15.0 - Introduction

Waves can be as plain to see as the ripples in a pond or as invisible as the

electromagnetic waves emanating from a cellular phone. Mechanical waves, like

those in a pond, require a medium in order to propagate. Electromagnetic waves í

including radio waves and light í require no medium and can travel in the near

vacuum of space. Electromagnetic waves rely on the interaction of electric and

magnetic fields to propagate through space.

In this chapter, we focus on mechanical waves. These are waves in which a

vibration causes a disturbance to travel through a medium. You are familiar with a

variety of mechanical waves: water waves in the ocean, sound waves in the air, or

waves along a string if you shake an end up and down. These waves exist due to

the movement of particles that make up a medium, such as water molecules in the

ocean or gas molecules in the air.

Waves carry energy from place to place: a relatively small amount with a sound

wave, a much larger amount with a tsunami wave.

Although many mechanical waves travel, sometimes across great distances, there

is no net movement of the medium through which they propagate. The 15th century Italian scientist and artist Leonardo da Vinci described this

key attribute when he said: “It often happens that the wave flees the place of its creation, while the water does not.”

Use the simulation to the right to begin your exploration of waves. It consists of a string stretched across the screen. A hand on the left holds

the string. By shaking the hand up and down, you can generate a variety of waves in the string.

When you open the simulation, press GO to send a wave down the string. You will see the hand begin to shake the string, causing a wave to

travel from left to right.

The control panel has two input gauges that allow you to vary the amplitude and frequency of the wave. As you may remember from your study

of simple harmonic motion, amplitude is the maximum displacement of a wave from equilibrium. Frequency is the number of cycles per second.

You can vary these parameters and observe changes in the shape of the wave. Also in the control panel is an output gauge that displays the

wavelength, the distance between successive peaks of the wave.

The string’s tension and other properties remain constant.

When you run the simulation, make sure you observe the differences between a wave with higher frequency and one with lower frequency.

This is an important fundamental characteristic of a wave.

Then try three quick experiments. First, does changing the frequency of a wave also change its wavelength? Change the frequency and

observe what happens to the wavelength.

Second, does changing the frequency result in any change in amplitude? Again, you can vary the frequency and note any change.

Finally, as you change the frequency and amplitude, does the wave travel down the string any faster or slower? For example, does a wave with

a very large amplitude travel noticeably faster than one with a very small amplitude?

The simulation is intended to let you conduct a preliminary exploration of topics that will be presented in this chapter. Answer what questions

you can above and then proceed to the rest of the chapter, which covers the topics in more depth.

15.1 - Mechanical waves

Mechanical waves: Vibrations in a medium.

Ocean breakers, the rolling wave of a crowd in a sports stadium, the back and forth

vibrations in a Slinky®: These are a few of the many kinds of waves you can see. Some

mechanical waves are invisible to the eye but detectable by the ear, such as the sound

waves generated by musical instruments.

Mechanical waves are vibrations in a medium, traveling from place to place without

causing any net movement of the medium. You may be familiar with “the wave” in a

football or baseball stadium. The wave travels around the stadium, the result of

spectators standing and then sitting in a rolling succession. As the fans oscillate up and

down, they create what is called a disturbance or waveform. The location of the

disturbance changes as the wave moves through the stadium, but the wave’s medium,

the crowd, stays put.

A wave in a stadium is a useful example, but it is not a true mechanical wave. Mechanical waves, such as a wave in a string, result from an

initial force (a vibration up and down or to and fro) followed by a continuing sequence of interactions between particles in the medium. In a

stadium wave, the particles of the medium (the people) do not typically exert physical forces on one another to propagate the wave (since peer

pressure is not a physical force).

Mechanical waves

Disturbances in a medium

(^294) Copyright 2007 Kinetic Books Co. Chapter 15