http://www.ck12.org Chapter 11. Wave Motion and Sound
a. Draw a side-view of the western section of the Bay Bridge and identify the seven nodes in this section
of the bridge.
b. Assume that the bridge is concrete (the speed of sound in concrete is 3200 m/s). What is the lowest
frequency of vibration for the bridge? (You can assume that the towers are equally spaced, and that the
central support is equidistant from both middle towers. The best way to approach this problem is by
drawing in a wave that “works.”)
c. What might happen if an earthquake occurs that shakes the bridge at precisely this frequency?- The simple bridge shown here oscillated up and down pretty violently four times every second as a result of
an earthquake.
a. What was the frequency of the shaking inHz?
b. Why was the bridge oscillating so violently?
c. Calculate two other frequencies that would be considered “dangerous” for the bridge.
d. What could you do to make the bridge safer? (Note that earthquakes rarely shake at more than 6 Hz)Answers- 390 Hz , 1170 Hz
- a. 3 wavelengths, 6thharmonic b. 3.56 Hz c. Resonance will occur, the amplitude will go out of control,
bridge will collapse - a. 4 Hz b. It was being driven near its resonant frequency. c. 8 Hz,12 Hz d. Put a support in the middle,
making the fundamental frequency 8 Hz
Summary
In this Chapter wave motion is studied in detail. Type’s of waves, the wave equation, interference, standing waves,
harmonics and sound are all covered.