题干

在其他条件不变的情况下,下列操作能改变滑动摩擦力大小的是(  )

A:在木块上加放砝码

B:改变木块的运动方向

C:使木块运动速度越来越大

D:使木块侧放减小接触面积

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答案(点此获取答案解析)

A

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    A small boy at summer camp received a large package of cookies (饼干) in the mail from his mother. He ate a few, and then 1 the rest under his bed. The next day, after lunch, he went to his tent to get a cookie. The box was2

That afternoon a camp teacher, who had been 3of the theft (失窃), saw another boy sitting behind a tree eating the stolen 4 “That young man,” he said to himself, “must be taught not to 5”

    He returned to the group and met the boy 6 cookies had been stolen. Billy, he said, “I know who stole your cookies. Will you 7 me teach him a lesson?”

    “Well, yes —8aren’t you going to punish him?” asked the puzzled boy.

    “No, that would only make him 9 you,” the teacher explained. “I want you to call your 10and ask her to send you another box of cookies.”

    The boy did 11the teacher asked and a few days 12 received another box of cookies in the mail.

    “Now,” said the teacher, ‘“the boy who stole your cookies is down by the lake.” Go down there and13your cookies with him.

    “But,” protested (抗议) the boy, “he’s the14”

    “I know. But 15 it - see what happens.”

    Half an hour later the camp teacher saw the two come up the hill,16 The boy who had stolen the cookies was 17 to get the other to accept his jackknife (折叠小刀) in18for the stolen cookies, and the victim was just 19the gift from his new friend, saying that a few old cookies weren’t that 20 anyway.