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motion (vibrations), and this periodic motion moved out in the that causes motion perpendicular to the direction that the wave
medium away from the point where the rock fell (the center or is moving. Particles responding to a transverse wave do not move

source of disturbance). The water did not move away from the cen- closer together or farther apart in response to the disturbance;
ter of disturbance, but the vibrations moved away. Th ese traveling rather, they vibrate up and then down in a direction perpen-
vibrations of the medium are called waves. Suppose you use your dicular to the direction of the wave motion (see Figure 5.6B).
finger or some other object to disturb the still water surface at

exactly one point not just once but many times, repeatedly, one
time after another in regular time intervals; for example, you tap CONCEPTS Applied

the surface every one-third of a sound. In this example, vibrations
of your finger are the source of periodic disturbances that travel in Making Waves

the medium. In short, you generated waves. The world around us

Obtain a Slinky or a long, coiled spring and stretch it out
provides us with many examples of waves or wave motion.
on the floor. Have another person hold the opposite end
stationary while you make waves move along the spring.
KINDS OF MECHANICAL WAVES Make longitudinal and transverse waves, observing how
the disturbance moves in each case. If the spring is long
If you could see the motion of an individual water molecule near
enough, measure the distance, then time the movement of
the surface as a water wave passed, you would see it trace out a each type of wave. How fast were your waves?
circular path as it moves up and over, down and back. Th is cir-
cular motion is characteristic of the motion of a particle reacting

to a water wave disturbance. There are other kinds of mechanical
waves, and each involves particles in a characteristic motion.
Whether you make mechanical longitudinal or transverse
A longitudinal wave is a disturbance that causes particles to
waves depends not only on the nature of the disturbance creat-
move closer together or farther apart in the same direction that
ing the waves but also on the nature of the medium. Mechanical
the wave is moving. If you attach one end of a coiled spring to
transverse waves can move through a material only if there is some
a wall and pull it tight, you will make longitudinal waves in the
interaction, or attachment, between the molecules making up the
spring if you grasp the spring and then move your hand back and
medium. In a gas, for example, the molecules move about freely
forth parallel to the spring. Each time you move your hand toward
without attachments to one another. A pulse can cause these mol-
the length of the spring, a pulse of closer-together coils will move
ecules to move closer together or farther apart, so a gas can carry
across the spring (Figure 5.6A). Each time you pull your hand
a longitudinal wave. But if a gas molecule is caused to move up
back, a pulse of farther-apart coils will move across the spring.
and then down, there is no reason for other molecules to do the

The coils move back and forth in the same direction that the wave
same, since they are not attached. Thus, a gas will carry mechani-

is moving, which is the characteristic for a longitudinal wave.
cal longitudinal waves but not mechanical transverse waves. Like-
You will make a different kind of mechanical wave in the

wise, a liquid will carry mechanical longitudinal waves but not
stretched spring if you now move your hand up and down. Th is
mechanical transverse waves because the liquid molecules simply
creates a transverse wave. A transverse wave is a disturbance
slide past one another. The surface of a liquid, however, is another

story because of surface tension. A surface water wave is, in fact,
a combination of longitudinal and transverse wave patterns that
A Longitudinal wave
Direction of wave motion produce the circular motion of a disturbed particle. Solids can and
do carry both longitudinal and transverse waves because of the
strong attachments between the molecules.
Direction of disturbance
WAVES IN AIR
Because air is fluid, mechanical waves in air can only be longitu-

dinal; therefore, sound waves in air must be longitudinal waves. A
B Transverse wave familiar situation will be used to describe the nature of a mechani-
Direction of wave motion cal longitudinal wave moving through air before we consider
Direction sound specifi cally. The situation involves a small room with no

of
disturbance open windows and two doors that open into the room. When you
open one door into the room, the other door closes. Why does this
happen? According to the kinetic molecular theory, the room con-
tains many tiny, randomly moving gas molecules that make up the
air. As you opened the door, it pushed on these gas molecules, cre-
ating a jammed-together zone of molecules immediately adjacent
FIGURE 5.6 (A) Longitudinal waves are created in a spring

when the free end is moved back and forth parallel to the spring. to the door. This jammed-together zone of air now has a greater
(B) Transverse waves are created in a spring when the free end is density and pressure, which immediately spreads outward from the

moved up and down. door as a pulse. The disturbance is rapidly passed from molecule to
5-5 CHAPTER 5 Wave Motions and Sound 119