PHY138 – Waves, Lecture 4
Today’s overview
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Sound as a pressure wave
The Doppler Effect
The Principle of Superposition
Constructive and Destructive Wave
Interference
Reading Assignment
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Please read the following from Serway
and Jewett before class on Monday:
Chapter 14, up to and including Section
14.6
A www.masteringphysics.com assignment
is due this Friday at 5:00 PM. It is based
on Chapter 13 material: mechanical
waves and sound.
Last day’s lecture: some comments
Regarding quiz on swings: by standing up
on a swing, you raise your centre of
mass closer to the pivot point, thereby
reducing L.
This increases the natural angular
frequency, ω.
This reduces the period, T.
Medical ultrasound imagers use sound with
frequencies in the range f =1-20 MHz.
Sound Waves
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Sound is produced by an oscillating
surface in a medium (ie a speaker in air)
Sound is a pressure wave:
P  Pmax cos( kx  t )
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Sound is also a longitudinal displacement
wave of individual molecules in the
medium :
s  smax cos(kx  t   2)
Doppler Effect for sound
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Creates a change in pitch or frequency
All speeds are measured relative to the
medium (ie air)
f goes up if source and observer are
approaching each other
f goes down if source and observer are
moving away from each other.
Chapter 14: Principle of Superposition
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If two or more waves combine at a given
point, the resulting disturbance is the sum
of the disturbances of the individual
waves.
Two traveling waves can pass through
each other without being destroyed or
even altered.
Quiz
Two traveling sinusoidal waves passing
the same point, each of wavelength λ,
result in total destructive interference if
the waves
1. are in phase.
2. are π/2 out of phase.
3. are π out of phase.
4. are traveling in opposite directions.
5. None of the above
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Wave Interference
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Two waves moving in the same direction
with the same amplitude and same
frequency form a new wave with amplitude
2Acos(ø/2), where ø is their phase
difference.