Physics 1240 Website: http://www.colorado.edu/physics/phys1240
Midterm Exam Physics 1240
Spring 2005
Date: Thursday 10 March
In all problems you can assume that the speed of sound at room temperature (20°C) is 344 m/s. Hint:
Do the easy problems first: if you cannot figure out how to do a particular problem right away, skip it
and come back to it later. Use only a #2 pencil to fill in the bubble sheets. Good luck!
1. Which of the following statements about sound is FALSE?
(a) sound waves are traveling disturbances that locally change the pressure
(b) sound waves travel with constant velocity
(c) the wavelength of a sound wave is inversely proportional to its frequency
L (d) sound propagates as transverse waves
sound waves are longitudinal
2. Radio waves travel with the speed of light, 3 x 10 8 m/s. What is the wavelength of the signal
from KCFC in Boulder, broadcasting at an AM carrier frequency of 1340 kHz ?
[1 kHz = 103 Hz]
J (a) about 220 m
(b) about 220 km
since v=λf we have
(c) about 0.45 m
λ=v/f=3x108/1.34x106 = 220 m
(d) about 4.5 cm
3. What is the frequency of a tone with a period of 0.002 seconds?
(a) 50 Hz
f=1/T=1/0.002=500 Hz
J (b) 500 Hz
(c) 5,000 Hz
(d) 50,000 Hz
4. Given that the speed of sound in air increases about 0.6 m/s per °C as it gets warmer, what is
the speed of sound on a very cold day in winter when the temperature is -10°C?
(a) –18 m/s
since the speed decreases by 18 m/s
(b) 0 m/s
when we are 30oC below room temperature
J (c) 326 m/s
(d) 362 m/s
the speed is (344 - 18) = 326 m/s
5. What is displayed on an oscilloscope?
(a) pressure vs. distance
J (b) voltage vs. time
(c) frequency vs. time
(d) waveform amplitude vs. frequency
6. What is pressure?
(a) the amount of force applied per unit length
J (b) the amount of force applied per unit area
(c) the number of air molecules per unit volume
(d) the air density divided by the air temperature
7. A Swiss tourist stands at the edge of the Grand Canyon and yodels (very loudly). If he hears
an echo 15 s later, how wide is the canyon?
(a) 258 m
since the sound travels for 15s, the total
(b) 516 m
distance (to the other side and back)
J (c) 2.58 km
(d) 5.16 km
is given by d = vt = 344 x 15 = 5160 m
the width is just half this, 2580 m = 2.58 km
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Physics 1240 Website: http://www.colorado.edu/physics/phys1240
8. The Doppler Effect. A loudspeaker emitting a 1 kHz tone is attached to a horizontal rotating
arm positioned at head height some distance in front of you. You will hear the lowest
frequency when the speaker
(a) moves towards you
the wavefronts are more spread out
J (b) moves away from you
when the source moves away from you
(c) is closest to you
so the frequency appears lower
(d) is furthest from you
9. A stationary bell at a railroad crossing emits a sound of frequency f 1 =400 Hz. A pedestrian
stands at the crossing as a train goes by and she listens to the bell. What does she hear?
J (a) a constant ringing at f 1
the listener is not moving toward
(b) a constant ringing at 2f 1
or away from the source
(c) the frequency goes up then down
(d) the beat frequency
so there is no frequency shift
10. If a spaceship fuel tanker were to crash a nd explode on the near side of the Moon
(a) the sound would take several days to reach us on Earth
(b) the sound would be too faint to hear on Earth
J (c) we would not hear any sound because sound does not travel through a vacuum
(d) we would hear a sound only if the explosion happened at night
11. Resonance is when
(a) there are simple harmonic oscillations observed
(b) the frequency of a vibrating object de pends on the amplitude of the oscillations
(c) a sound wave has a definite musical pitch
J (d) an object is driven at or near its natural frequency and responds with sympathetic
vibrations
12. A complex periodic waveform may be thought of as the sum of the fundamental f 1 and its
harmonics. Which of the following statements is NOT true?
(a) the harmonics are integer multiples of the fundamental (2f 1 , 3f1 , 4f 1, etc.)
(b) some musical instruments produce only some of the harmonics present
(c) the harmonics can be viewed with a spectrum analyzer
L (d) the longer the period of the fundamental the higher the perceived pitch
13. If the frequency of the fundamental of some musical waveform is 220 Hz what is the
frequency of the 2 nd harmonic (i.e. the octave, sometimes called the first overtone)?
(a) 110 Hz
if the fundamental is f1
(b) 220 Hz
(c) 330 Hz
then the second harmonic is 2f1
(d)
440
Hz
J
14. The pitch of a complex periodic waveform comprising several harmonics is perceived as
J (a) the lowest frequency component
(b) the highest frequency component
(c) the average frequency component
(d) the beat frequency
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Physics 1240 Website: http://www.colorado.edu/physics/phys1240
15. Which of the following statements about hearing is FALSE?
(a) for quiet sounds, the human ear is more sensitive in the middle of the hearing range
(b) elephants can hear lower sounds than humans, even sensing vibrations through their feet
L (c) women’s hearing improves with age but not as much as men’s hearing
(d) even when sound echoes off an ideally hard, flat, smooth wall, the echo still seems quieter
because the sound has spread out
nobody’s hearing improves with age!
16. Which of the following statements about waves and their properties is FALSE?
(a) both longitudinal and transverse waveforms can travel on a slinky
L (b) two identical sources of sound placed side by side produce no interference
(c) two tones of slightly different frequency produce a ringing effect called beats
(d) interference of two sound sources can in gene ral be either constructive or destructive,
depending on the phase difference between the two sources
we heard in class (and saw in the ripple tank applet) that identical sources do interfere
17. Which of the following statements about air and sound is FALSE?
(a) mechanical vibrations of a violin string couple to resonances of the air-filled body of the
instrument to produce loud musical sounds
(b) objects like trees and bridges can resonate dramatically when the wind blows
L (c) pressure waves cannot propagate through the air because it is not elastic
(d) the sound of a siren results from periodic puffs of air
air is elastic: remember the bicycle pump!
18. The two waveforms show on the oscilloscope trace below differ in
(a) frequency only
(b) amplitude only
(c) frequency and amplitude
J (d) amplitude and phase
they are different in size and offset in time
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Physics 1240 Website: http://www.colorado.edu/physics/phys1240
19. Which of these waveforms has the lowest pitch?
P
t
J (a)
this one has the longest period
hence the lowest frequency fundamental
P
t
(b)
P
t
(c)
P
(d)
t
20. The following oscilloscope trace shows the sound made by a violin. What is the pitch
perceived by the listener? [1 ms = 0.001 s]
(a) about 10 Hz
(b) about 100 Hz
J (c) about 550 Hz
(d) about 5500 Hz
the period T of the fundamental
(the longest wavelength) is about 1.8 ms
so the pitch is f= 1/T = 555 Hz
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