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.

Why we wrote ‘Cracking Key Concepts in Secondary Science’

From the Introduction

“We strongly believe that the central part of any science lesson or learning sequence is a well-crafted and executed explanation.

“But we are also aware that many – if not most – teachers have had very little training in how to actually go about crafting or executing their explanations. As advocates of evidence-informed teaching, we hope to bring a new perspective and set of skills to your teaching and empower you to take your place in the classroom as the imparter of knowledge.

“We do, however, wish to put paid to the suspicion that we advocate science lessons to be all chalk and talk: we strongly urge that teachers should use targeted and interactive questioning, model answers, practical work, guided practice and supported individual student practice in tandem with ‘teacher talk’. There is a time when the teacher should be a ‘guide on the side’ but the main focus of this book is to enable you to shine when you are called to be a science ‘sage on the stage’.

[…] “For many years, it seems that teacher explanation has been taken for granted. In a nation-wide focus on pedagogy, activity, student-led learning and social constructivism, the role of the teacher in taking challenging material and explaining it has been de-emphasised, with discovery, enquiry, peer-to-peer tuition and ‘figuring things out for yourself’ becoming ascendant. Not only that, but a significant number of influential organisations and individuals championed the cause of ‘talk-less teaching’ where the teacher was relegated to a near-voiceless ‘guide on the side’, sometimes enforced by observers with a stopwatch and an inflexible ‘teacher talk’ time limit.

“We earnestly hope that such egregious excesses are now a thing of the past; but we must admit that all too often, the mistakes engendered by well-meaning edu-initiatives live on, while whatever good they achieved lies composting with the CPD packs from ancient training days. Even if they are a thing of the past, there has been a collective deskilling when it comes to the crafting of a science explanation – there is little institutional wisdom and few, if any, resources for teachers to use as a reference.”

And that is one reason why we wrote the book.

What follows is an example of how we discuss a teaching sequence in the book.

Viewing waves through the lens of concrete to abstract progression

Many students have a concrete idea of a wave as something ‘wavy’ i.e. something with crests and troughs. However, in a normal teaching sequence we often shift from a wave profile representation to a wavefront representation to a ray diagram representation with little or no explanation — is it any wonder that some students get confused?

I have found it useful to consider the sequence from wave profile to wavefront to ray as representations that move from the concrete and familiar representation of waves as something that looks ‘wavy’ (wave profile) to something that looks less wavy (wavefront) to something more abstract that doesn’t look at all ‘wavy’ (ray diagram) as summarised in the table below.

Each row of the table shows the same situation represented by different conventions and it is important that students recognise this. You can quiz students to check they understand this idea. For example:

  • Top row: which part of the wave do the straight lines in the middle picture represent? (The crests of the waves.)
  • Top row: why are the rays in the last picture parallel? (To show that the waves are not spreading out.)
  • Middle row: compare the viewpoints in the first and middle picture. (The first is ‘from the side’, the middle is ‘from above, looking down.’)
  • Middle row: why are the rays in the last picture not parallel? (Because the waves are spreading out in a circular pattern.)

Once students are familiar with this shift in perspective, we can use to explain more complex phenomena such as refraction.

For example, we begin with the wave profile representation (most concrete and familiar to most students) and highlight the salient features.

Next, we move on to the same situation represented as wavefronts (more abstract).

Finally, we move on to the most abstract ray diagram representation.


‘Cracking Key Concepts in Secondary Science’ is available in multiple formats from Amazon and Sage Publishing. You can also order the paperback and hardback versions direct from your local bookshop 🙂

We hope you enjoy the book and find it useful.

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This is only available if you order directly from SAGE Publishing before 31/12/2021 and some terms and conditions apply (see SAGE website).

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