问题 选择题

(2011年镇海中学高三月考)如图,一磁铁用细线悬挂,一个很长的铜管固定在磁铁的正下方,开始时磁铁上端与铜管上端相平,烧断细线,磁铁落入铜管的过程中,下列说法正确的是(  )

①磁铁下落的加速度先增大,后减小

②磁铁下落的加速度恒定

③磁铁下落的速度先增大后减小

④磁铁下落的加速度一直减小最后为零

⑤磁铁下落的速度逐渐增大,最后匀速运动

A.只有②正确

B.只有①③正确

C.只有①⑤正确

D.只有④⑤正确

答案

答案:D

磁铁下落过程中,将在铜管中产生感应电流,由楞次定律可知,磁铁将受到阻碍,磁铁下落速度越大,阻力越大,加速度越小,当阻力与重力相等时,加速度减为0,速度达到最大,磁铁以此速度匀速运动,故只有D项正确.

填空题
单项选择题

A child who has once been pleased with a tale likes, as a rule, to have it retold in almost the same words, but this should not lead parents to treat printed fairy stories as formal texts. It is always much better to tell a story than read it out of a book, and, if a parent can produce what, in the actual situation of the time and the child, is an improvement on the printed text, so much the better.

A charge made against fairy tales is that they harm the child by frightening him or making him sad thinking. To prove the latter, one would have to show in a controlled experiment that children who have read fairy stories were more often sorry for cruelty than those who had not. As to fears, there are, I think, some cases of children being dangerously terrified by some fairy story. Often, however, this arises from the child having heard the story once. Familiarity with the story by repetition turns the pain of fear into the pleasure of a fear faced and mastered.There are also people who object to fairy stories on the grounds that they are not objectively true, that giants, witches, two-headed dragons, magic carpets, etc. do not exist; and that, instead of being fond of the strange side in fairy tales, the child should be taught to learn the reality by studying history. I find such people, I must say so peculiar that I do not know how to argue with them. If their case were sound, the world should be full of mad men attempting to fly from New York to Philadelphia on a stick or covering a telephone with kisses in the belief that it was their beloved girl-friend. No fairy story ever declared to be a description of the real world and no clever child has ever believed that it was.

The advantage claimed for repeating fairy stories to young children is that it()

A. makes them less fearful

B. develops their power of memory

C. makes them believe there is nothing to be afraid of

D. encourages them not to have strange beliefs