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Exam I – Physics 1240 – Fall 2010
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1. Professor Betterton puts a strobe light in front of a spinning fan and adjusts the frequency of the
strobe to the highest possible frequency so that what you see in class is the fan blade “frozen” in one
position. The strobe is flashing with a frequency of 50 Hz. How many rotations per minute does the
fan make?
A) 25/min
B) 600/min C) 1500/min
D) 3000/min
E) 9000/min
2. Suppose the strobe frequency is doubled (compared to the previous problem). What will you see
when you look at the fan?
A) It will look like it would with normal room illumination, you'll see the fan blade spinning as usual.
B) Exactly the same as before, the fan blade is “frozen” in one position.
C) The fan blade “frozen” in the upside-down (opposite) position compared to before.
D) The fan blade rotates around very slowly.
E) A kind of “double exposure” – you see the fan blade froze in the original position and in the
upside-down (opposite) position compared to before.
3. A signal generator drives a speaker. Next to the speaker is
a microphone, which is connected to an oscilloscope. The
oscilloscope shows a pattern (see diagram on the right). The
vertical axis measures Volts (which is what the microphone
produces), the horizontal axis measures time in ms
(milliseconds). What is the frequency of this sound wave?
A) 0.05 Hz
B) 20 Hz
C) 25 Hz
D) 50 Hz
E) 100 Hz
Voltage
1V
0 s
-1 V
10
20
30
40
time (in ms)
4. If a tuning fork vibrates steadily at 1.5 kHz, what is the approximate physical distance in the room
between one high pressure peak, and the next one, at any given instant in time? (Assume the
temperature is 20 °C.)
A) 344 m
B) 23 m
C) 4.4 m
D) 23 cm
E) 44 mm
The next two questions address what happens when the tuning fork of the previous problem is moved
to a much warmer room where the temperature is 40 °C. The tuning fork still vibrates steadily at 1.5
kHz.
5. What happens to the speed of the sound wave produced because of the increase in temperature?
A) It goes down.
B) It stays the same.
C) It goes up.
D) Not enough information to determine.
6. What happens to the wavelength of the sound produced because of the increase in temperature?
A) It goes down.
B) It stays the same.
C) It goes up.
D) Not enough information to determine.
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7. An object is oscillating back and forth on an spring, undergoing simple harmonic motion at some
frequency. You then make two changes: first, you replace the mass with one that is nine times lighter
(the new mass is 9 times smaller). Second, you pull the object three times as far from equilibrium
before letting it go.
What happens to the frequency of oscillation as a result of these two combined changes?
A) It is a 9 times lower frequency than before.
B) It is a 9 times higher frequency than before.
C) It is a 3 times lower frequency than before.
D) It is a 3 times higher frequency than before.
E) None of the above is correct.
The next four questions refer to the diagrams shown below. The diagrams show snapshots of two
waves, labeled (i) and (ii), on two wiggled strings, at some given time. The two strings are identical.
There is a small black circle which labels a particular spot on each string but does not alter the strings’
motion.
(i)
(ii)
Transverse displacement, in m
.01
0
-.01
Transverse displacement, in m
.01
1
2
3
4
position along string, in m
0
-.01
1
2
3
4
position along string, in m
8. Consider wave (i). The time for one point on the string to undergo one full cycle of oscillation is 0.5
s. What is the speed of the traveling wave?
A) 0.5 m/s
B) 1 m/s
C) 2 m/s
D) 4 m/s
E) 344 m/s
9. Consider waves (i) and (ii). These waves are:
A) (i) is longitudinal and (ii) is transverse. B) (i) is transverse and (ii) is longitudinal.
C) both transverse waves.
D) both longitudinal waves.
E) None of the above is a correct classification of these waves.
10. Compare waves (i) and (ii). Which wave travels fastest?
A) (i)
B) Both have the same speed
C) (ii)
D) Not enough information is given to decide.
11. Consider wave (ii). Which of the following best describes the motion of the labeled spot on the
string?
A) The spot moves to the right continuously.
B) The spot vibrates left and right repeatedly at a much lower frequency than the oscillation of the
string.
C) The spot vibrates left and right repeatedly at the same frequency as the oscillation of the string.
D) The spot vibrates up and down repeatedly at a much lower frequency than the oscillation of the
string.
