar things (trolleys and springs)
  • The idea that there is no net movement of the ‘particles’ as energy is transferred is much more directly observable using this arrangement rather than the slinky or elastic rope.
  • The frequency of a wave (which in some ways is a more fundamental measurement than wavelength) can be associated with the repeating motion of a single ‘particle’ and extended outwards to the whole system, rather than vice versa.

You can read more in Chapter 25 of Cracking Key Concepts in Secondary Science.

Conclusion

I hope readers will try this demonstation: hopefully introducing students to a thing which is already recognisable as a thing will make wave behaviour more comprehensible and less like an unwelcome diversion into terra incognita.

Readers who are ‘rich in years’ like myself will recognise this demonstration as being adapted from the old Nuffield linear A-level Physics course.

You can listen to Richard Burton’s great reading of Robert Graves’ Welsh Incident here.