Physics 2C, Winter 2014
Prof. Massimiliano Di Ventra
Quiz 2
Jan 24, 2014
Instructions
A. Write your name and student ID number at the top of this sheet. You must show all work here, and turn this in
with your scantron.
B. Academic Integrity Announcement: I understand that cheating is an academic FELONY, which is taken very seriously
by UCSD. Punishment includes an F grade in the class, and possible suspension or expulsion from the University. I
understand that the TA and proctors have the right to randomly check my ID, and that it is their duty to report any
suspicious activity. I understand that I will turn in this exam, and that it will be kept as a record of my work. There will
be both random and non-random integrity checks. I understand that many methods will be used to detect cheating. I
understand that enabling another student to copy my answers is also considered cheating, with the same consequences.
YES, I UNDERSTAND, AND WILL OF COURSE FOLLOW THE UNIVERSITY'S RULES ABOUT ACADEMIC
INTEGRITY (NO CHEATING).
PLEASE SIGN:
C. No calculators or electronic devices.
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3
3
2
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Approximate π ≈ 3. Other possibly useful approximate facts: 1atm ≈ 10 Pa ; ρH2 ≈ 10 kg/m ; g = 9.8m/s ≈ 10m/s
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INTENTIONALLY LEFT BLANK. USE THIS SIDE FOR ANY OF YOUR ADDITIONAL CALCULATIONS.
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MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question.
1) Which one of the following statements is true?
A) Both the intensity level (in dB) and the sound intensity can be negative.
B) Both the intensity level (in dB) and the sound intensity can never be negative.
C) Both intensity level (in dB) and sound intensity obey inverse-square distance laws.
D) The sound intensity can never be negative, but the intensity level (in dB) can be negative.
E) The intensity level (in dB) obeys an inverse-square distance law, but the sound intensity does not.
2) A pipe that is 120 cm long resonates to produce sound of wavelengths 480 cm, 160 cm, and 96 cm but
does not resonate at any wavelengths longer than these. This pipe is
A) closed at both ends.
B) open at one end and closed at the other end.
C) open at both ends.
D) We cannot tell because we do not know the frequency of the sound.
3) When a rocket is traveling toward a mountain at 100 m/s, the sound waves from this rocket's engine
approach the mountain at speed V. If the rocket doubles its speed to 200 m/s, the sound waves from the
engine will now approach the mountain at speed
A) 4V
B) 0.5V
C) V
D) 2V
4) A person is hearing two sound waves simultaneously. One has a period of 1 ms and the other one a
period of 2 ms. What is the period of the beat due to these two waves?
A) 2 ms
B) 0.040 ms
C) 1.5 ms
D) 330 ms
E) 3.0 ms
5) A bat is flying, at a velocity v = vsound/2, directly at a flat wall of a cave. The bat hears its own bleep,
which was emitted at frequency 6kHz, after it was reflected off the cave wall. What frequency does the
bat hear for this reflected wave?
A) 3kHz B) 10 kHz
C) 2kHz
D) 18 kHz
E) 42 kHz
6) Consider a pipe of length L that is open at both ends. What are the wavelengths of the three lowest-pitch
tones produced by this pipe?
A) 4L, 2L, L
B) 2L, L, L/2
C) 2L, L, L/2
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D) 2L, L, 2L/3
E) 4L, 4L/3, 4L/5
7) As you stand by the side of the road, a car approaches you at a constant speed, sounding its horn, and
you hear a frequency of 80.0 Hz. After the car goes by, you hear a frequency of 60.0 Hz. What is the speed
of the car? The speed of sound in the air is 343 m/s.
A) 64.0 m/sB) 36.0 m/s C) 16.0 m/s
D) 25.0 m/s
E) 49.0 m/s
8) At a distance of L = 20 m from one speaker, the decibel level is β1 = 100dB. What would be the decibel
level at a distance L = 20 m from 100 of these speakers (all identical and in phase)?
A) 20dB
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B) 104 dB
C) 200 dB
D) 140 dB
E) 1200 dB
1) D
2) B
3) C
4) A
5) D
6) D
7) E
8) D
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