问题 多选题

如图所示,水平放置的两个用相同材料制成的轮P和Q靠摩擦传动(不打滑),两轮的半径R:r=2:1.当主动轮Q匀速转动时,在Q轮边缘上放置的小木块恰能相对静止在Q轮边缘上,此时Q轮转动的角速度为ω1,木块的向心加速度为a1;若改变转速,把小木块放在P轮边缘也恰能静止,此时Q轮转动的角速度为ω2,木块的向心加速度为a2,则(  )

A.

ω1
ω2
=
2
2

B.

ω1
ω2
=
2
1

C.

a1
a2
=
1
1

D.

a1
a2=
1
2

答案

在Q轮边缘上放置的小木块恰能相对静止在Q轮边缘上.则有最大静摩擦力提供向心力.即为μmg=mω12r,当木块放在P轮也静止,则有μmg=mωP2R

解得:

ω1
ωP
=
2
1

因为线速度相等,

ω2r=ωPR

解得:ω2=2ωP

所以

ω1
ω2
=
2
2

因为a112r,a2P2R,

所以

a1
a2
=
1
1

故选AC

阅读理解

One girl decided to study judo(柔道)although she had lost her left arm in a car accident.

The girl began lessons with an old Japanese judo instructor. The girl was doing well. So she couldn’t understand why, after three months of training, the instructor had taught her only one move.

“Instructor,” the girl finally said, “Shuldn’t I be learning more moves?”

“This is the only move you know, but this is the only move you’ll ever need to know,” the instructor replied.

Not quite understanding, but believing in her teacher, the girl kept training.

Several months later, the instructor took the girl to her first tournament. Surprising herself, the girl easily won her first two matches. The third match proved to be more difficult, but after some time, her opponent became impatient and charged. The girl skillfully used her one move to win the match. Still amazed by her success, the girl was now in the finals.

This time, her opponent was bigger, stronger and more experienced. For a while, the girl appeared to be overmatched. Concerned that the girl might get hurt, the referee called a time-out. She was about to stop the match when the instructor intervened(干预).

“No,” the instructor insisted, “Let her continue.”

Soon after the match restarted, her opponent made a serious mistake; she dropped her guard. Instantly, the girl used her move to pin her opponent. The girl had won the match and the tournament. She was the champion.

One the way home, the girl and her teacher reviewed every move in each and every match. Then the girl gathered the courage to ask what was really on her mind.

“Instructor, how did I win the tournament with only one move?”

“You won for two reasons,” the teacher answered. “First, you’ve almost mastered one of the most difficult throws in all of judo. Second, the only known defense for that move is for your opponent to grab your Left arm.”

The girl’s biggest weakness had become her biggest strength.

小题1:What can we learn about the girl?

A.She was disabled in an accident.

B.She disliked judo training.

C.She learnt several moves.

D.She won the first two matches hard.小题2:The underlined word “overmatched” probably means         .

A.impatient

B.depressed

C.defeated

D.trapped小题3:The girl won the championship because of          .

A.her bravery

B.her skills

C.her tricks

D.her strength小题4:Which of the following is probably the best title of the story?

A.The Story of a Girl.

B.A Disabled Girl.

C.Defense Matters.

D.Weakness Becomes Strengh.

单项选择题