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5.22: Mechanical Wave

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No doubt you’ve seen this happen. Droplets of water fall into a body of water, and concentric circles spread out through the water around the droplets. The concentric circles are waves moving through the water.

Waves in Matter

The waves in the picture above are examples of mechanical waves. A mechanical wave is a disturbance in matter that transfers energy through the matter. A mechanical wave starts when matter is disturbed. A source of energy is needed to disturb matter and start a mechanical wave.

Q: Where does the energy come from in the water wave pictured above?

A: The energy comes from the falling droplets of water, which have kinetic energy because of their motion.

The Medium

The energy of a mechanical wave can travel only through matter. The matter through which the wave travels is called the medium ( plural , media). The medium in the water wave pictured above is water, a liquid. But the medium of a mechanical wave can be any state of matter, even a solid.

Q: How do the particles of the medium move when a wave passes through them?

A: The particles of the medium just vibrate in place. As they vibrate, they pass the energy of the disturbance to the particles next to them, which pass the energy to the particles next to them, and so on. Particles of the medium don’t actually travel along with the wave. Only the energy of the wave travels through the medium.

Types of Mechanical Waves

There are three types of mechanical waves: transverse, longitudinal, and surface waves. They differ in how particles of the medium move. You can see this in the Figure below and in the animation at the following URL. http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html

  • In a transverse wave, particles of the medium vibrate up and down perpendicular to the direction of the wave.
  • In a longitudinal wave, particles of the medium vibrate back and forth parallel to the direction of the wave.
  • In a surface wave, particles of the medium vibrate both up and down and back and forth, so they end up moving in a circle.

Q: How do you think surface waves are related to transverse and longitudinal waves?

A: A surface wave is combination of a transverse wave and a longitudinal wave.

Summary

  • A mechanical wave is a disturbance in matter that transfers energy through the matter.
  • The matter through which a mechanical wave travels is called the medium ( plural , media).
  • There are three types of mechanical waves: transverse, longitudinal, and surface waves. They differ in how particles of the medium move when the energy of the wave passes through.

Vocabulary

  • mechanical wave : Disturbance in matter that transfers energy from one place to another.
  • medium (plural, media ): Matter through which a mechanical wave moves.

Practice

At the following URL, read the short introduction to waves and watch the animations. Then answer the questions below. http://www.acs.psu.edu/drussell/Demos/waves-intro/waves-intro.html

  1. The article gives a dictionary definition of wave. What is the most important part of this definition?
  2. What happens to particles of the medium when a wave passes?
  3. How is “doing the wave” in a football stadium like a mechanical wave?

Review

  1. Define mechanical wave.
  2. What is the medium of a mechanical wave?
  3. List three types of mechanical waves.
  4. If you shake one end of a rope up and down, a wave passes through the rope. Which type of wave is it?

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Difficulty Level:

At Grade

Grades:

7 , 8

Date Created:

Nov 01, 2012

Last Modified:

Sep 05, 2014
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