Transverse Pulses and Waves
Stretch a slinky along the floor, pull it sideways and let go. A shape runs the whole length of the spring, bounces off the far hand and comes back.
That shape is a pulse, and everything in this topic is built out of it. Five experiments, two chapters, one spring and a piece of wool.
A pulse is one disturbance. A wave is a series of them
A pulse is a single disturbance that moves along or through a medium, while the overall position of the medium does not change.
That last clause is the half everybody skips, and it is the interesting half. When the pulse has been and gone, the spring is exactly where it started. Nothing has been carried from one end to the other except the disturbance itself.
A wave is a regular, continuous disturbance that travels from one place to another even though the particles of the material do not travel. So a wave is a succession of pulses. Understand the pulse and the wave comes free.
The medium is the material the pulse travels through. Here it is the spring. For sound it is the air. For a stadium wave it is people.
Why it is called transverse, and how to prove it in one minute
In a transverse pulse the particles of the medium move at right angles to the direction the pulse travels.
Every textbook draws that. Here is how to show it.
Tie a piece of wool to a single coil of the spring, well ahead of where the pulse starts. Send a pulse. Watch the wool, not the pulse.
The wool moves up and back down, and stops where it began. The pulse crosses the room; the wool goes nowhere. The pulse moves along the spring, the wool moves across it, and the two directions are at right angles.
That is the whole definition, demonstrated instead of asserted, and it costs a piece of wool.
Tell the class to watch the wool before they start. Everyone instinctively follows the pulse, and they will have to run it again.
And it shows what a pulse actually carries
The wool came back to its starting position, so no material travelled along the spring. A pulse transports energy, not matter. That sentence earns marks and this experiment is where it stops being something to memorise.
Two velocities, and they are not the same thing
| What it is | Behaviour | |
|---|---|---|
| Pulse velocity | How fast the disturbance travels along the spring | Constant |
| Particle velocity | How fast one coil of the spring moves up and down | Changes as different parts of the pulse go past |
The pulse crosses the room. The particle goes up and comes back down. Confusing the two is one of the standard errors in this topic.
What sets the speed, and what does not
Not the amplitude, and not the pulse length. A big pulse and a small pulse travel at exactly the same speed, which surprises people.
The speed depends on two properties of the spring itself:
- The tension in the spring
- The mass of the spring divided by its length
Both change when you stretch the spring more or less, so a class comparing trials has to keep the stretch the same or nothing is comparable.
Superposition: what happens when two pulses meet
Send a pulse from each end at the same time and watch where they cross.
| Pulses sent | Where they meet |
|---|---|
| On the same side | The displacement is bigger. The two add together |
| On opposite sides | The spring goes momentarily straight. Equal and opposite displacements cancel to zero |
Superposition is the addition of two effects at the same place at the same time. That is the whole definition. The displacements the two pulses would each produce simply add.
And then the pulses carry on, unchanged.
This is the bit learners get wrong. When two pulses cancel, they have not destroyed each other. The cancellation lasts an instant, and both pulses emerge from the other side with their original shape and amplitude. Watch past the meeting point and you can see them come out again.
Stand a few paper cups alongside the spring where the pulses will meet. The eye needs something to judge the displacement against, and without a reference the whole thing happens too fast to read.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 10 |
| Term | 2 |
| Topic | Waves, sound and light |
| Status | Class experiments. Not a formal assessment |
| Marks | 40 on our worksheet. None is prescribed |
From pulses to waves
Stop making single pulses and move your hand from side to side at a steady rate. A repeating pattern travels along the spring. That is a wave.
The vocabulary, and one definition that costs marks every year
| Term | Definition |
|---|---|
| Crest | Where the displacement is a maximum in the positive direction |
| Trough | Where the displacement is a maximum in the negative direction |
| Amplitude (A) | The maximum distance a particle moves from its REST position |
| Wavelength (λ) | The distance between two consecutive points exactly in phase. Crest to crest, or trough to trough |
| Period (T) | The time for one full wave to pass a given point |
| Frequency (f) | Waves per second. f = 1/T |
Amplitude is measured from the rest position, not from crest to trough.
Crest to trough is two amplitudes. It is the most common error in the whole topic, it looks right on a diagram, and it costs a mark every single time.
Move your hand faster and the wavelength gets shorter. Worth doing in front of the class, because it makes f = 1/T something they have watched rather than something they were given.
