单选题 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.
单选题 An object, if moving rhythmically when stimulated in a natural period, is said to ______.
【正确答案】 B
【答案解析】[解析] 细节题。答案在第一段最后一句话：“This ability of an object to move periodically or to vibrate when stimulated by a force operating in its natural period is called resonance. ”意思是：当一个物体受到一个自然周期内力的刺激时进行的有规律的运动和震动被称做共振。
单选题 The distance a swing moves from its resting position is called its ______.
【正确答案】 D
【答案解析】[解析] 细节题。第一段的第三句：“In physics, increasing the swing is increasing the amplitude; the length of the rope on the swing determines its natural oscillation period. ”意思是：在物理学中，增加摆动就是增加振幅，摆上的绳索长度决定着摆的摆动周期。所以D为答案。
单选题 A tachometer is an instrument that uses resonance to determine ______.
【正确答案】 A
【答案解析】[解析] 细节题。最后一段最后一句“These bars begin to move, and the resulting dial may be read to find the revolutions per minute of the motor very quickly. ”意思是：这些针开始运动，可以读刻度盘迅速找出电动机每分钟所转的数。
单选题 An annoying vibration can be caused at the natural resonance of the car"s engine mount ______.
【正确答案】 C
【答案解析】[解析] 细节题。第二段第一句话：“...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. ”意思是：人们发现，汽车内由于松动了的发动机悬置随着增加的振幅的振动而产生的恼人的振动或摇晃是因为不圆的车胎造成的。out-of-round tire=tire out of balance.
单选题 In which of the following cases is it useful to consider the relationship between the length of an oscillating object and its natural period?
【正确答案】 B
【答案解析】