Transcript – Waves, Lecture 3 PHY138

PHY138 – Waves, Lecture 3
“..imagine a stadium full of people doing "the wave"
with a giant sheet covering them so that none of
the spectators can be seen. To someone
watching such an event it would appear that
something was moving around the stadium under
this sheet. It would look as if some giant creature
was running in circles. In reality no one moves
around the stadium; each spectator simply stands
up and sits back down in the same seat. There is
energy moving around the stadium while the
people remain stationary.”
- thinkquest.org
Reading Assignment
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Please read the following from Serway
and Jewett before class on Wednesday:
Chapter 13, Sections 13.7 and 13.8, and
Chapter 14, Sections 14.1 and 14.2
A Web-CT quiz is due Wednesday
morning, which tests your basic familiarity
with Chapter 14 assigned reading.
McDermott Tutorials book, page 149
regarding Electromagnetic Waves
Written Homework
Please complete Problems 8, 22, 34
and 41 from Chapter 13. They are
due in your tutorial drop-box by
Monday, November 22 at 5:00 PM.
Note from David Harrison:
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Regarding Test 1: Problem involving two
horse teams pulling a set of connected
hemispheres:
Both the correct answer and “Answer C”
were marked as correct. Students who
interpreted the question to be about net-force
were correct to reason that the net-force on a
non-accelerating object is always zero.
Harrison will not answer emails about this
question
Last day’s lecture: some comments
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The simple pendulum was modeled as a
point-mass on a massless string. Moment
of inertia was therefore I=mr2=mL2.
Objects such as bridges, trumpets, radio
antennae, etc, all have natural oscillation
frequencies. Resonance occurs when an
outside driving force is close to one of
these frequencies. At resonance, the
amplitude of oscillations is very high.
First mini-quiz
A person swings on a swing. When the
person sits still, the swing oscillates back
and forth at its natural frequency. If,
instead, two people sit on the swing, the
natural frequency of the swing is
1. greater
2. the same
3. smaller
Second mini-quiz
A person swings on a swing. When the
person sits still, the swing oscillates back
and forth at its natural frequency. If,
instead, the person stands on the swing,
the natural frequency of the swing is
1. greater
2. the same
3. smaller
PHY138 – Waves, Lecture 3
Today’s overview
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Mathematical Description of a Wave
Speed of Waves on a String
Reflection and Transmission of Waves
Sound, Ultrasound
Sinusoidal Wave snapshot
k=2π/λ
Sinusoidal Wave at a point in space
ω=2π/T
Reflection of Transverse Wave Pulse
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A pulse traveling to
the right on a heavy
string attached to a
lighter string
Speed suddenly
increases
Reflection of Transverse Wave Pulse
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A pulse traveling to
the right on a light
string attached to a
heavier string
Speed suddenly
decreases
Reflection of Transverse Wave Pulse
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Reflection of a
traveling pulse at
the free end of a
stretched string.
Reflection of Transverse Wave Pulse
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Reflection of a
traveling pulse at
the fixed end of a
stretched string.
Physics of Ultrasound
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Speed of sound in bone, flesh and blood
are all different
When the speed of any wave suddenly
changes, there is a reflection and
transmission
Ultrasound images are formed from
reflected high frequency sound
Image resolution is set by wavelength, λ
λ=v/f, so higher frequency yields smaller λ,
and better resolution
Speed of sound in humans
Tissue
Sound Speed (m/s)
Air
350
Fat
1450
Brain
1540
Blood
1570
Bone
4080
Muscle
1585