题干

绳中有列正弦横波,沿x轴传播,图1中a、b是绳上两点,它们在x轴方向上的距离小于一个波长.a、b两点的振动图象如图2所示.试在图上a、b之间画出t=1.0s时的波形图.

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答案(点此获取答案解析)

解:由振动图象2可知:t=1.0s时,质点a处于正向最大位移处(波峰处),质点b处于平衡位置且向下振动.先画出一列沿x轴正方向传播的波形图,如图所示.在图左侧波峰处标出a点.b点在a的右测,到a点距离小于1个波长的平衡位置,即可能是b1、b2两种情况.而振动方向向下的点只有b2.题中所求沿x轴正方向传播的波在a、b之间的波形图即为图中ab2段所示.画到原题图上时波形如图甲(实线)所示.

同理可以画出波沿x轴负方

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    It is quite reasonable to blame traffic jams, the cost of gas and the great speed of modern life, but manners on the road are becoming horrible. Everybody knows that the nicest men would become fierce tigers behind the wheel. It is all right to have a tiger in a cage, but to have one in the driver's seat is another matter.

    Road politeness is not only good manners, but a good sense. It takes the most cool-headed drivers great patience to give up the desire to beat back when forced to face rude driving. On the other hand, a little politeness goes a long way towards reducing the possibility of quarreling and fighting. A friendly nod or a wave of thanks in answer to an act of politeness helps to create an atmosphere of good will and becomes so necessary in modern traffic conditions. But such behavior of politeness is by no means enough. Many drivers nowadays don't even seem able to recognize politeness when they see it.

    However, misplaced politeness can also be dangerous. A typical example is the driver who waves a child crossing the street at a wrong place into the path of oncoming cars that may not be able to stop in time. The same goes for encouraging old ladies to cross the road wherever and whenever they want to.

An experienced driver, whose manners are faultless, told me it would help if drivers learnt to correctly join in a traffic stream without causing total blockages that give rise to unpleasant feelings. Unfortunately, modern drivers can't even learn to drive, let alone master the roadmanship (公路驾车技能). Years ago, experts warned us that the fast increase of the car ownership would demand more give-and-take (互谅互让) from all road users. It is high time for all of us to take this message to heart.