E) The spot vibrates up and down repeatedly at the same frequency as the oscillation of the string.
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12. Two big boxes are sitting on the floor. One of them weighs 5 Newtons (that's about 1 pound), and
has a base which is a square shape, 1 meter by 1 meter. The other is bigger – it weighs 20 Newtons,
and has a base which is a square 2 meters by 2 meters.
Comparing the two, which one exerts more pressure on the floor, and by how much?
A) The smaller one exerts 4 times more pressure. B) The smaller one exerts twice the pressure.
C) They both exert the same pressure.
D) The larger one exerts 4 times more pressure.
E) The larger one exerts 4 times more pressure.
13. Which one of the following best describes the physical nature of sound in air?
A) When sound is produced, sound particles are emitted by the source. These sound particles collide
many times with air molecules on the way to our ears, where the sound particles are detected.
B) When sound is produced, sound particles are emitted by the source. These sound particles travel to
our ears in straight lines, without colliding with anything on the way.
C) There are no special sound particles. Sound is a traveling pressure disturbance created by the source,
and it is this disturbance that propagates through the air to our ears.
D) There are no special sound particles. When sound is produced the source emits regular air molecules
and those same molecules propagate through the room and arrive at our ears where they are detected.
E) Sound is a type of light, technically known as an electromagnetic wave, just very low frequency so
that we can hear it.
14. When you turn up the volume on your stereo, how does the motion of the speaker cone change?
A) The speaker cone makes larger oscillations, traveling back and forth farther on each cycle.
B) The speaker cone makes smaller oscillations, traveling back and forth less on each cycle.
C) The speaker cone oscillates back and forth more often, completing more cycles per second.
D) The speaker cone oscillates back and forth less often, completing fewer cycles per second.
E) None of the above.
15. Can sound travel through solids? Why or why not? Choose the answer with the best explanation.
A) Yes, because the sound is small enough that it can go in between the atoms of the solid
B) Yes, because solids can support a longitudinal pressure wave just as air can.
C) No, because sound would always reflect from a solid, and therefore none would go through.
D) No, because sound is a longitudinal wave, and you can't have longitudinal waves propagating
through solids.
E) No, because you need air for sound to propagate through, and solids have no air in them
16. The period of an oscillating object is doubled. What happens to its frequency?
A) It becomes ¼ of what it was (becomes 4 times smaller).
B) It quadruples (becomes 4 times larger).
C) It is unchanged (frequency and period are independent).
D) It doubles (becomes 2 times larger).
E) It halves (becomes 2 times smaller).
17. You use an electronic signal generator to play a pure tone of frequency 100 Hz. If you want to play
the same musical note two octaves lower, what frequency should you set on the signal generator?
A) 25 Hz
B) 50 Hz
C) 100 Hz, with the volume increased by a factor of 2
D) 200 Hz
E) 400 Hz
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For the last 3 questions, you are to select from a list of some of the experiments we have done in class.
Here is the list; the questions about them follow below.
A) We watched a movie video of a speaker producing a high-pitched tone and a wine glass placed near
the speaker. After about a minute, the wine glass shattered.
B) We placed a ringing bell inside a glass jar. When a vacuum pump removed the air from inside the
jar, the sound made by the bell went away.
C) We used a drill to rotate a series of disks with different numbers of tabs, all rotating at the same
speed. We observed how the sound was different, depending on which disk (i.e., different numbers of
tabs) created the sound when a stick was held against it.
D) We hung a mass on a spring and observed the oscillations. When we switched to a larger mass, the
period of the oscillation increased.
18. Of the experiments listed above, which ONE best demonstrated the relationship between frequency
(a physically measurable quantity) and pitch (human perception)?
Bubble “E” if you think none of these experiments was well suited to demonstrate this relationship.
(If you think more than one showed this relationship, pick the one which showed it most directly)
C
19. Of the experiments listed above, which ONE would provide the best direct evidence that sound
waves travel through a medium?
Bubble “E” if none of these experiments provides any evidence that sound waves travel through a
medium. (If you think more than one provided evidence, pick the one which showed it most directly)
B
20. Of the experiments listed above, which ONE would provide the best direct evidence that sound is
a longitudinal wave?
Bubble “E” if none of these experiments provides direct evidence that sound is a longitudinal wave.
(If you think more than one provided evidence, pick the one which showed it most directly)
E