问题 问答题

如图所示,一横波的波源在坐标原点,x轴为波的传播方向,y轴为振动方向.当波源开始振动1s时,形成了如图所示的波形(波刚传到图中P点).试求:

(1)从图示位置再经多长时间波传到Q点?

(2)波传到Q点时质点P的位移.

答案

(1)根据图象,波的波长λ=4m,周期为T=1s,则波速为v=

λ
T
=4m/s

故波传的到Q点用的时间t=

xPQ
v
=
7-4
4
s=0.75s

(2)质点振动的周期:T=1s

P质点开始向下振动,波经P点传到Q点的时间为

3
4
周期,则此时质点P到达波峰,其位移为10cm.

答:

(1)从图示位置再经0.75s长时间波传到Q点.

(2)波传到Q点时质点P的位移是10cm.

选择题
阅读理解

B

YOU CAN HELP!

Everyone was born with his own built-in burglar(窃贼) alarm. It’s called the sense of sight and sound.Unfortunately, many of us go around with the alarm switched off.

We don’t see the stranger wandering outside the house next door.

We don’t notice the sounds from the flat upstairs.

(Weren’t they supposed to be on holiday?)

The police can only do so much to prevent crime. There never can be enough of them to guard every home in every town. So they need your help in fighting with the burglars,the vandals(恣意破坏者), and the car thieves.

Not, of course, by setting out to have a go every time you see something suspicious. It’ll always be the job of the police to arrest criminals.

But by acting as a line of communication between them and your community, for instance, you probably know far more about your immediate neighborhoods than the police ever could.

A stranger in someone’s garden would probably be far more obvious to you than it would to even the local police, if, of course, you were on the look-out.

That’s the whole idea behind the Neighborhood Watch schemes, springing up around the country to create a spirit of watchfulness within a community, anything suspicious being reported to the police.

It’s early days yet, but results so far are very encouraging. The crime figures are already dropping in many of the areas running the scheme. And all due to people like you.

61. The underlined word “them” refers to “______”.

A. criminals          B. the police   C. neighbors   D. strangers

62. The advertisement points out that many people______.

A. are not ready to help the police   

B. are not as watchful as they could be

C. don’t look after their gardens well      

D. don’t tell their neighbors about their holidays

63. One of the ways we could help prevent crime is to______.

A. turn on the alarm system in our home 

B. try to stop criminals from escaping

C. look out for people behaving suspiciously 

D. inform the police if we hear noises upstairs

64. The purpose of the advertisement is to __________in their neighborhood.

A. ask people to join the police force

B. advise people how to protect their homes

C. warn people about the increasing risk of crime

D. encourage people to be on watch for possible crime