The
BOSS Model: Name: _______________ E.N.___
Building Oscillation Seismic Simulation Period: _________ Due Date: ____
RATIONALE
During
an earthquake, buildings oscillate. If the frequency of this oscillation is
close to the natural frequency of the building, resonance may cause severe
damage. The BOSS model allows students to observe the phenomenon of resonance.
FOCUS QUESTIONS
Why do
buildings of different heights respond differently in an earthquake?
OBJECTIV ES
Students
will:
1.
Predict how a structure will react to vibrations (oscillations) of different
frequencies.
2.
Perform an experiment to establish the relationship between the height of a
structure and its natural frequency.
3.
Describe the phenomenon of resonance.
Demo:
Oscillation
Discussion
Notes:
Procedure:
a. Hold
the base stationary, pull the wooden number 1 out several centimeters to the
side, and release it. As the rod oscillates, use a stopwatch to measure the
time for 10 oscillations.
Record this number
b.
Practice until you can get almost the same swing each time, then repeat the measurement four
times. Calculate the average of these four times. Now calculate the natural
frequency of the number by dividing 10 cycles by the average time.
Record it. Repeat this procedure for the other
three numbers.
c.
Measure the height of each assembly from the base to the top, and record it.
d. Plot
height versus natural frequency on the graph provided.
Post Lab
Questions:
1. From
what you have learned, do the earthquakes with the highest numbers on the
Richter Scale always do the most damage?
2.
Discuss your results. Be sure to point out the connection between the
experimental results and the way real buildings resonate. Other things being
equal, do taller buildings have lower natural frequencies than short buildings?
3. Explain how seismic waves caused by
earthquakes produce oscillations, or vibrations, in materials with many
different frequencies.
Include: Natural frequency, Resonance, ground shaking,
4. Hypothesize
what would happen when buildings of two different heights, standing next to
each other, resonate from an earthquake.
5. “How
could you add structural elements to reduce resonance in a building?”
ADAPTATIONS AND EXTENSIONS
One way
to protect a building from resonating with an earthquake is to isolate its
foundation, or base, from the ground with devices much like wheels. This
technique is called base isolation. Structural engineers are now developing the
technology to place buildings on devices that absorb energy, so that ground shaking
is not directly transferred to the building. Add standard small wheels from a hardware
store to your models as an illustration of one of the many base isolation
technologies, or add wheels to your own BOSS model, then shake the table.
Better yet, place the model in a low box or tray and shake it. Then take out
the model, fill the box with marbles or BBs, and
replace the model on this base. Now shake the box. Can you come up with other
base isolation techniques?
Observations:
Inference(s):
Record
oscillation times in the data table below in the appropriate place for each rod
assembly. These times are measured in seconds per 10 cycles. Repeat the
measurement four times to minimize human error, then record.
Caution: Start
the stopwatch as a numbered block reaches its maximum swing and start counting
with zero, otherwise you will end timing only nine swings. Practice this until
your times for 10 oscillations are fairly close to each other.
Calculate
the average
time for each oscillation by adding four measurements and dividing
by four. Record.
Calculate
the natural frequency. Divide 10 cycles by the average
time (do not simply move the decimal point). Frequency is measured in hertz, or
cycles per second. Record.
1. How
much variation do you notice among the four trials?
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2. What
relationship do you notice, if any, between the height
of the rods and their natural frequencies?
________________________________________________________________________________________________________________________________________
B.
Heights of the Rod Assemblies
1.
Measure the height of each rod assembly from the base to the top of the block
and record it.
Rod Assembly 
Height (cm) 
#1 

#2 

#3 

#4 

2. What
is the approximate difference in height between #1 and #2, #2 and #3, and so
on?
________________________________________________________________________________________________________________________________________
3. Plot
the height versus the natural frequency of each rod assembly on the graph
provided. You should have four data points. Connect the points with the best fitting
straight or curved line you can.
4. What
kind of line did you get from your data?
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5. As
the height of the rod assemblies gets larger, what happens to their natural
frequency?
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C.
Summary
1. What
variable is manipulated in this experiment? (How do the four rod assemblies
differ from each other?)
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2. What
is the responding variable in this experiment? (What did you measure?)
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3. What
does oscillate, or vibrate, mean?
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4.
Define frequency.
____________________________________________________________________
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5. Why
does only one rod assembly oscillate greatly (or resonate) when you wiggle the
base?
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6. What
is resonance?
____________________________________________________________________
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7. How
are the rod assemblies like buildings?
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8. (extra credit) How
can a building be protected from resonating with seismic vibrations?
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