Concept explainers
BIO Resonance vibration transfer and the ear When you push a person on a swing. a series of snail pushes limed to match the swinger's swinging frequency makes the person swing with larger amplitude. If timed differently, the pushing is ineffective. The board shown in Figure 10.17 (from the Exploratorium in San Francisco) is made of rods of different length with dentinal balls on the ends of each rod Each rod vibrates at a different natural frequency, the long rod on the left at lower frequency and the short rod on the right at higher frequency if you shake the board at the high frequency at which the short rod vibrates, the short rod swings with large amplitude while the others swing a little. If you shake the board at the middle frequency at which the two center rods vibrate. the center rods undergo large-amplitude vibrations and the rods on each end do not vibrate imagine now that you have a fancy board with 15.000 rods, each of slightly different length, the shortest on the left and the longest on the right Shaking the board at a particular frequency causes the rods in one small region of the board to vibrate at this frequency and has little effect on the others.
The inner ear (the cochlea)
You hang four pendulum bobs from strings connected to a wooden dowel. The strings are different lengths. How can you get the second longest pendulum bob to vibrate while the other three do not—without touching the pendulums?
a. Shake the dowel back and forth.
b. Shake the dowel back and forth at the resonant frequency of that pendulum.
c. Move the dowel sideways at any frequency.
d. Blow air on that bob.
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