The five experiments, and they all use one spring
| Experiment | What it needs beyond the spring |
|---|---|
| 1. Make a pulse | A tape measure |
| 2. Watch one coil as the pulse passes | A piece of wool. The best one |
| 3. Superposition, same side | Paper cups |
| 4. Superposition, opposite sides | Paper cups |
| 5. Make waves | A stopwatch |
You also need three learners and a smooth floor. Both are genuinely on the apparatus list. Carpet kills a pulse, so if the classroom is carpeted, use the passage.
Making the pulse
- Two learners hold the ends and stretch the spring along the floor, as far as it will go without damage
- A third places a foot about 50 cm from one end, touching the side of the spring but not standing on it
- The third learner pulls the spring sideways a measured distance and lets go, keeping the foot in place
- Watch, and record
Two measurements come out of it: the pulse length, front to back, and the amplitude, how far it was pulled aside.
If it does not work
| What you see | What caused it |
|---|---|
| The pulse dies before it reaches the far end | Carpet, or the spring is not stretched enough. Find a smooth floor |
| A pulse reflects halfway along | There is a kink in the spring. Uncoil it fully and check before starting |
| The pulse is a mess, not a clean shape | The spring was jerked rather than pulled aside and released |
| The two pulses never meet in the middle | The releases were not simultaneous. It takes several attempts and that is fine |
| Learners think the cancelling pulses were destroyed | They pass through and carry on. Keep watching past the meeting point |
| The wave has no clear wavelength | The hand is moving irregularly. A steady rate matters more than a fast one |
| Nobody can see the displacement where the pulses meet | Use the paper cups. The eye needs a reference |
| The spring is permanently stretched out of shape | Overstretched, and it cannot be repaired. Say so before you hand it out |
The failure worth keeping
The whole class will watch the pulse during Experiment 2 instead of the wool.
Let it happen once. When they run it again watching the wool, the point lands much harder, because they have just felt how strong the instinct is to follow the thing crossing the room.
If you have a ripple tank
The spring shows superposition along a line. A ripple tank shows it in two dimensions, with two dippers producing circular waves that interfere across the whole surface, giving the familiar pattern of constructive and destructive lines.
It is the better demonstration of interference if your school owns one. It is not what this chapter asks for, and a spring costs a fraction of the price, so the spring method above is the one to plan the lesson around.
How the 40 marks are made up
| Section | Marks |
|---|---|
| Pulse: measurements and definitions | 6 |
| Watching one coil, and why it matters | 8 |
| Pulse velocity versus particle velocity | 5 |
| Superposition, both cases | 8 |
| Crest, trough, wavelength and amplitude on a diagram | 8 |
| Period and frequency | 3 |
| Conclusion | 2 |
No mark allocation is prescribed. The source sets five numbered experiments with no marks attached, so the worksheet and this split are ours.
The marks most often dropped: measuring amplitude crest to trough, naming amplitude as something that affects pulse speed when it does not, and saying that two cancelling pulses destroy each other.
If you have time
Change the stretch and time the pulse again. A tighter spring carries a faster pulse, which is one of the two factors that actually matter. Time it over a measured distance and you have a speed.
Send a big pulse and a small one. They arrive together. Nobody believes amplitude has no effect on speed until they watch it.
Do a stadium wave across the classroom. People are the medium, each person moves only up and down, and the wave travels round the room. Same physics, no apparatus, and it is the version they will still remember in the exam.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, with the pulse measurements, the wool observation, both superposition cases and a wave diagram to label
- Marking memorandum, with model answers and the five places learners most often drop marks
Related practicals
- The Doppler effect, Grade 12. Where waves go next
- Snell's Law, Grade 11. Waves and light
- What is a tuning fork. Sound waves, and how to make one visible
Buy this experiment
We are putting together a Waves and Pulses Kit for Grade 10 with the spring, tape measure, stopwatch, wool and cups in one box, plus a printed teacher guide and the marking memo. Coming shortly.
The spring is the only thing that matters. A slinky helical spring runs all five experiments across both chapters, and everything else in the practical is wool, paper cups and a tape measure you already own. Springs and the rest of the mechanics range are in force, motion and dynamics.
Buy two if you can. One spring runs one group, and a class of thirty working in eight groups will otherwise spend the period queueing.