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## VIBRATION AND WAVES Sound and Light

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### VIBRATION AND WAVES Sound and Light Pendulum Pendulum s Period: The time required for one complete vibration, for example, from one crest to the next crest, is ... – PowerPoint PPT presentation

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Title: VIBRATION AND WAVES Sound and Light

1
VIBRATION AND WAVES Sound and Light
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Pendulum
4
Pendulums Period
• The time required for one complete vibration, for
example, from one crest to the next crest, is
called the pendulum's period and is measured in
seconds. The formula to calculate this quantity is

5
The formula to calculate this quantity is
• where
• L is the length of the pendulum in meters
• g is the gravitational field strength, or
acceleration due to gravity

6
FREQUENCY
• The frequency of a pendulum represents the number
of vibrations per second. This quantity is
measured in hertz (hz) and is the reciprocal of
the pendulum's period.

7
EXAMPLE 1
• What Would Be the Period of a Pendulum Located at
Sea Level If It Is 1.5 Meters Long?

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Solution 1
• 2.46 seconds
•

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EXAMPLE 2
• If the pendulum's length were to be shortened to
one-fourth its original value, what would be its
new period?

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SOLUTION 2
• 1.23 Seconds

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EXAMPLE 3
• At sea level, how long would a pendulum be if it
has a frequency of 2 Hz?

12
SOLUTION 3
• 6.21 cm

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EXAMPLE 4
• The Sears Building in Chicago sways back and
forth at a frequency of 0.1 Hz.
• What is the period of its vibration?

15
EXAMPLE 4
• The period is 1/frequency.
• 1 vib / 0.1 Hz 1 vib/0.1vib/sec
• 10 sec.

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TYPES OF WAVES
• Transverse Wave
• Longitudinal Wave

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Longitudinal Wave wave particles vibrate back
and forth along the path that the wave
travels. Compressional Wave
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Transverse waves wave particles vibrate in an
up-and-down motion.
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Longitudinal Wave
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Transverse Wave
http//www.phy.ntnu.edu.tw/ntnujava/index.php?topi
c14.0
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Wave Speed
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Wave Speed
• Speed Wavelength Frequency

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Example 5
• 4. A ruby-throated hummingbird beats its wings at
a rate of about 70 wing beats per second.
• What is the frequency in Hertz of the sound wave?
• b. Assuming the sound wave moves with a velocity
of 350 m/s, what is the wavelength of the wave?

24
Solution 5
• f 70 Hz and
• wavelength 5.0 m

25
Example 6
• Ocean waves are observed to travel along the
water surface during a developing storm. A Coast
Guard weather station observes that there is a
vertical distance from high point to low point of
4.6 meters and a horizontal distance of 8.6
meters between adjacent crests. The waves splash
into the station once every 6.2 seconds.
Determine the frequency and the speed of these
waves.

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Solution 6
• The wavelength is 8.6 meters and the period is
6.2 seconds.
• The frequency can be determined from the period.
If T 6.2 s, then
• f 1 /T 1 / (6.2 s)
• f 0.161 Hz

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Solution 6 Contd
• Now find speed using the
• v f wavelength equation.
• v f wavelength
• (0.161 Hz) (8.6 m)
• v 1.4 m/s

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Example 7
• Two boats are anchored 4 meters apart. They bob
up and down, returning to the same up position
every 3 seconds. When one is up the other is
down. There are never any wave crests between the
boats. Calculate the speed of the waves.

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Solution 7
The wavelength must be 8 meters The period is 3
seconds so the frequency is 1 / T or 0.333
Hz. Now use speed f wavelength Substituting
and solving for v, you will get 2.67 m/s.
30
Wave Interference
• Constructive
• And
• Destructive

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Constructive Wave Interference
• When the crest of one wave passes through, or is
superpositioned upon, the crest of another wave,
we say that the waves constructively interfere.
• Constructive interference also occurs when the
trough of one wave is superpositioned upon the
trough of another wave.

http//id.mind.net/zona/mstm/physics/waves/interf
erence/constructiveInterference/InterferenceExplan
ation2.html
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Destructive Wave Interference
• When the crest of one wave passes through, or is
superpositioned upon, the trough of another wave,
we say that the waves destructively interfere.
• During destructive interference, since the
positive amplitudes from one crest are added to
the negative amplitudes from the other trough,
this addition can look like a subtraction.

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Doppler Effect
• Some Examples

http//www.wfu.edu/physics/demolabs/demos/3/3b/3B4
0xx.html
http//www.walter-fendt.de/ph11e/dopplereff.htm
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Doppler Effect
• When a source of waves and an observer of waves
are getting closer together, the observer of the
waves sees a frequency for the waves that is
higher than the emitted frequency.

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antenna. Objects in the air, such as raindrops,
snow crystals, hailstones or even insects and
dust, scatter or reflect some of the radio waves
back to the antenna. All weather radars,
including Doppler, electronically convert the
reflected radio waves into pictures showing the
location and intensity of precipitation.
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