PhysicsWaves Carry Energy, Not Matter
Lumi the glowing guide floats above a calm blue pool, gently tapping the surface so bright ripple rings spread outward toward a tiny floating leaf that bobs gently in place.
- Explain that a wave moves energy from one place to another while the matter stays roughly in place.
- Identify amplitude as the distance the medium moves from its resting position.
- Identify wavelength as the distance between two matching points on a wave, such as crest to crest.
- Describe frequency as the number of full waves that pass a point each second.
- Predict whether an object on a wave surface will travel with the wave or return near its starting position.
Key terms
- Wave
- A disturbance that carries energy from place to place without carrying the matter along.
- Medium
- The matter, such as water or air, through which a wave's disturbance travels.
- Amplitude
- How far the medium moves from its resting position, signaling the energy carried.
- Frequency
- The number of full waves that pass a single point each second.
Energy Travels, Matter Stays
The most surprising idea about waves is that they move energy without moving the matter along with it. When ripples cross a pond, a floating leaf only bobs up and down and settles back near where it began — the water itself does not rush across. Each bit of water simply nudges its neighbor, passing the disturbance along while staying roughly in place. So the energy reaches the far side, but the matter stays home, which is the defining feature of every wave.
Describing a Wave
Three measurements capture a wave's behavior. Amplitude is how far the medium moves from its resting spot, and a bigger disturbance carries more energy. Wavelength is the distance between two matching points in a row, such as crest to crest. Frequency is how many full waves pass a point each second; a higher frequency packs the waves closer together. Importantly, raising the frequency does not speed the wave up — wave speed depends on the medium, not on how often the waves arrive.
Worked examples
Explain what reaches a floating leaf when ripples cross a pond.
- Watch the leaf: it bobs up and down and settles near its start.
- The water does not flow across, so matter is not what arrives.
- The disturbance passes from bit to bit, carrying only energy.
Answer: Energy travels to the leaf while the water stays in place.
Wave A is 2 Hz and Wave B is 8 Hz in the same pool; compare them.
- Frequency counts waves passing a point each second.
- Wave B's 8 Hz means more waves pass per second than Wave A's 2 Hz.
- Wave speed depends on the medium, so both travel at the same speed.
Answer: Wave B has more waves per second but the same speed.
Activity
Predict what happens to each object when a water wave passes by it.
Practice
Predict whether a floating boat travels with a passing wave or stays in place.
Explain why a wave with bigger amplitude is not necessarily moving faster.
Common mistakes to avoid
- Waves carry matter across the pond.Waves carry only energy; the water bobs in place rather than flowing across to the other side.
- Bigger amplitude means a faster wave.Amplitude signals how much energy the wave carries, while wave speed depends on the medium instead.
Check your understanding
You drop a pebble near a floating leaf across a pond. Ripples reach the leaf and it bobs, then settles. What actually traveled from the pebble to the leaf?
A student says 'a wave with bigger amplitude must be moving faster.' What is wrong with this statement?
On a diagram of a water wave, what does wavelength measure?
Wave A has a frequency of 2 Hz and Wave B has a frequency of 8 Hz. Both travel through the same pool of water. Which statement is true?
Recap
A wave carries energy from place to place while the matter stays roughly in place; amplitude shows the energy carried, wavelength is the distance between matching points, and frequency counts waves per second without changing the wave's speed.
Reflect
How does the bobbing leaf help you remember that a wave moves energy rather than matter?