Push and pull along the line

The idea

In a longitudinal wave the particles of the medium vibrate parallel to the direction of wave propagation; sound waves are longitudinal.

A slinky pulse

You push and pull one end of a slinky along its length. A disturbance runs down the slinky.

As the disturbance passes, how does a single turn of the slinky move?

• Back and forth along the slinky
• Up and down across the slinky
• Round and round in a circle

Show the directions

Turn on Show the directions, then press Play. Watch the red turn.

Parallel to the wave

In the slinky, the turns move along the same line in which the disturbance travels. The air in a sound wave does the same. Such waves are called longitudinal waves.

In a longitudinal wave the particles of the medium vibrate parallel to the direction of wave propagation. Sound waves are longitudinal.

Notes

In a longitudinal wave the particles vibrate parallel to the direction of propagation. Sound waves are longitudinal.

Check yourself

In a longitudinal wave, how do the particles of the medium vibrate?

Which kind of wave is a sound wave?

In the tube with the oscillating piston, how do the air particles move?

A mark on a slinky turn moves to and fro along the slinky as a disturbance passes. What does this show?

• Parallel to the direction of wave propagation — correct. Yes!
• Perpendicular to the direction of wave propagation. Particles that vibrate perpendicular to the direction of propagation belong to a transverse wave.
• In the direction of propagation, travelling with the wave. The particles only vibrate about their mean positions. They do not travel with the wave.
• A wave where particles vibrate across the direction of travel. In sound waves the particles vibrate back and forth parallel to the direction of propagation.
• A longitudinal wave — correct. Yes!
• A wave with no particles moving at all. The particles of the medium do vibrate in a sound wave. They vibrate parallel to the direction of propagation.
• Up and down across the tube. The particles vibrate parallel to the direction of propagation, which is along the tube.
• Round and round in circles. In sound waves the particles vibrate back and forth parallel to the direction of propagation.
• Back and forth along the tube — correct. Yes!
• The turns vibrate parallel to the direction of the disturbance — correct. Yes!
• The turns vibrate perpendicular to the direction of the disturbance. The mark oscillates parallel to the direction of the disturbance.
• The marks travel from one end of the slinky to the other. Each turn oscillates about its own position while the disturbance travels from one end to the other.