A Wave Moves, the Rope Doesn't

Lesson 1 of 10 · 5 min · Free, no account needed

What you will learn

A wave is a disturbance that travels: the hump moves, the rope stays where it is.
Each bit of the medium (rope, air, water, people in seats) only moves back and forth around its own place.
A wave carries energy from one place to another, not stuff.

A stadium wave goes all the way round the arena. Did anyone leave their seat?
The wave is coming from the left. Make your guess on the picture.
Flick the end of the rope. Where does the red bead go?
Drag the hand up and back down, quickly. Do it a few times.
Back to the stadium. Play 5 seconds of the wave.
Keep your eyes on the red cap.
The words for what you saw.
A wave is a disturbance that travels from place to place: the hump.
The medium is the stuff it travels through: the rope, the people, water, air. Each bit only moves back and forth around its own spot.
So a wave carries energy (it can ring a bell far away), but no stuff travels with it.
Simplified model: an ideal rope, the hump keeps its shape. A real rope loses a little energy on the way.
Game: Ring the bell. Send a hump that rings it, but does not knock it off.
The green band on the post is the bell's range. The ruler by the hand shows how high you lift.
Draw the path of the red bead while this hump passes it.
Each bit of rope is pulled up by its neighbour, then pulled back down. It never gets pulled along, so its path is a short up-and-down line. Only the shape travels.
Your bell hump against a train of humps. Which delivers more?
One hump carries one parcel of energy. Shaking again and again sends a new parcel each time, so more energy arrives at the bell, but the rope itself still goes nowhere.

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