DATE: Wednesday April 29, 2020
Spring 2020 PHYS 120 Test 3
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1. [9 Points] Adult humans are able to exhale a volume of air of
about 1.5 × 10−3 m3 (1.5 liters) in 1.0 second.
a. Suppose you exhale this volume of air through a straw
with radius 0.40 cm. Calculate the speed of the air as it
leaves the straw. the straw. (3 pts)
𝑣 = _____________________m/s
Since the air leaves flows horizontally over the second straw, calculate the difference in pressure
between the pressure in the room (𝑃1 ) and the pressure at the top of the second straw (𝑃2 ). Notice
that the speed of the air in the rest of the room is zero. Also, you are given that the density of air
is 𝜌𝑎𝑖𝑟 = 1.29 𝑚3. (3 pts)
𝑃1 − 𝑃2 = __________________ N/m2.
c. Calculate the height the water rises to as shown in the figure. Remember that 𝜌𝑤𝑎𝑡𝑒𝑟 = 1000 𝑚3.
ℎ = ________________m
2. [6 Points] A block of mass m = 1.4 kg is attached to a spring with a force constant k = 370 N/m. The
block is in simple harmonic motion. At the moment that the block has zero instantaneous speed, the
magnitude of the force acting on it is F = 15 Newtons.
a. Find the maximum speed of the block during the vibrations. (3 points)
b. Find the magnitude of the net force acting on the block at the moment when the speed is one-half
of the maximum speed. (3 points)
𝐹 = ________________N
3. [11 Points] The position vs. time for a mass on
a spring is shown in the figure below. The
attached mass is 8 kg (8.000 kg to be more
a. What is the amplitude of these oscillations?
Use the graph shown to answer this. (1 pt)
𝐴 = _____________________m
b. What is the period of these oscillations?
Use the graph shown to answer this. (1 pt)
𝑇 = _____________________s
c. Calculate the angular frequency of oscillations. (1 pt)
𝜔 = _________________rad/s
d. Calculate the force constant of the spring. (2 pts)
𝑘 = _________________N/m
e. Calculate the total mechanical energy stored this mass-spring system. (2 pts)
𝐸 = __________________J
What is the position of the mass when 𝑡 = 4 seconds? (2 pts)
𝑥 = __________________m
g. What is the velocity of the mass when 𝑡 = 4 seconds? (2 pts)
𝑣 = __________________m
4. [6 Points] Two simple pendulums of lengths 𝐿1 = 𝐿 = 0.9 meters and 𝐿2 = 0.64𝐿 =
0.576 meters are suspended from a ceiling. Both pendulums are gently pulled sideways so
that the strings are parallel to each other (the side view of this situation is shown). The
pendula are then released simultaneously. The acceleration due to gravity is 9.80 m/s2.
a. Calculate the period for the pendulum of length 𝐿1 . (2 pts)
𝑇1 = ______________s
b. Calculate the period for the pendulum of length 𝐿2 . (2 pts)
𝑇2 = ______________s
c. How long would it be before the pendula both simultaneously come back to the position shown in
the diagram? (2 pts)
𝑡 = ________________s
5. [10 Points] You take 5 m of wire that
has a mass of 4 grams and use it to
set up a system of standing wave
patterns. You tie one end of the wire
to a post on your lab bench, 4 meters
of the wire runs horizontally to a
pulley, and after the pulley the
remaining meter of wire is used to
support a 1.5 kg hanging mass. Now,
an apparatus is used to cause the wire
between the post and the pulley to
vibrate with a frequency such that 6
bumps in the standing wave pattern
are seen. A sketch of the wave
pattern is shown below:
a. Calculate the mass per unit of length for the given wire. (2 points)
𝜇 = ___________________kg/m
b. Calculate the Tension of the wire. (2 points)
𝑇 = _________________N
c. Calculate the speed of waves in this wire under tension. (2 points)
𝑣 = __________________m/s
d. Calculate the wavelength of the standing waves generated in the above wire. (2 points)
𝜆 = __________________m
e. Calculate the frequency used to generate the standing waves in the above wire. (2 points)
𝑓 = _________________Hz
6. [8 Points] At 10 meters from a loud rock band you can hear a sound level = 100 decibels.
a. Calculate the intensity 10 meters away from this rock band. (2 points)
𝐼 = _____________________W/m2
b. Calculate the power produced by the rock bands. (2 points)
𝑃 = _____________________W
c. How far from the rock band do you have to move in order for = 0 decibels? (2 points)
𝑟 = __________________m
d. If 16 such bands play next to each other, by how much will the sound level () increase? (2
The sound level will increase by _________________dB
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