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14 Modern Physics Questions

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1. A certain string on a piano is tuned to produce middle C
(f = 261.63 Hz) by carefully adjusting the tension is the string. For a
fixed wavelength, what is the frequency when this tension is
a. 130.08 Hz
b. 185.00 Hz
c. 370.00 Hz
d. 446.63 Hz
e. 523.26 Hz

2. A bell is ringing inside of a sealed glass jar that is connected to a
vacuum pump. Initially, the jar is filled with air. What does one
hear as the air is slowly removed from the jar by the pump?
a. The sound intensity gradually decreases
b. The frequency of the sound gradually increases
c. The frequency of the sound gradually decreases
d. The speed of the sound gradually increase
e. The sound intensity of the bell does not change

3. The Young's modulus of aluminum (density = 2700 kg/m3) is
6.9 × 1010 N/m2. Determine the speed of sound in an aluminum rod.
a. 1.4 × 103 m/s
b. 2.5 × 103 m/s
c. 5.1 × 103 m/s
d. 6.3 × 103 m/s
e. 7.0 × 103 m/s

4. Two boys are whispering in the library. The radiated sound power
from one boy's mouth is 1.2 × 10-9 W and it spreads out uniformly in
all directions. What is the minimum distance the boys must be
away from the librarian so that she will not be able to hear them?
The threshold of hearing for the librarian is 1.00 × 10-12 W/m2/
a. 100 m
b. 35 m
c. 23 m
d. 16 m
e. 9.8 m

5. At a distance of 5.0 from a point sound source, the sound intensity
level is 110 dB. At what distance is the intensity level 95 dB?
a. 5.0 m
b. 7.1 m
c. 14 m
d. 28 m
e. 42 m

6. The car in the figure is moving to the left at 35 m/s. The car's horn
continuously emits a 2.20 × 102 Hz sound. The figure also shows
the first two regions of compression of the emitted sound waves.
The speed of sound is 343 m/s. How far does the car move in one
period of the sound emitted from the horn?
a. 0.08 m
b. 0.16 m
c. 8 m
d. 16 m
e. 35 m

7. Two pulses of identical shape travel toward each other in opposite
directions on a string, as shown in the figure. Which one of the
following statements concerning the situation is true?
a. The pulses will reflect from each other
b. The pulses will diffract from each other
c. The pulses will interfere to produce a standing wave
d. The pulses will pass through each other and produce beats
e. As the pulses pass through each other, they will interfere

8. Sound waves are emitted from two speakers. Which one of the
following statements about sound wave interference is false?
a. In a region where both destructive and constructive
interference occur, energy is not conserved
b. Destructive interference occurs when two waves are exactly
out of phase when they meet
c. Interference redistributes the energy carried by the
individual waves.
d. Constructive interference occurs when two waves are exactly
in phase when they meet
e. Sound waves undergo diffraction as they exit each speaker

9. Two loudspeakers are located 3 m apart on the stage of an
auditorium. A listener at point P is seated 29.0 m from one speaker
and 25.0 from the other. A signal generator drives the speakers in
phase with the same amplitude and frequency. The wave amplitude
at P due to each speaker alone is A. The frequency is then varied
between 20 Hz and 300 Hz. The speed of sound is 343 m/s.
At what frequency or frequencies will the listener at P hear a
maximum intensity?
a. 170 Hz only
b. 113 Hz and 226 Hz
c. 86 Hz, 170 Hz, 257 Hz
d. 57 Hz, 113 Hz, 170 Hz, 227 Hz, and 284 Hz
e. 43 Hz, 85 Hz, 128 Hz, 170 Hz, 213 Hz, 257 Hz, and 298 Hz

10. For a diffraction horn loudspeaker, the sound emerges through a
rectangular opening. The opening of a diffraction horn has a width
of 0.15 m. If the speaker emits a continuous tome with a
wavelength of 0.11 m, at what angle does the first minimum occur?
a. 47°
b. 39°
c. 23°
d. 12°
e. 8.4°

11. A guitar string produces 4 beats/s when sounded with a 250 Hz
tuning fork and 9 beats per second when sounded with a 255 Hz
tuning fork. What is the vibrational frequency of the string?
a. 240 Hz
b. 246 Hz
c. 254 Hz
d. 259 Hz
e. 263 Hz

12. One string on a guitar is exactly in tune. The guitarist uses this
string to produce a tone with a frequency of 196 Hz by pressing
down at the proper fret. An adjacent string can also be used to
produce this tone without being pressed against a fret. However,
this adjacent string is out of tune and produces a tone that sounds
lower in frequency than the other tone. When the tones are
produced simultaneously, the beat frequency is 5.0 Hz. What
frequency does the adjacent string produce?
a. 196 Hz
b. 191 Hz
c. 171 Hz
d. 201 Hz
e. 186 Hz

13. A cylindrical tube sustains standing waves at the following
frequencies: 600 Hz, 800 Hz, and 1000 Hz. The tube does not
sustain standing waves at 500 Hz, at 900 Hz, at any frequencies
between 600 and 800 Hz, or at any frequencies between 800 and
1000 Hz. Determine the fundamental frequency of the tube and
whether the tube is open at both ends or has only one end open.
a. 50 Hz, both ends
b. 100 Hz, one end
c. 100 Hz, both ends
d. 200 Hz, one end
e. 200 Hz, both ends

14 . What is the electric flux passing through a Gaussian surface that
surrounds a +0.075 C point charge?
a. 8.5 × 109 N?m2/C
b. 6.8 × 108 N?m2/C
c. 1.3 × 107 N?m2/C
d. 4.9 × 106 N?m2/C
e. 7.2 × 105 N?m2/C

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Solution Preview

Here are the explanations of the answers:
1c = 261.63 * sqrt(2)
2a : the speed, and therefore the frequency, are determined by the ...

Solution Summary

Very brief explanation is given in addition to the answers.