Progressive Waves
Progressive Waves
- A wave is a disturbance that travels, carrying energy from one place to another.
- The material it travels through does not travel with it. Each small part only vibrates about a fixed point.
What a wave moves
- Pluck a guitar string and it shakes the air next to it. That shaking spreads out as sound.
- Drop a stone in a pond and ripples move outwards, but a floating cork only moves up and down as they pass. It is not carried to the edge.
- So the water is not travelling. What travels is the energy: each particle pushes the next one, and the disturbance moves forward.
cork oscillates
about one pointenergy travels →A wave carries energy, not material. The medium does not travel; it only vibrates.
Definition
2 marksWhat is meant by progressive wave?
Model answer: A travelling disturbance that transfers energy from one point to another without a net transfer of matter.
Your turn9702/12/M/J/22 Q21
Which statement is correct for every progressive wave? (9702/12/M/J/22 Q21)
Show worked answer
It transfers energy from one place to another, without a net transfer of the medium.
Waves that need a medium, and waves that do not
- Mechanical waves need a material to travel through: sound, waves on a string, water waves, seismic waves. No material, no wave, which is why sound cannot cross empty space.
- Electromagnetic waves (light, radio, X-rays) are different: they can travel through a vacuum. We come back to these near the end of the chapter.
Common mistake
Saying “the water moves along with the wave” loses the mark. The particles only oscillate about a fixed point; the wave (the energy) is the thing that travels.