Mechanical Waves
Lesson 1
Objectives- What are mechanical waves and what causes them; describe two types of mechanical
waves; explain the relationship between frequency and wavelength
Waves are an important part of everyday life. Waves are used to help us communicate, produce
color, fight infection or cancer and make music. Waves are a rhythmic disturbance that carries energy
through matter or space. The material the wave travels through is called the medium. A medium can
be a solid, liquid, gas or even a vacuum like empty space. Even though the wave travels through the
medium, the particles in the medium do not travel with the wave.
A mechanical wave is a type of wave that requires a medium. These cannot travel through
empty space. There are three types of mechanical waves. We will study two of these. They differ in
what happens to the matter (medium) when the energy travels to it.
The type of wave most people are most familiar with is called a transverse wave. See Figure 1.
In a transverse wave, the particles in the medium move at right angles to the direction the wave is
traveling. Water resembles of a transverse wave, though it isn’t. But we will use it as an example. As
water wave moves towards the shoreline, a bobber moves up and down on the water (right angle) but it
does not travel along with the water. This shows the medium (bobber) travelling at right angles to the
direction the wave is travelling (towards the shore).
Figure 1
The parts of a wave are pictured in Figure 1. The crest is the highest part of the wave. The
trough is the lowest part. The distance these are from the equilibrium line is a measure of how much
energy the wave has or its amplitude. A wavelength is the distance (a measurement) from one point on
a wave to the next identical point. On the wave shown, A to E represents one wavelength. Typically,
wavelengths are determined by going from one crest to the next. The frequency refers to the number
of complete wavelengths that pass per second.
The other type of mechanical wave is called a compression wave. Another name for a
compression wave is longitudinal wave. A compression wave is the type of wave that produces sound.
The particles in the medium move in the same direction as the wave is moving. A slinky is a good
example of what a compression wave looks like. See Figure 2.
FIGURE 2
Figure 2 shows the parts of a compression wave. The compressions are areas where particles
are closer together. The areas where particles are spread apart are called rarefactions. The more
energy the wave has, the closer the compressions are. One wavelength is the distance from the
beginning of one compression to the end of one rarefaction. The frequency is how many cycles of
compressions/rarefactions pass in one second.
Frequency and wavelength show an inverse proportion. When one goes up, the other goes
down. See Figure 3. This relationship will be important when we discuss speed in the next section and
electromagnetic waves later.
Figure 3
Define the following terms:
Wave
Medium
Transverse wave
Crest
Trough
Amplitude
Wavelength
Frequency
Compression wave
Compression
Rarefaction
Describe the relationship between wavelength and frequency.