Multiple choiceQuestion 1 · Recall
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By the end you can explain what wave motion means, describe it with a rope, a spring and a ripple tank, and state clearly what a progressive wave transfers and what it does not.
The main symbols used in this lesson:
| Symbol | Meaning |
|---|---|
| distance travelled by the wave | |
| time | |
| speed of the wave |
Drop a stone into a pond. Rings spread outwards across the surface.
Put a rubber duck where the rings will pass. The duck bobs up and down as each ring goes by, but it does not get carried to the edge of the pond.
The pattern moves outwards. The water does not.
That is the central idea of a wave: a disturbance travels through a medium, while the particles of the medium only vibrate about their own resting positions.
Progressive wave
A progressive wave is a wave that transfers energy from one place to another without transferring matter.
As it passes, each point of the medium oscillates about its equilibrium position. The oscillation reaches neighbouring points a little later, so the disturbance appears to travel.
Hold one end of a rope and flick it. The flick lifts the first short section of rope. That section pulls on the next one, lifting it slightly later, and so on down the rope.
Each section does exactly what its neighbour did, just a moment later. The shape moves; the rope does not.
A line of people passing a bucket along works the same way: the bucket moves from person to person, but each person stays put and only moves their arms.
The analogy stops at the bucket, because a wave has no single object being passed — what is passed is energy and the pattern of vibration.
The syllabus names three standard demonstrations. They all show the same physics.
| Demonstration | What you do | What the medium does |
|---|---|---|
| Rope | Flick or shake one end | Rope moves across the wave direction |
| Spring | Push and pull one end along the spring | Coils move along the wave direction |
| Ripple tank | Dip a vibrating bar into shallow water | Water surface moves up and down |
In the rope, the oscillation is at right angles to the direction the wave travels. That is a transverse wave.
In the spring, the coils oscillate along the same line as the wave travels. That is a longitudinal wave.
Both are progressive waves, and both carry energy outwards.
Worked example: Worked example: how fast does the disturbance travel?
A pulse travels along a rope. It passes a mark from the flicked end after .
Calculate the speed of the pulse.