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Longitudinal Wave

Waves oscillating in the same direction they are traveling.

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Longitudinal Wave

You’ve probably played with Slinky spring toys like these. They’re simple toys, but they can move in very interesting ways. Pushing in on the end of a spring toy, for example, gives it energy that moves through the spring in a longitudinal wave.

What Is a Longitudinal Wave?

A longitudinal wave is a type of mechanical wave. A mechanical wave is a wave that travels through matter, called the medium. In a longitudinal wave, particles of the medium vibrate in a direction that is parallel to the direction that the wave travels. You can see this in the Figure below. The person’s hand pushes and pulls on one end of the spring. The energy of this disturbance passes through the coils of the spring to the other end. You can see a video of a longitudinal wave in a spring at this URL: http://www.youtube.com/watch?v=ubRlaCCQfDk.

Diagram of a longitudinal wave

Compressions and Rarefactions

Notice in the Figure above that the coils of the spring first crowd closer together and then spread farther apart as the wave passes through them. Places where particles of a medium crowd closer together are called compressions, and places where the particles spread farther apart are called rarefactions. The more energy the wave has, the closer together the particles are in compressions and the farther apart they are in rarefactions.

P Waves

Earthquakes cause longitudinal waves called P waves. The disturbance that causes an earthquake sends longitudinal waves through underground rocks in all directions away from the disturbance. P waves are modeled in the Figure below.

Primary waves are longitudinal waves

Q: Where are the compressions and rarefactions of the medium in this model of P waves?

A: The compressions are the places where the vertical lines are closest together. The rarefactions are the places where the vertical lines are farthest apart.

Summary

  • A longitudinal wave is a type of mechanical wave, or wave that travels through matter, called the medium. In a longitudinal wave, particles of the medium vibrate in a direction that is parallel to the direction that the wave travels.
  • Places where particles of the medium crowd closer together are called compressions. Places where particles of the medium spread farther apart are called rarefactions.
  • Earthquakes cause longitudinal waves called P waves, which pass through underground rocks.

Explore More

Use these resources of longitudinal waves to answer the questions that follow: http://www.physicsclassroom.com/mmedia/waves/lw.cfm and http://www.physicsclassroom.com/Class/waves/u10l1c.cfm.

  1. What is a wave?
  2. Describe a transverse wave.
  3. Define and describe a longitudinal wave.
  4. How is energy transported in a longitudinal wave?
  5. Describe a longitudinal wave flowing through a slinky.
  6. Give examples of a longitudinal wave.

Review

  1. What is a longitudinal wave?
  2. Draw a sketch of a longitudinal wave. Use arrows to show the direction of the wave and the direction of the vibrating particles of the medium. Add labels to identify the compressions and rarefactions.
  3. Describe P waves.

Vocabulary

longitudinal wave

longitudinal wave

Wave in which particles of the medium vibrate in the same direction that the wave travels.

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