Physics (Part 1 - Part 3)

456 CHAPTER 13 | Vibrations and Waves
What Is a Wave?
When you drop a pebble into a pool of water, the disturbance produces water
waves, which move away from the point where the pebble entered the water. A leaf
floating near the disturbance moves up and down and back and forth about its
original position, but doesn’t undergo any net displacement attributable to the disturbance.
This means that the water wave (or disturbance) moves from one place
to another, but the water isn’t carried with it.
When we observe a water wave, we see a rearrangement of the water’s surface.
Without the water, there wouldn’t be a wave. Similarly, a wave traveling on a string
wouldn’t exist without the string. Sound waves travel through air as a result of pressure
variations from point to point. Therefore, we can consider a wave to be the
motion of a disturbance. In Chapter 21 we discuss electromagnetic waves, which don’t
require a medium.
The mechanical waves discussed in this chapter require (1) some source of disturbance,
(2) a medium that can be disturbed, and (3) some physical connection
or mechanism through which adjacent portions of the medium can influence each
other. All waves carry energy and momentum. The amount of energy transmitted
through a medium and the mechanism responsible for the transport of energy
differ from case to case. The energy carried by ocean waves during a storm, for
example, is much greater than the energy carried by a sound wave generated by a
single human voice.
■ APPLYING PHYSICS 11.2 Burying Bond
At one point in On Her Majesty’s Secret Service, a James Bond
film from the 1960s, Bond was escaping on skis. He had a
good lead and was a hard-to-hit moving target. There was
no point in wasting bullets shooting at him, so why did the
bad guys open fire?
EXPLANATION These misguided gentlemen had a good
understanding of the physics of waves. An impulsive sound,
like a gunshot, can cause an acoustical disturbance that
propagates through the air. If it impacts a ledge of snow
that is ready to break free, an avalanche can result. Such
a disaster occurred in 1916 during World War I when Austrian
soldiers in the Alps were smothered by an avalanche
caused by cannon fire. So the bad guys, who have never
been able to hit Bond with a bullet, decided to use the
sound of gunfire to start an avalanche.
The shape of the pulse is
approximately unchanged
as it travels to the right.
Active Figure 13.21 A hand
moves the end of a stretched string
up and down once (red arrow), causing
a pulse to travel along the string.
Types of Waves
One of the simplest ways to demonstrate wave motion is to flip one end of a long
string that is under tension and has its opposite end fixed, as in Active Figure
13.21. The bump (called a pulse) travels to the right with a definite speed. A disturbance
of this type is called a traveling wave. The figure shows the shape of the
string at three closely spaced times.
As such a wave pulse travels along the string, each segment of the string that is
disturbed moves in a direction perpendicular to the wave motion. Figure 13.22
illustrates this point for a particular tiny segment P. The string never moves in
the direction of the wave. A traveling wave in which the particles of the disturbed
medium move in a direction perpendicular to the wave velocity is called a transverse
wave. Figure 13.23a illustrates the formation of transverse waves on a long
spring.
In another class of waves, called longitudinal waves, the elements of the
medium undergo displacements parallel to the direction of wave motion. Sound
waves in air are longitudinal. Their disturbance corresponds to a series of highand
low-pressure regions that may travel through air or through any material
medium with a certain speed. A longitudinal pulse can easily be produced in a
stretched spring, as in Figure 13.23b. The free end is pumped back and forth along
the length of the spring. This action produces compressed and stretched regions
of the coil that travel along the spring, parallel to the wave motion.
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