Course 14.1Lesson 1 of 3
A wave carries energy and a pattern from place to place, while the medium only moves about its own place. In a transverse wave the particles move across the direction of travel. In a longitudinal wave they move along it. Mechanical waves need a medium.
Try the code and the simulations beside the lesson.
Tie one end of a long rope to a post. Hold the other end and flick your hand up and down once. A hump runs along the rope to the post. Did the rope travel to the post? No. Each bit of rope went up and came back down. Only the hump travelled.
This is a wave. A wave is a disturbance that moves through a medium. It carries energy and a pattern from one place to another. The medium itself does not travel along. Each part of it only moves about its own place.
Drop a stone in a pond. Rings of ripples spread out. A leaf on the water bobs up and down, but it does not ride out with the rings. A stadium wave works in the same way. Each person stands up and sits down in turn. The wave goes round the stadium, but every person stays in their own seat.
The parts of a medium are joined to their neighbours. Pull one bit of rope up, and it pulls the next bit up a moment later. That bit pulls the next one, and so on. Two things decide how a wave moves. The medium must pull back when it is disturbed, and it must have inertia.
A wave like this needs a medium. We call it a mechanical wave. Waves on ropes, ripples on water and sound are mechanical waves. Sound cannot cross empty space. Light can, because light is not a mechanical wave. It is an electromagnetic wave, and it needs no medium.
Look at the rope again. Each bit of rope moves up and down, while the wave moves along the rope. The motion of the particles is at right angles to the direction of travel. This is a transverse wave. The highest points are crests and the lowest points are troughs.
Now stretch a slinky along a table and push one end forward and back. Regions where the coils crowd together run along it. Between them are regions where the coils are spread out. Each coil moves to and fro along the slinky, in the same direction as the wave. This is a longitudinal wave. The crowded regions are compressions. The spread-out regions are rarefactions.
Sound in air is a longitudinal wave. The air in front of a drum skin is squeezed, then spread out, again and again. These compressions and rarefactions travel to your ear.
A transverse wave needs a medium that resists a change of shape. A solid does this. So a solid can carry both kinds of wave. A gas or a liquid resists being squeezed, but it does not resist a change of shape. So in the bulk of a gas or a liquid, only longitudinal waves travel.
Earthquakes show this well. They send out P waves, which are longitudinal, and S waves, which are transverse. P waves are faster, so they arrive first. S waves cannot pass through the liquid outer core of the Earth. So stations on the far side of the Earth receive P waves but no S waves.
What about ripples on water? The surface of water is a special case. Each bit of water moves in a small circle, partly up and down and partly to and fro. So a water wave is a mix of both kinds.
Every particle in a wave repeats its motion. The time for one repeat is the period . The frequency is , in hertz. The distance from one crest to the next, or from one compression to the next, is the wavelength .
In one period, the wave moves on by one wavelength. So its speed is
Ripples whose crests are apart, passing a leaf times each second, travel at .
The program finds where five points of a rope are at one moment, and again a quarter period later.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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