Jelly Baby Wave Machine

My last six weeks have been spent in placement at Ross High School, Tranent. They were probably the most exhausting, grueling yet rewarding six weeks of my academic life so far. It’s felt like trying to squash all four years of my undergrad degree into a month and a half of intense planning, designing and writing, whilst testing my own personal character in endurance, patience and development. The light at the end of the tunnel has been met with mixed emotions – on the one hand, I’m very excited to be getting back to Moray House and seeing friends and colleagues again, swapping stories and sharing experiences, whilst on the other, I’m sad to be leaving Ross High. Only in the last week or so can I say I found myself really getting to know the pupils, finding my groove and establishing myself in the role of ‘teacher’. However, the next few weeks will involve some more ‘traditional’ work for a student, with the second Curriculum Studies Task seeing us through to the new year, before commencing placement two in January.


I was lucky enough to have a timetable consisting of one class from each year group first to fourth, with varied abilities between and within classes. This provided me with a wonderfully rounded experience, bursting with highlights. One that cries out to be shared comprises a single task carried out by my third years, but served as an excellent demonstration tool for S2 and was shared across the department. This is a STEM derived wave generator, which is cheap and easy to build, and has a wealth of uses in a broad range of curricular areas: the Jelly Baby Wave Machine!


I had a class of 13 mixed ability pupils at the time of construction. The machine itself couldn’t be easier to build – it’s simply a strip of duck tape hung sticky side up between two clamp stands fixed to the bench with G-Clamps. Onto the tape go wooden kebab skewers, about 5cm apart, with jelly babies attached to both ends. I decided to start the lesson with a quick revision of wave characteristics and properties of transverse wave propagation, which we had already covered in previous lessons – alternatively, this would work as a precursor to theory, engaging pupils in the topic with a visual stimulus. I showed a short video clip of how to build it, during which they took notes on what tasks had to be undertaken and their order. We wrote these on the whiteboard and groups were assigned tasks such as:


  1. Organisation – since there is a strong interdependency between groups, it’s important to have some pupils in charge of exchanging information and making sure groups are working at a suitable pace.
  2. Calculations – working out the limiting factor, be it the number of jelly babies, skewers or length of tape available, then using this to determine the parameters of our wave machine. This involved taking information from other groups.
  3. Construction – While some of the more technical, planning elements were being undertaken, other pupils began constructing the clamp stands ready for the tape to be attached. They were fixed to two different benches so that their separation could be increased or decreased easily.
  4. Counting – Jelly Babies and skewers had to be counted out and attached together. To make it easier to position the skewers on the tape, their midpoint was marked with a pen.


During the exercise, two pupils were very reserved and withdrawn from the groups. It was apparent they felt vulnerable about the task – they were unconfident in the subject already and although they didn’t say anything, I think they felt like they were being put on the spot. Asking them to perform with their peers would highlight their knowledge gap, which made them embarrassed. To overcome this, I gave them the role of Director and Producer and asked them to record the task, from construction through to demonstration. This was a no pressure task which facilitated a more intimate role in the proceedings than simply standing at the side. They could listen and interact as they wished and on several occasions, spotted errors made by other students, boosting their self esteem.


The number of tasks related to making the machine is really only limited by your imagination, there is something for every type of learner – hands on construction, mathematical calculations, organisation and some elements of art and design. Even before we have demonstrated the properties of a wave, we have developed some important learning techniques in collaboration and problem solving. Differentiation by task and allowing pupils to establish their own experimental steps and success criteria (AifL), not only gives them the motivation to complete the task, but breaks the learning down into manageable chunks that don’t intimidate them.


Once all the elements were complete, the pupils began constructing the wave machine. The tape, carefully measured to fit all the skewers with a 5cm gap between them, was attached to the clamp stands. Extra tape was left on the ends to create little tabs. A good tip here, to make the model reusable, is to put a small strip of paper on the inside of the loops. This way, they will slip on and off the stands more easily by covering up the sticky part. The skewers were laid on using a ruler as a guide. The emphasis at this point was not in the precision of gap size, but making sure the skewers were aligned perpendicular to the tape. When finished, a second layer of tape was laid over the first, securing the skewers in place and making the whole model more robust. And that’s it. It is simple and quick to make, but the pupils really engaged with the task. Having set their own goals at the beginning, I was able to stand back and watch the task evolve with few interjections.


The wave machine demonstrates the propagation of a transverse wave – one where the particles of the medium bob up and down, but the transfer of energy moves along the wave, side to side. The Jelly Babies are our particles – moving one of the skewers near the end, in a swift up-down motion, twists the tape. This twisting causes the next skewer to move in the same way as the first, and so on, giving us a visual representation of a smooth, sinusoidal wave traveling horizontally along the tape, comprised only of Jelly Babies moving vertically up and down. During the demonstration, I came back to a question I had been asked at the beginning of the task – “Why are we using Jelly Babies?”. It was a good question, which I hoped they would come to answer for themselves on completing the task. Seeing the wave machine in action, pupils came up with several answers: “They’re colourful, which helps us see them better”, “They’re all the same size”, “They’re gooey, making them stick on!”. I was very happy to hear these statements, as this showed their awareness of the mechanics behind the model – the uniformity of the Jelly Babies makes it easier to find a balance point on the skewer, so that the model at rest lies perfectly flat. Corrections can be made by simply sliding the sweets in or out to adjust the centre of mass. A strong enough wave will also reflect at the boundary, displaying the nature of a wave as it rebounds from a fixed end. You can also create two waves, one at each end, and watch as they interact in the middle. Pupils might expect them to cancel out, so are generally surprised to see each wave survive the ‘collision’ and continue down the tape.


In the context of the waves lesson, this was about as much as I wanted to cover. However, I used the wave machine again with a second year class, setting it up myself before the lesson. We discussed many of the same points – energy traveling down the wave, particles up and down – even the specific use of Jelly Babies! Having just introduced refraction in the previous lesson, the class were able to relate the mass of the sweets to the density of the medium through which the wave travels. I had some fantastic questions about replacing the Jelly Babies with elephants and how that would affect the wave. This sparked a discussion where pupils imagined how much more energy it would take to lift an elephant, whether the wave would therefore travel faster or slower and how far would it get before it ran out of energy completely. Next, using their well rehearsed definition of refraction – ‘the changing speed of light as it passes from one material to another’ – we adjusted the model to demonstrate this. By removing Jelly Babies from around about the last third of the tape, you simulate two materials of differing densities. The end with sweets is heavier, like a perspex block, whereas the skewers-only end would represent the air. Now when a wave travels down the tape, pupils can observe any changes as it passes between the two media. The model is good enough to see a clear change in wave speed as it crosses the boundary. In the less dense medium, the wave shape isn’t as smooth, but improves when it reenters the denser material. Though difficult to observe directly, pupils were able to infer this had something to do with a change in wave properties. This can lead to discussions on how an angled transition between materials would cause a direction change – in the case of the wave machine, leave Jelly Babies attached further down one side of the tape than the other – leading to the wave traveling further down the side with fewer sweets, effectively ‘bending’ the wave.


All in all, I got about 4 lessons use out of the wave machine, over two year groups in the Waves and EM Spectrum topics. The pupils loved the interactive nature of the model and questions relating to how waves move could be explored physically rather than simply being told the answer. The cost of materials came to less than ten pounds and the G-Clamps were borrowed from CDT, making this an extremely effective, low cost practical with a huge variety of applications in the science classroom. It is one I will most certainly be carrying forward through my teaching career!


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