问题 选择题

质量为m的物体,从静止开始以g/2的加速度竖直下落h的过程中,下列说法中正确的是(  )

A.物体的机械能守恒

B.物体的机械能减少mgh/2

C.物体的重力势能减少mgh/2

D.物体克服重力做功为mgh

答案

因物体的加速度为

g
2
,故说明物体受阻力作用,由牛顿第二定律可知,mg-f=ma;

解得f=

1
2
mg

重力做功WG=mgh; 阻力做功Wf=-

1
2
mgh

A、物体下落过程中除重力还有阻力做功,机械能不守恒,故A错误;

B、机械能的变化量等于除重力以外的力做的功,所以机械能减少

1
2
mgh,故B正确;

C、重力做功等于重力势能的改变量,重力做正功,故重力势能减小mgh,故C错误;

D、重力做正功,物体不是克服重力做功,故D错误.

故选B

阅读理解

D

Influenced by their families and society, many youths are infatuated with expensive famous brands. But not every youth is as rich as a millionaire and people are getting worried about this.

According to a recent survey, one out of five pupils wears clothes of well-known brands, including a belt worth 560 Yuan, more than an ordinary worker’s salary. Among the 100 middle school students surveyed, as many as 70 own famous-brand clothes, 60 have famous-brand watches and 45 possess famous-brand bicycles. Two of them even wear over 1500 Yuan worth of suits.

Most young people who come from ordinary families will try to steal money to have better food and clothing if their parents don’t satisfy their needs. A boy named Chen even killed another boy simply for 500 Yuan and was sentenced to nine years’ imprisonment.

People wonder how these teenagers can be independent in the future, while parents are calling for help from schools, governments and the society.

which of the following is not true according to the passage?

A.More than 50 percent of the students surveyed possess things of famous brands.

B.An ordinary worker owns less than 560 Yuan each month.

C.Suits may cost as much as 1500 Yuan.

D.Young people now are as rich as millionaires.

68 What does “be infatuated with” mean according to the context?

A.be anxious about

B.be filled with a strong unreasonable feeling of love for

C.have a strange and unusual way of

D.be low in spirits; sad for

69 As a result, the boy mentioned in the passage will       .

A.steal money for better food and clothing

B.kill another boy for 500 Yuan

C.stay in prison for nine years

D.ask his parents to satisfy his needs

70 The best title for the passage would be        .

A.Youths Run After Famous Brands. 

B.Youths Should Own More Money

C.Youths As Rich As Millionaires     

D.Youths Must Be Taken Good Care of

问答题