Physics 115 Lecture 4
Resonance and introduction
to waves
January 31, 2017
1
Driven oscillators



A force must act on an oscillator to
either add or subtract energy
If a periodic force acts on an
oscillator, it is a driven oscillator
The energy transfer is very efficient
when the driving frequency
matches the natural frequency of
vibration




Child on a swing
TV or radio
Microwave oven
Animated applet
or energy
Resonance
Resonance


The Tacoma Narrows bridge is the poster child for
the power of resonance
Video of its collapse
Possible cause:
vortex shedding
Likely cause:
aeroelastic flutter


The narrower the
resonance curve, the
more selective the
response of the system
The selectivity of a
system is expressed by
its quality factor (Q).
or energy
Quality factor
The period of the Tacoma Narrows bridge twisting
oscillations was 5.0 s. With what frequency did the
aerodynamic process push the bridge?
A. 0.20 Hz
B. 1.0 Hz
C. 5.0 Hz
D. 20 Hz
91%
0%
A.
B.
3%
C.
6%
D.
The period of the Tacoma Narrows bridge twisting
oscillations was 5.0 s. With what frequency did the
aerodynamic process push the bridge?
A. 0.20 Hz
B. 1.0 Hz
C. 5.0 Hz
D. 20 Hz
The driving frequency must match
the natural frequency of the bridge
1
1
f  
 0.20 Hz
T 5.0 s
How should you design an audio microphone so
that it will resonate at a very wide range of
frequencies?
A. It should have a high Q factor at74%
high frequency
B. It should have a low Q factor in the middle of the
audio frequency range.
C. It should have a high Q factor in the middle of
the audio frequency range.
D. It should have a low Q factor at low frequency
15%
12%
0%
A.
B.
C.
D.
How should you design an audio microphone so
that it will resonate at a very wide range of
frequencies?
A. It should have a high Q factor at high frequency
B. It should have a low Q factor in the middle of
the audio frequency range.
C. It should have a high Q factor
in the middle of the audio
frequency range.
D. It should have a low Q factor
at low frequency
A high Q factor would make the
microphone too selective
Waves and oscillations

Waves make oscillations, and
oscillations make waves

Animated applet

String waves example

Radio waves example
Describing waves

Wave features
Describing waves





Animated applet
Frequency
Wavelength applet
Speed
distance 
v
  f c
time
T
The speed of a wave is determined by
the properties of the medium, NOT
either frequency or wavelength
How far does a wave travel in one period?
A. It depends on the frequency
56%
B. 0.50λ
C. 1.0λ
D. 2.0λ
44%
0%
A.
B.
0%
C.
D.
How far does a wave travel in one period?
A. It depends on the frequency
B. 0.50λ
The speed of a wave depends only on
the properties of the medium, not on
C. 1.0λ
the frequency or wavelength.
D. 2.0λ
A wave always travels one wavelength
during one oscillation period.
A wave oscillates 4.0 times a second and has a
wavelength of 3.0 m. What is its speed?
79%
A. 4.0 m/s
B. 0.75 m/s
C. 1.33 m/s
D. 12 m/s
12%
9%
0%
A.
B.
C.
D.
A wave oscillates 4.0 times a second and has a
wavelength of 3.0 m. What is its speed?
A. 4.0 m/s
B. 0.75 m/s
C. 1.33 m/s
D. 12 m/s
c  f    4.0 Hz  3.0 m 
 12 m/s