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Intro to Waves

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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? – 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
    students at your table.
  • 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
    your notes
  • 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
    in your notes
  • 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
    frequency of about .1 Hz.
  • 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
    cicada?
  • 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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