Longitudinal Waves

Longitudinal Waves

Middle School

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Learning Objectives

• Define a longitudinal wave and identify it as a type of mechanical wave.

• Describe how particles move when a longitudinal wave passes through a medium.

• Differentiate between compressions and rarefactions in a longitudinal wave.

• Explain how wave energy affects its compressions and rarefactions.

• Recognize earthquake P waves as an example of longitudinal waves.

Key Vocabulary

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Longitudinal Wave

A mechanical wave where medium particles vibrate parallel to the direction the wave travels.

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Mechanical Wave

A wave that requires a medium, which is the matter it travels through, to transfer energy.

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Compression

The part of a longitudinal wave where particles of the medium are crowded closely together.

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Rarefaction

The part of a longitudinal wave where the particles of the medium are spread farther apart.

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P Waves

Longitudinal earthquake waves that pass through underground rocks in all directions away from the disturbance.

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What Is a Longitudinal Wave?

• A longitudinal wave is a mechanical wave that travels through a medium.

• The medium's particles vibrate parallel to the direction the wave travels.

• A Slinky spring toy is a great example of this wave.

• Its coils move back and forth along the spring's length.

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Multiple Choice

In a longitudinal wave, how do the particles of the medium vibrate relative to the direction of wave travel?

• Perpendicular to the direction of the wave

• In a circular motion

• Parallel to the direction of the wave

• They do not vibrate at all

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Compressions and Rarefactions

Compressions

• Compressions are the parts of a longitudinal wave where particles are crowded together.

• These are areas of high pressure and density within the wave's medium.

• A wave with more energy will have particles that are packed more tightly.

Rarefactions

• Rarefactions are the parts of a longitudinal wave where particles are spread apart.

• These are areas of low pressure and density within the wave's medium.

• A wave with more energy will have particles that are spread farther apart.

Multiple Choice

What is the name for the part of a longitudinal wave where particles of the medium are crowded closest together?

• A rarefaction

• A compression

• A crest

• A trough

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P Waves: An Example of Longitudinal Waves

• Earthquakes provide a powerful, real-world example of longitudinal waves.

• Earthquakes cause longitudinal waves called P waves, or primary waves.

• These are the first waves to be detected from an earthquake.

• P waves cause underground rock particles to move back and forth.

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Multiple Choice

What is the name of the longitudinal waves caused by earthquakes that travel through underground rocks?

• S waves

• Surface waves

• P waves

• Ocean waves

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Common Misconceptions

Misconception	Correction
Waves transport matter from one place to another.	Waves transfer energy. Particles of the medium vibrate but stay in their original position.
All waves move in an up-and-down pattern.	This describes transverse waves. Longitudinal waves move in a back-and-forth motion.
Longitudinal waves have crests and troughs.	They have compressions and rarefactions, not the crests and troughs of transverse waves.

Multiple Choice

Why is a P wave classified as a mechanical wave?

• Because it travels in a straight line.

• Because it is caused by a machine.

• Because it requires a medium like rock to travel through.

• Because it has a high amount of energy.

Multiple Choice

If you push the end of a Slinky with a greater amount of energy, how does this affect the rarefactions?

• The particles in the rarefactions will get closer together.

• The particles in the rarefactions will spread farther apart.

• The rarefactions will disappear.

• The rarefactions will change into compressions.

Multiple Choice

An earthquake sends a wave through underground rock. A scientist observes that the rock particles are vibrating parallel to the direction the wave is moving. What can the scientist conclude?

• It is not a wave.

• It is a P wave.

• It is a transverse wave.

• It is a wave that can travel in a vacuum.

Multiple Choice

A geologist studies two P waves. Wave 1 has very dense compressions. Wave 2 has less dense compressions. What can be inferred about the energy of the waves?

• Wave 1 has more energy than Wave 2.

• Wave 2 has more energy than Wave 1.

• The waves have equal energy.

• Energy cannot be determined from compressions.

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Summary

• In longitudinal waves, particles vibrate parallel to the direction of wave motion.

• They are mechanical waves and require a medium like a solid, liquid, or gas.

• They consist of compressions and rarefactions.

• More energy results in more intense compressions and rarefactions.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1

• 2

• 3

• 4