| 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 ampl 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 [单项选择]She often says her greatest happinessconsists in helping the disadvantaged children.
A. is proportionate to B. is composed of C. lies in D. relies on [单项选择]What does the mother ask her son to do
[填空题]
What happened when a mother and her daughter were sitting in their room ______ broke out. 我来回答: 提交
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