# Mechanical Waves: Production & Propagation

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• 0:00 Definition of Mechanical Waves
• 0:42 Production of Mechanical Waves
• 1:12 Mechanical Wave Propagation
• 2:52 Factors That Affect…
• 3:58 Lesson Summary

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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

What is a mechanical wave? Learn about the types of mechanical waves, how they are produced, and what affects their propagation, then test yourself by taking a quiz.

## Definition of Mechanical Waves

A mechanical wave is a wave that is a vibration in matter, transferring energy through a material. Not all waves are like this. For example, electromagnetic waves such as visible light are not mechanical because they can travel through the vacuum of space to reach us from the sun. Mechanical waves include water waves, sound waves, earthquake waves, and many more. Like all waves, those of the mechanical variety have peaks, or crests, and troughs. They also have a frequency, which is the number of waves that pass by per second, and a wavelength, which is the distance from one peak to the next, or one trough to the next.

## Production of Mechanical Waves

To create a mechanical wave, some initial energy has to be put into it. This is the energy that will then be transferred by the wave. How you provide this energy depends on the medium and type of wave. For example, you could drop a stone in some water to create a water wave. You could also speak loudly to create a sound wave, or you might shake a Slinky up and down to create a wave in the Slinky. But either way, the energy you expend to do the action is what creates the wave and gives it the energy to transfer.

## Mechanical Wave Propagation

Mechanical waves can be divided into three main categories according to the ways in which they travel, known as propagation. The three propagation types are:

A transverse wave is one that vibrates at 90 degrees to the direction the wave is moving. For example, if you hold a Slinky between two hands and shake it up and down, you'll get a wave that moves along the Slinky, but the vibrations will still be up and down. Underwater waves are also transverse.

A longitudinal wave is one in which the vibrations are parallel to the direction the wave is moving. That's like sending a pulse along the length of a Slinky, pushing it lengthwise. Instead of peaks and troughs, longitudinal waves have compressions, areas where the Slinky is bunched together, and rarefactions, areas where the Slinky is spread apart. Another example of a longitudinal wave is a sound wave. Although you can't see air molecules, if you could, you would notice that sound involves air molecules hitting each other, thereby producing areas with high densities of molecules (compressions) and areas with low densities of molecules (rarefactions).

Last of all, a surface wave is a wave that travels along the boundary between two materials. For example the kind of water wave you most often see, along the top of water, is an example of a surface wave. Surface waves move in similar ways to transverse waves but are a bit more complicated in their behavior.

In the case of an earthquake, you get a mixture of all three types of waves. The initial earthquake, called the primary wave, is longitudinal, but the aftershock that comes later, the secondary wave, is transverse. Extra surface waves are also created.

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