intro to waves - tfn physicstfnphysics.weebly.com/uploads/1/1/1/4/1114892/wavesintro.pdfa wave is a...
TRANSCRIPT
a wave
is a rhythmic disturbance that
carries/transfers energy through
matter or space
A repeating movement
Mechanical Waves
require a medium
medium: the matter through which a
wave travels examples: water, air, anything made of matter
waves transfer energy from one point to
another with little or no permanent
displacement to the particles in the medium
Molecules pass energy along to neighbor,
who then passes energy to neighbor, etc.
All waves are produced by VIBRATIONS
Two Types of Mechanical
Waves:
1. Transverse Waves
2. Longitudinal/Compressional
Waves
Transverse Waves
waves that propagate (or move)
perpendicular to the direction of travel
crest
the point on the medium that exhibits the
maximum amount of positive or upward
displacement from the rest position
trough
the point on the medium that exhibits the
maximum amount of negative or
downward displacement from the rest
position
amplitude
the maximum displacement of a particle
on the medium from its equilibrium
position
wavelength
the length of one wave is measured
from a point on one wave to the same
point on the next wave
trough to trough
crest to crest
midpoint to midpoint
Anatomy of a Transverse Wave
Label the following parts of a transverse
wave in your notes:
equilibrium
crest
trough
amplitude
wavelength
Longitudinal Waves
waves that propagate (or move) parallel
to the direction of motion
compression
a point on a medium through which a
longitudinal wave is traveling that has
the maximum density
rarefaction
a point on a medium through which a
longitudinal wave is traveling that has
the minimum density
Anatomy of a Longitudinal Wave
Label the following parts of a
longitudinal wave in your notes:
compression
rarefaction
wavelength
Describing a Wave & Wave Speed
Frequency (f): the number of vibrations
an object makes in a unit of time
(generally one second)
the unit of frequency is Hz = 1
𝑠
𝑓 =𝑣𝑖𝑏
𝑡
Period (T): the time needed to complete
one vibration (measured in seconds)
𝑇 =𝑡
𝑣𝑖𝑏
The Sears Tower sways back and forth at
a frequency of about .1 Hz.
What is the period of vibration?
Speed Review
an object’s speed is described by the
distance traveled in a specific amount of
time (generally measured in m/s)
Wave Speed
v = λ ● f
wave speed = wavelength ● frequency
Lambda
A wave with a frequency of 60.0 Hz
travels through vulcanized rubber with a
wavelength of 0.90 m.
What is the speed of this wave?
The lowest pitch that the average
human can hear has a frequency of
20.0 Hz.
If sound with this frequency travels through
air with a speed of 343 m/s, what is its
wavelength?
A ship anchored at sea is rocked by
waves that have crests 14 m apart. The
waves travel at 7.0 m/s.
How often do the wave crests
reach the ship?
Earthquakes generate shock waves that
travel through Earth’s interior to other parts
of the world. A shock wave has a very low
frequency, typically around 0.050 Hz.
If the speed of a p-wave with this frequency is
8.0 km/s, what is its wavelength?
A dolphin can typically hear sounds with
frequencies up to 150 kHz.
What is the speed of sound in water if a
wave with this frequency has a wavelength
of .01 m?
Sonar is a device that uses reflected sound
waves to measure underwater depths.
There is a sonar signal that has a
frequency of 288 Hz. If the speed of sound
in water is 1.45x103 m/s,
what is the wavelength of
the sonar signal?
Cicadas produce a buzzing sound that has a
wavelength in air of 2.69 m. If the speed of
sound in air is 343 m/s,
what is the frequency of the sound produced
by a cicada?
What is its period?
A hiker shouts toward a vertical cliff 685 m
away. The echo is heard 4 s later.
What is the speed of sound of the hiker’s voice
in air?
The wavelength of the sound is .75 m. What is
its frequency?
What is the period of the wave?
Wave Behavior
What happens when a ball is thrown
against a wall?
Predict what would happen if a wave
encounters a rigid boundary?
When a wave pulse is sent down a medium
connected to a rigid wall,
the energy transmitted
is reflected.
The reflected wave
is inverted due to
Newton’s Third Law of
Motion.
Predict what would happen if two waves were
in the same place on a medium.
Principle of Superposition
the displacement of a medium caused by two or
more waves is the algebraic sum of the
displacements of the individual waves two or more waves can combine to form a new wave
the result of the superposition of two or more
waves is called interference
Two Types of Interference
1. Constructive
2. Destructive
Constructive Interference
occurs when wave displacements are in
the same direction
the two waves pass through each other
without changing their shapes or sizes
the resultant pulse at the overlap of the
two waves is the algebraic sum
Constructive Interference
Destructive Interference
the superposition of waves with equal
but opposite amplitudes
when the pulses meet at the same
location, the displacement of the
medium is zero
just as constructive interference, the
waves pass through one another
unchanged
Destructive Interference
Standing Waves
a wave pattern that remains in a
constant position
the result of reflection and wave
interference
characterized by two fixed points on the
wave
Tacoma Narrows Bridge
In an effort to save money, corners were
cut in the design of the bridge thus
compromising its stability
On November 7, 1940 the Tacoma
Narrows suspension bridge collapsed
due to 42 mph wind induced vibrations.