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  3. If a mother pushes her small son in a swing, giving only a light force each time he returns, eventually he will be swinging quit

If a mother pushes her small son in a swing, giving only a light force each time he returns, eventually he will be swinging quit

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问题     If a mother pushes her small son in a swing, giving only a light force each time he returns, eventually he will be swinging quite high. The child can do this for himself by using his legs to increase the motion, but both the mother’s push and the child’s leg movements must occur at the proper moment, or the extent of the swing will not increase. In physics, increasing the swing is increasing the amplitude; the length of the rope on the swing determines its natural oscillation period. This ability of an object to move periodically or to vibrate when stimulated by a force operating in its natural period is called resonance.
    Resonance is observed many times without consciously thinking about it; for example, one may find an annoying vibration or shimmy in an automobile, caused by a loose engine mount vibrating with increasing amplitude because of an out-of-round tire. The bulge on the tire slaps the pavement with each revolution; at the natural resonance point of the engine mount, it will begin to vibrate. Such vibrations can result in considerable damage if allowed to persist. Another destructive example of resonance is the shattering of a crystal goblet by the production of a musical tone at the natural resonant point of goblet. The energy of the sound waves causes vibration in the glass; as its amplitude increases, the motion in the glass exceeds the elasticity of the goblet, and it shatters.
    An instrument called a tachometer makes use of the principle of resonance. It consists of many tiny bars, loosely fastened together and arranged so that each bar can slide independently of the others. Movement of the bars causes changes in a dial. When placed next to a rotating motor or engine, the tachometer picks up slight vibrations which are transferred to the resonant bars. These bars begin to move, and the resulting dial may be read to find the revolutions per minute of the motor very quickly.
An annoying vibration can be caused at the natural resonance of the car’s engine mount______.
选项 A、if the engine moves too fast
B、if the engine’s amplitude increases
C、if a tire gets out of balance
D、if a damage occurs in the engine
答案C
解析 答案在第二段第一句话：“…one may find an annoying vibration or shimmy in anautomobile，aused by a loose engine mount vibrating with increasing amplitude because of anout—of-round tire．”其中，out—of-round tire=tire out of balance。
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