Physics 41 Exam 1 Spring 2012
Print Name: ________________________________
Circle Lab:
1
2
3
4
Short Questions and Problems: 2 points each. Circle the BEST answer.
1. Frequencies that are too high for humans to hear are called:
a) supersonic
b) infrasonic
c) hypersonic
d) ultrasonic e) ubersonic
f) funkasonic
2. Amy and Zack are both listening to the source of sound waves that is moving to the right.
How do the wave speeds compare?
a. vAmy < vZack
b. vAmy = vZack
c. vAmy > vZack
3. A tube open at one end and closed at the other produces sound of fundamental frequency of 350 Hz.
If you now open the closed end, the fundamental frequency becomes 175 Hz.
a) True
b) False
4. Two tuning forks produce sounds of wavelengths 3.4 meters and 3.3 meters. Approximately what beat
frequency is produced?
a. 0.1 hertz
b. 1.0 hertz
c. 2.0 hertz
d. 3.0 hertz
5 Two harmonic waves traveling in opposite directions interfere to produce a standing wave described by
y = 2 sin (πx) cos (3πt) where x is in m and t is in s. What is the distance (in m) between the first two
antinodes?
a. 2
b. 4
c. 1
d. 0.5
6. Circle the points that have constructive interference:
7. and destructive interference:
P
Q
R
P
Q
R
8. What is the phase difference between the crest of a wave and the adjacent trough?
a. 0
b. π /4
c. π /2
d. 3 π /2
e. π
9. Find the speed of an ocean wave whose displacement is given by y = 3.7 cos(2.2x – 5.6t) where x and y
are in meters and t is in seconds.
a. 0.4 m/s
b. 1.5 m/s
c. 2.5 m/s
d. 1.7 m/s
10. The mass is displaced from equilibrium and set into SHM. The effective
spring constant for the system is:
a. k1 + k2
b.
k1 + k2
2
c.
k1k2
k1 + k2
d.
k1 + k2
k1k2
Problems: 10 points each. You MUST show work for any credit! BOX final answers! Neatness Counts!
1. The siren of an ambulance wails at 1395 Hz when the ambulance is stationary. What frequency will you
hear after this ambulance passes you while traveling at 33.60 m/s? The speed of sound under the
prevailing conditions is 343.0 m/s.
A) 152 Hz
B) 1271 Hz
C) 1325 Hz
D) 1546 Hz
Two loudspeakers are 3.20 m apart as shown. Both emit identical sound
waves which are in phase and of equal intensity. Assume the speed of
sound to be 343 m/s.
2. If the observer at C hears the first maximum (n = 1, constructive
interference), what is the frequency of the sound?
a.
b.
c.
d.
e.
86 Hz
143 Hz
214 Hz
107 Hz
428 Hz
3. If each speaker alone creates sound with an average power output of 50 Watts, what is the total intensity
of sound observed at C in dB?
a.
b.
c.
d.
e.
98 dB
112 dB
120 dB
137 dB
140 dB
The Long Problems ( 25 points each) Show all your work. Neatness Counts! Units Count! Details Count!
Box final answers. Assume and report results to 3 significant figures.
A uniform disk (R = 1.00 m, m = 2.00 kg) is suspended from a pivot a distance 0.250 m above its center of
mass. Ignore air resistance and any other frictional forces. Starting from Newton’s Second Law and
assuming SHM and small oscillations, derive an expression for the angular frequency in terms of the given
variables: (R, m, g), and simplify it. Box it. Then solve for a numerical value of the period and box that.
Show all your work. Draw a detailed diagram, showing angle, forces, lengths, lever arms, etc, and explain
whatever is needed for a fantastic solution, just like we did in lab.
For a uniform disk: I cm =
1
mR 2
2
2. A string mass-pulley system like the one we studied in lab has a vibrating length of 1.75 m and a mass
density of 0.0015 g/cm. The hanging mass is 250.0 g and the string forms a standing wave in the 4th
harmonic with maximum amplitude of 3.00mm.
a) Sketch the setup showing standing wave of the 4th harmonic and labeling the wavelength.
b) Find the speed of the wave on the string.
c) Find the frequency of the 4th harmonic.
d) Find the power transmitted down the string in this mode. Show explicitly how the units work out!
NEATNESS COUNTS!!! BOX YOUR FINAL NUMERICAL ANSWERS!