题目内容
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 the 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. The distance a swing moves from its resting position is called its ______.
查看答案
搜索结果不匹配？点我反馈
更多问题
Already lasers can obliterate skin blemishes, topically applied drugs can smooth facial lines and injected agents can remove deep wrinkles. Future products will be faster, better and longer lasting. "New substances will be developed by entrepreneurs," says Brian Mayou, an aesthetic plastic surgeon, "that will be more successful than liquid silicone that we use today to eradicate wrinkles." The next major breakthrough, says Mel Braham, plastic surgeon and chief executive of the Harley Medical Group, will be laser treatment that needs no recovery period. Nicholas Lowe, clinical professor of dermatology at the University of Los Angeles, adds: "There will be more efficient anti-oxidants to help reduce sun damage and aging. There will also be substances that increase the production of new collagen and elastic tissue to maintain the elasticity of youthful skin." Lee Shreider, a research cosmetic chemist, says that we may be able to look better without any kind of operation as semi-permanent make-up gets better. "Crooked noses will be improved by effectively sealing on shaded colors that either enhance or subdue areas of the face. We will be able to straighten eyebrows and lips making the face more metrical—which remains one of the keys to beauty, and even close blocked pores with permanent, custom-designed foundation." The development of the safe Sun tan is a potential gold mine. Being researched at the University of Arizona, but a long way from reality, is the injectable tan. Professor Lowe is optimistic: "There will almost certainly be a safe way of developing a sunless tan that protects against sun damage. In animal research, we’ve applied creams to guinea pigs that can actually ’turn on’ some of the genes that produce pigmentation without any sunlight exposure.\ Paragraph 4 suggests that one could improve effectively one’s appearance concerning the nose, eyebrows, lips, etc. by ______.
分析方法评价的效能指标为 以信噪比3：1确定
关于妊娠合并急性肾盂肾炎的说法正确的是
When we think about addiction to drugs or alcohol, we frequently focus on negative aspects, ignoring the pleasures that accompany drinking or drug-taking. (21) the essence of any serious addiction is a pursuit of pleasure, a search for a "high" that normal life does not (22) . It is only the inability to function (23) the addictive substance that is dismaying, the dependence of the organism upon a certain experience and a(n) (24) inability to function normally without it. Thus a person will take two or three (25) at the end of the day not merely for the pleasure drinking provides, but also because he "doesn’t feel (26) " without them. (27) does not merely pursue a pleasurable experience and need to (28) it in order to function normally. He needs to repeat it again and again. Something about that particular experience makes life without it (29) complete. Other potentially pleasurable experiences axe no longer possible, (30) under the spell of the addictive experience, his life is peculiarly (31) . The addict craves an experience and yet he is never really satisfied. The organism may be (32) sated, but soon it begins to crave again. Finally a serious addiction is (33) a harmless pursuit of pleasure by its distinctly destructive elements. A heroin addict, for instance, leads a (34) life: his increasing need for heroin in increasing doses prevents him from Working, from maintaining relationships, from developing in human ways. (35) an alcoholic’s life is narrowed and dehumanized by his dependence on alcohol.
微信支付 
支付宝支付