问题 选择题

如图9所示,表面粗糙的斜面固定于地面上,并处于方向垂直纸面向外、磁感应强度为B的匀强磁场中.质量为m、带电荷量为+Q的小滑块从斜面顶端由静止下滑.在滑块下滑的过程中,下列判断正确的是      (  )

A.滑块受到的摩擦力不变

B.滑块到达地面时的动能与B的大小无关

C.滑块受到的洛伦兹力方向垂直斜面向下

D.B很大时,滑块可能静止于斜面上

答案

答案:C

由左手定则知C正确.而Ff=μFN=μ(mgcosθ+BQv)要随速度增加而变大,A错误.若滑块滑到斜面底端已达到匀速运动状态,应有Ff=mgsinθ,可得v= (-cosθ),可看到v随B的增大而减小.若滑块滑到斜面底端时还处于加速运动状态,则在B越强时,Ff越大,滑块克服阻力做功越多,到达斜面底端的速度越小,B错误.当滑块能静止于斜面上时应有mgsinθ=μmgcosθ,即μ=tanθ,与B的大小无关,D错误.

单项选择题
阅读理解

People have long debated the essence(本质)of human nature.Some people think humans are

born good.In San Zi Jing(Three Chara~er Classic),a Chinese children’s educational book from

the 13th century, we read:“Men at birth are naturally good.Their natures are similar, their habits

become different.’’

Yet some philosophers disagree with this.They think men are born selfish and vicious(恶的).

For example,Thomas Hobbes,the 1 7th century English philosopher argued that men are born self-

interested and with a liking for war.

Over the centuries,different philosophers have argued their cases.Now the results of a new

study,which contrasts the behavior of very young children and young chimpanzees(黑猩猩)

suggests that human beings are born sociable and helpful to others,according to a New York Times

report.

“Of course every animal must to some extent be selfish to survive.But the biologists also see

in humans a natural willingness to help.”The New York Times science reporter Nicholas Wade writes.

Wade quotes a book published in October by American psychologist Michael Tomasello.

Tomasello writes:“When infants(婴儿)18 months old see an unrelated adult whose hands are

full and who needs help opening a door or picking up a dropped clothespin,they will immediately

help.The helping behavior seems to be innate because it appears so early and before many parents

start teaching children the rules of polite behavior.’’

Tomasello finds that this behavior is not the result of an incentive(动机)of a

reward,suggesting training does not influence it.And it seems to happen across cultures,with their different timetables for teaching social rules.For these reasons,Tomamsello concluded that helping is natural,not something taught by parents or culture.

And it seems that infants cannot only help in practical ways,it can also help with information,

Tomasello writes.From the age of 1 2 months they will point at objects thin all adult pretends to

have lost.Chimpanzees,by contrast,never point at things for each other, and when they point for

people.it seems to be as a command to go fetch something rather than to share information.

49.What is the article mainly about?

A.The behavior of young children.

B.Former philosophies about human nature.

C.The difference between babies and baby chimpanzees.

D.A new study of human nature.

50.According to Michael Tomasello, human beings are born           .

A.similar to chimpanzees in nature       B.selfish and Vicious

C.sociable and helpful       D.with a liking for war

51.Of all the following,      has a negative attitude towards human nature.

A.Three Character          B.Thomas Hobbes

C.Nicholas Wade    D.Michael Tomasello

52.According to the study,it's safe to say that          .

A.human beings are not selfish

B.children like to help others when they get a reward

C.different countries teach social rules at different times

D.children tend to be helpful if they are taught early