Chapter 18525Q18.16Walking makes the person’s hand vibrate a little. If the frequency of this motion is equal to thenatural frequency of coffee sloshing from side to side in the cup, then a large–amplitude vibration ofthe coffee will build up in resonance. To get off resonance and back to the normal case of a small-amplitude disturbance producing a small–amplitude result, the person can walk faster, walk slower,or get a larger or smaller cup. Alternatively, even at resonance he can reduce the amplitude byadding damping, as by stirring high–fiber quick–cooking oatmeal into the hot coffee.Q18.17Beats. The propellers are rotating at slightly different frequencies.Q18.18Instead of just radiating sound very softly into the surrounding air, the tuning fork makes thechalkboard vibrate. With its large area this stiff sounding board radiates sound into the air withhigher power. So it drains away the fork’s energy of vibration faster and the fork stops vibratingsooner. This process exemplifies conservation of energy, as the energy of vibration of the fork is
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