Mechanical waves have some common properties. First, they require a physical
medium, such as air, a string or a body of water. Mechanical waves cannot move
through a vacuum.
Second, mechanical waves require a driving excitation to get the wave started. The
vibrations then propagate, via interactions between particles, through the medium.
In this fashion, waves transfer energy from place to place. When you hear a sound, you
are hearing energy that has been transferred by a wave through air or water (or even, if
you are listening for buffalo, the ground).
All of the waves in this chapter are traveling waves in which the disturbance moves from
one point to another. Concept 2 shows a wave moving down a string, caused by a hand
shaking the string. The illustration shows three successive moments in time. You can
track the position of the first peak as it moves down the string over time. It moves with a
constant speed v. The other peaks also move down the string with the same speed.
The peaks do not move through the medium in all waves. In what are called standing
waves, the locations in the medium where peaks and troughs (the "low" parts of the wave) occur are fixed. These waves, caused by the
reflection or interaction of traveling waves, are discussed in a later chapter.
Traveling waves
Vibrations that travel through a medium
15.2 - Transverse and longitudinal waves
Transverse wave: Particles in a medium
vibrating perpendicular to the direction the
wave is traveling.
Longitudinal wave: Particles in a medium
vibrating parallel to the direction the wave is
traveling.
Waves can be classified by the relationship between their direction of travel, and the
direction of the motion of the particles in the medium.
Imagine that two people stretch a Slinky between them, and one shakes the Slinky up
and down. This causes a wave to move along the Slinky, as shown in Concept 1. The
wave moves to the right with a velocity called vwave in the diagram.
Although the wave moves to the right, the particles that make up the medium move up
and down. An individual particle of the Slinky is highlighted in red in the diagram to the
right, and its movement is shown with the vertically directed arrows. The direction of the
wave is perpendicular to the motion of the particles of the medium. This type of wave
is called a transverse wave.
Many types of mechanical waves are transverse waves, including those caused by
shaking a Slinky up and down, the vibrations of a violin string, and certain types of
earthquake waves.
Now imagine that instead of shaking the Slinky up and down, a person pulls the Slinky
to the left and then pushes it to the right, as shown in Concept 2. This causes the spring
to be stretched and then compressed.
This disturbance again travels horizontally along the Slinky, and again we show its
velocity as vwave. The wave consists of regions in which the coils of the spring are tightly
packed, followed by regions in which the coils are widely spaced. A particle of the
Slinky, again marked with a red dot, oscillates horizontally, parallel to the direction the wave is traveling. This type of wave is called a
longitudinal wave.
Sound is a longitudinal wave that consists of alternate compressions and rarefactions of air. Individual air particles oscillate back and forth, and
a sound wave travels through the air, where it can be detected by a sophisticated instrument: the human ear.
In both transverse and longitudinal waves, the particles do move, but there is no net motion of the particles after each cycle. A particle moves
up and down, or back and forth, but it returns to its initial position. It oscillates like a mass attached to a spring.
A single source of vibration, such as an earthquake, can create both transverse and longitudinal waves. In an earthquake, the longitudinal
waves (P waves, for primary waves) travel at about 8 km/s, while the transverse waves (S waves, for secondary waves) are slower, moving at
about 5 km/s. By noting when each type of wave arrives at a given seismographic station, a seismologist can determine the distance of the
earthquake from that station. Using data from several stations, the seismologist can triangulate the location of the earthquake’s epicenter.
The motion of the particles that make up a wave can be complex, with ocean waves serving as one example. You can see this in Concept 3,
where the wave moves to the left and the water molecules near the surface move in circles. This means the molecules’ motion involves vertical
and horizontal components. Their motion is both perpendicular and parallel to the direction of the wave. An ocean wave displays both
Transverse waves
Particles vibrate perpendicular to
direction of waveLongitudinal waves
Particles vibrate parallel to direction of
wave