TEACHING CLUES AND CUES
You may choose to have
students build models in
class. In that case, you
will need to make stu-
dent copies of Master 4.3a, and
assembly will become step 1 under
Lesson Development. You will also
need more materials.
Instruct students to start
the stopwatch as a
numbered block reaches
its maximum swing and
story buildings. The effect was severe damage to medium-height
buildings that had the same frequency as the ground shaking and
resonated with it. Higher and lower buildings were hardly damaged.)
Use the BOSS model as a visual aid when describing this event. You
may also want to draw attention to the photos or books you used in
Lesson 1 of this unit.
4. Divide students into seismic engineering teams (SETs) and
distribute one copy of Master 4.3b, BOSS Worksheet, to each group.
Tell students that they will take turns performing an experiment with
the model, recording their data, and providing the answers asked for
on the data sheet. Give these directions:
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.
(Students should come up with a hyperbola, a curve representing an
inverse relationship in which, as the height of the structure increases,
its natural frequency decreases.)
e. Ask the class: From what you have learned, do the earthquakes with
the highest numbers on the Richter Scale always do the most damage?
(Students should already know that the amount of damage has to do
with population density and other factors, but now they will be aware
of something new. To illustrate the relationship of frequency and
resonance, use the example of someone pushing a child in a swing.
The person pushes a little at a time, over time, and soon the swing
goes very high without a big push. Each small push is at the right
frequency. Similarly, a building may vibrate with a great amplitude
without big earthquake vibrations because the smaller vibrations came
at that structure’s natural frequency.)
5. Ask the SETs to share and discuss their results. Again point out the
connection between the experimental results and the way real
buildings resonate. Other things being equal, taller buildings have
lower natural frequencies than short buildings.
C. Conclusion
Review the terms and concepts introduced in this lesson. Explain that
seismic waves caused by earthquakes produce oscillations, or
vibrations, in materials with many different frequencies. Every object
has a natural rate of vibration that scientists call its natural frequency.
The natural frequency of a building depends on its physical
start counting with zero. Often
students will start counting with one
when they start the stopwatch and
end up with only nine swings.
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F E M A
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