Waves: Transverse and Longitudinal
You are floating on a pool raft at a Fourth of July party when a cannonball splash sends ripples across the water. A moment later your raft bobs up and down. The splash happened far away, so what actually reached you? The water did not rush across the pool. Something else did. Let's find out what.
Waves transfer energy, not matter
A wave is a disturbance that carries energy from one place to another. The material the wave moves through is called the medium. Water, air, a rope, and a spring can all be mediums.
Try this with a long rope tied to a post. Shake one end up and down once. A bump travels along the rope toward the post. Each piece of rope moves up and back down, but it stays near where it started. The bump, which is the energy, is what travels.
Why does it work this way? Each particle pushes or pulls on the next one, like a row of dominoes. The push moves down the line, but each domino stays put.
Does a wave transfer energy, matter, or both from one place to another? Explain your answer using a rope shaken up and down as an example.
Does a wave transfer energy, matter, or both from one place to another? Explain your answer using a rope shaken up and down as an example.
Transverse waves
In a transverse wave, the particles of the medium move at right angles (perpendicular) to the direction the wave travels.
Shaking a rope up and down makes a transverse wave. The rope moves up and down, and the wave moves forward. Waves on a guitar string are another example.
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Crest: the highest point of the wave.
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Trough: the lowest point of the wave.
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Wavelength: the distance from one crest to the next crest.
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Amplitude: how far the medium moves from its rest position. A bigger amplitude means the wave carries more energy.
Name the parts of a transverse wave called the crest and the trough. In a longitudinal wave, what are the regions called where the particles are squeezed together and…
Name the parts of a transverse wave called the crest and the trough. In a longitudinal wave, what are the regions called where the particles are squeezed together and spread apart?
Liam shakes one end of a long rope up and down while the other end is tied to a post. A wave travels along the rope toward the post. (a) What direction does the rope…
Liam shakes one end of a long rope up and down while the other end is tied to a post. A wave travels along the rope toward the post. (a) What direction does the rope move? (b) What direction does the wave energy travel? (c) What type of wave is this? Explain how you know.
Longitudinal waves
In a longitudinal wave, the particles move back and forth parallel to the direction the wave travels.
Push and pull one end of a Slinky along its length. The coils bunch up, then spread out, and that pattern moves down the spring. Sound is a longitudinal wave: a speaker pushes air particles together and pulls them apart.
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Compression: a region where the particles are squeezed together.
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Rarefaction: a region where the particles are spread apart.
After the wave passes, each particle returns to about where it started. The particles are not used up or carried away.
| Transverse | Longitudinal | |
|---|---|---|
| Particles move | perpendicular to wave direction | parallel to wave direction |
| Features | crests and troughs | compressions and rarefactions |
| Example | rope, water surface | sound, Slinky pushed and pulled |
Define a transverse wave and a longitudinal wave. Give one example of each.
Define a transverse wave and a longitudinal wave. Give one example of each.
A speaker plays a low note. Air particles near the speaker move back and forth, forming compressions and rarefactions that travel to Priya's ear. Explain why this is a…
A speaker plays a low note. Air particles near the speaker move back and forth, forming compressions and rarefactions that travel to Priya's ear. Explain why this is a longitudinal wave, state the direction the particles vibrate compared with the direction the energy travels, and explain what happens to the air particles after the wave passes.
Olivia pushes and pulls one end of a long coiled spring toy (a Slinky) along its length, and then, in a second trial, she shakes the end side to side. Compare the two…
Olivia pushes and pulls one end of a long coiled spring toy (a Slinky) along its length, and then, in a second trial, she shakes the end side to side. Compare the two trials. For each, draw or describe the motion of the coils, name the wave type, label the features (compressions and rarefactions, or crests and troughs), and state the direction of energy transfer. Explain one way the two waves are similar.
Measuring waves and following the energy
Two numbers describe how fast a wave repeats. Treat these as vocabulary: you only need to read and use them in simple cases.
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Wavelength: the distance between two crests, in meters (m).
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Frequency: how many waves pass a point each second, in hertz (Hz). 1 Hz = 1 wave per second.
Now follow the energy. A cork on a lake near Lake Michigan's shore bobs up and down while the waves pass. The cork stays in about the same place, which shows the water itself is not moving toward shore. The energy travels, passing from water particle to water particle, until the wave breaks on the beach. There the energy turns into motion of sand and water, sound, and a little thermal energy.
A wave on a stretched spring has crests that are $0.40\ \text{m}$ apart. Ethan counts $3$ crests passing a point every second. (a) What is the wavelength? (b) What is…
A wave on a stretched spring has crests that are $0.40\ \text{m}$ apart. Ethan counts $3$ crests passing a point every second. (a) What is the wavelength? (b) What is the frequency in hertz? (c) Is a wave with a larger amplitude carrying more or less energy than one with a smaller amplitude, if the wavelength is the same?
A cork floats on the surface of a lake near Lake Michigan's shore. Waves pass by, and Mateo notices the cork bobs up and down but stays in about the same place. (a)…
A cork floats on the surface of a lake near Lake Michigan's shore. Waves pass by, and Mateo notices the cork bobs up and down but stays in about the same place. (a) Identify the wave type. (b) Explain what the cork's motion tells you about whether matter travels with the wave. (c) Describe how the wave's energy reaches the shore and what happens to the energy when the wave breaks on the beach.
Back to the start: the cannonball splash sent energy across the pool, not water. Your raft bobbed because the energy reached it.
Summary
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A wave transfers energy from place to place, not matter.
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The particles of the medium vibrate in place and return to their starting positions.
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Transverse waves: particles move perpendicular to the wave direction; they have crests and troughs.
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Longitudinal waves: particles move parallel to the wave direction; they have compressions and rarefactions. Sound is one.
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Wavelength is measured in meters, frequency in hertz, and a larger amplitude means more energy.