Page 143 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch05_115-138.indd Page 120 9/1/10 9:39 PM user-f465
tiL12214_ch05_115-138.indd Page 120 9/1/10 9:39 PM user-f465 /Users/user-f465/Desktop

molecule, and the pulse of compression spreads through the room.
In the example of the closing door, the pulse of greater density and
increased pressure of air reached the door at the other side of the

room, and the composite effect of the molecules hitting the door,
that is, the increased pressure, caused it to close.
If the door at the other side of the room does not latch, you
can probably cause it to open again by pulling on the fi rst door
quickly. By so doing, you send a pulse of thinned-out molecules

of lowered density and pressure. The door you pulled quickly
pushed some of the molecules out of the room. Other mole-
cules quickly move into the region of less pressure, then back
to their normal positions. The overall effect is the movement

of a thinned-out pulse that travels through the room. When
the pulse of slightly reduced pressure reaches the other door, A
molecules exerting their normal pressure on the other side of
the door cause it to move. After a pulse has passed a particular

place, the molecules are very soon homogeneously distributed
again due to their rapid, random movement.
If you swing a door back and forth, it is a vibrating object.
As it vibrates back and forth, it has a certain frequency in terms
of the number of vibrations per second. As the vibrating door
moves toward the room, it creates a pulse of jammed-together
molecules called a condensation (or compression) that quickly
moves throughout the room. As the vibrating door moves
away from the room, a pulse of thinned-out molecules called a
rarefaction quickly moves throughout the room. Th e vibrating
door sends repeating pulses of condensation (increased den- B
sity and pressure) and rarefaction (decreased density and pres- FIGURE 5.7 (A) Swinging the door inward produces pulses of
sure) through the room as it moves back and forth ( Figure 5.7). increased density and pressure called condensations. Pulling the door
You know that the pulses transmit energy because they pro- outward produces pulses of decreased density and pressure called
duce movement of, or do work on, the other door. Individual rarefactions. (B) In a condensation, the average distance between
molecules execute a harmonic motion about their equilibrium gas molecules is momentarily decreased as the pulse passes. In a
rarefaction, the average distance is momentarily increased.
position and can do work on a movable object. Energy is thus
transferred by this example of longitudinal waves.

5.3 DESCRIBING WAVES
CONCEPTS Applied A tuning fork vibrates with a certain frequency and ampli-
tude, producing a longitudinal wave of alternating pulses of
A Splash of Air? increased-pressure condensations and reduced-pressure rar-

In a very still room with no air movement whatsoever, efactions. The concept of the frequency and amplitude of the
place a smoking incense, punk, or appropriate smoke vibrations is shown in Figure 5.8A, and a representation of the
source in an ashtray on a table. It should make a thin condensations and rarefactions is shown in Figure 5.8B. Th e
stream of smoke that moves straight up. Hold one hand wave pattern can also be represented by a graph of the changing
flat, fingers together, and parallel to the stream of smoke.
air pressure of the traveling sound wave, as shown in Fig-
Quickly move it toward the smoke for a very short distance
ure 5.8C. This graph can be used to define some interesting

as if pushing air toward the smoke. Then pull it quickly
concepts associated with sound waves. Note the correspon-
away from the stream of smoke. You should be able to
dence between (1) the amplitude, or displacement, of the
see the smoke stream move away from, then toward your
vibrating prong, (2) the pulses of condensations and rarefac-
hand. What is the maximum distance from the smoke that
you can still make the smoke stream move? There are at tions, and (3) the changing air pressure. Note also the corre-
least two explanations for the movement of the smoke spondence between the frequency of the vibrating prong and
stream: (1) pulses of condensation and rarefaction or the frequency of the wave cycles.
(2) movement of a mass of air, such as occurs when you Figure 5.9 shows the terms commonly associated with
splash water. How can you prove one explanation or the waves from a continuously vibrating source. Th e wave crest is
other to be correct without a doubt? the maximum disturbance from the undisturbed (rest) posi-
tion. For a sound wave, this would represent the maximum
120 CHAPTER 5 Wave Motions and Sound 5-6