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## Intro to Waves

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### Constructive Interference Destructive Interference the superposition of waves with equal but opposite amplitudes when the pulses meet at the same location, the ... – PowerPoint PPT presentation

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Title: Intro to Waves

1
Intro to Waves
2
What is a wave?
• Take a moment to answer this question with the
• As a group, create a definition and give an
example of a wave.
• (Be a bit more original than an ocean wave.
• I know you can think of a better example)

3
• a wave
• is a rhythmic disturbance that carries energy
through matter or space

4
Mechanical Waves
• require a medium
• medium the matter through which a wave travels
• examples water, air, anything made of matter

5
• waves transfer energy from one point to another
with little or no permanent displacement to the
particles in the medium

6
Two Types of Mechanical Waves
• Transverse Waves
• Longitudinal Waves

7
Transverse Waves
• waves that propagate (or move) perpendicular to
the direction of travel

8
crest
• the point on the medium that exhibits the maximum
amount of positive or upward displacement from
the rest position

9
trough
• the point on the medium that exhibits the maximum
amount of negative or downward displacement from
the rest position

10
amplitude
• the maximum displacement of a particle on the
medium from its equilibrium position

11
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

12
Anatomy of a Transverse Wave
• Label the following parts of a transverse wave in
• equilibrium
• crest
• trough
• amplitude
• wavelength

13
Longitudinal Waves
• waves that propagate (or move) parallel to the
direction of motion

14
compression
• a point on a medium through which a longitudinal
wave is traveling that has the maximum density

15
rarefaction
• a point on a medium through which a longitudinal
wave is traveling that has the minimum density

16
Anatomy of a Longitudinal Wave
• Label the following parts of a longitudinal wave
• compression
• rarefaction
• wavelength

17
Describing a Wave
• frequency the number of vibrations an object
makes in a unit of time (generally one second)
• the unit of frequency is Hz
• period the time needed to complete one vibration
(measured in seconds)

18
• The Sears Tower sways back and forth at a
• What is the period of vibration?

19
Speed Review
• an objects speed is described by the distance
traveled in a specific amount of time (generally
measured in m/s)

20
Wave Speed
• v ? ? f
• wave speed wavelength ? frequency

21
• 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?

22
• 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?

23
• 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?

24
• Earthquakes generate shock waves that travel
through Earths interior to other parts of the
world. The fastest of these waves are
longitudinal waves, like sound waves, and are
called primary waves, or just p-waves. A p-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?

25
• 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?

26
• 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?

27
• 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
• What is its period?

28
• 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 hikers voice
in air?
• The wavelength of the sound is .75 m. What is its
frequency?
• What is the period of the wave?

29
Wave Behavior
• What happens when a ball is thrown against a
wall?
• Predict what would happen if a wave encounters a
rigid boundary?

30
• 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
• Newtons Third Law of
• Motion.

31
• 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

32
Two Types of Interference
1. Constructive
2. Destructive

33
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

34
Constructive Interference
35
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

36
Destructive Interference
37
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

38
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39
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.

40
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