Waves and Sound · Grade 9

If a wave travels towards the shore, why does a cork not travel with it?

Because a wave carries energy, not material. The cork rises and falls on the spot, and so does the water around it: each particle vibrates about its resting position and passes the disturbance to its neighbour. What moves towards the shore is the pattern itself. If the water travelled, the sea would empty.

Learning objectives

What moves and what stays

A wave is a disturbance that travels through a medium and carries energy with it. The important word is disturbance: what advances is a shape, not a substance.

Shake a rope tied to a wall. A pattern runs along the rope, but the rope itself stays where it was — mark a point with a pen and it only goes up and down. No part of the rope ever arrives at the wall.

Exactly the same thing happens at sea. The cork marks one point in the water, and it bobs on the spot while the wave passes through. The energy is handed from particle to neighbour all the way to the shore.

Transverse and longitudinal

In a transverse wave the particles vibrate at right angles to the direction of travel. The shaken rope is the cleanest example: your hand moves up and down while the pattern moves forward. Water waves and light waves are transverse too.

In a longitudinal wave the particles vibrate along the same line the wave travels. A spring pushed and pulled at one end shows it well: squashed and stretched regions form and run along the coil. Sound is a longitudinal wave.

You do not have to memorise a list to classify a wave. Ask one question: which way does a particle vibrate relative to the direction the wave is going? At right angles means transverse; along the same line means longitudinal.

With a medium and without

Most waves need material to travel through, and that material is called the medium. Sound needs air, water or a solid; sea waves need water; a wave on a rope needs the rope.

Electromagnetic waves are the exception. Light, radio waves and heat radiation cross a perfect vacuum, which is why we can see the Sun and the stars but never hear them.

So the loud explosion in a space film is wrong. The light of the blast would reach us; the sound would not arrive at all, because there is no material between the stars to carry it.

Worked examples

  1. A wave runs along a shaken rope. What reaches the far end?

    1. Every point on the rope only moves up and down
    2. No part of the rope travels forward
    3. What advances is the pattern, and with it the energy

    Answer: The energy reaches the end; the material stays where it was

  2. A spring is pushed and pulled at one end. What kind of wave is made?

    1. The coils move back and forth
    2. The wave advances in that very same direction
    3. Vibration is parallel to travel

    Answer: A longitudinal wave, like sound

  3. Why would an explosion on the Moon not be heard on Earth?

    1. Sound is a wave that needs a medium
    2. Between the Moon and the Earth there is vacuum
    3. With no particles, there is nothing to pass the disturbance along

    Answer: There is no medium; the light would reach us and the sound would not

Common mistakes

Thinking the water travels towards the shore
The water oscillates in place. If it really moved with the wave, the whole sea would pour onto the beach within minutes and leave its bed empty.
Assuming every wave needs a medium
Electromagnetic waves cross a complete vacuum. That is precisely why sunlight reaches us across space while sound does not.
Classifying a wave from how it is drawn
Sound is always drawn as an up-and-down curve, yet it is longitudinal. The classification comes from the direction the particles vibrate relative to travel, not from the picture.

What to remember

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