Waves Carry Energy, Not Water
- Physics
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- A wave carries energy from place to place without carrying the material along.
- Amplitude, wavelength and frequency describe a wave; bigger amplitude means more energy.
- For a wave moving at a fixed speed, higher frequency means shorter wavelength.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
The wave moves on; the water stays
Drop a stone into a still pond and rings of ripples spread outwards. It looks as if water is flowing away from the splash, but it is not. Put a cork on the water and watch it as the ripples pass. It bobs up and down but stays in about the same place. What travels outwards is energy, not water. Each bit of water pushes on the next and passes the push along, like people doing a wave in a stadium without leaving their seats. That is what a wave is. In some waves, like a wiggle sent along a rope, the material moves up and down. In others, like sound in air, it is pushed back and forth.
Measuring a wave
Waves can be measured. The amplitude is how high the wave rises above the flat level. A bigger amplitude means more energy, which is why a storm wave can smash a sea wall. The wavelength is the distance from one top, or crest, to the next. The frequency is how many waves pass each second. Shake a spring toy slowly and you get long waves. Shake it quickly and the waves bunch up and get shorter. Sound is a wave that travels about 340 metres every second, and a high note has shorter waves than a low one.
Curriculum links: NGSS MS-PS4-1 · NGSS MS-PS4-2