一端固定在光滑面O点的细线，A、B、C各处依次系着质量相同的小球_爱下问搜题

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问题填空题

一端固定在光滑面O点的细线，A、B、C各处依次系着质量相同的小球A、B、C，如图所示，现将它们排列成一直线，并使细线拉直，让它们在桌面上绕O点作圆周运动，如果增大转速，细线将在OA、AB、BC三段线中______段先断掉．

答案

设A、B、C三个球的角速度为ω，质量都为m．根据牛顿第二定律得

对C：F_BC=mω²r_c

对B：F_AB-F_BC=mω²r_B

对A：F_OA-F_AB=mω²r_A

由以上式可知，F_OA＞F_AB＞F_BC，所以在OA、AB、BC三段线中OA段先断掉．

故答案为：OA

完形填空

完形填空。

     Xiao Qiang,a l 2-year-old Chinese boy,has too many activities．His father    1  him away from school four
afternoons a week and drives him across town to practice．"I do most of my  2    in the car when we are  3    to
practice,and I have to do the rest (剩下的)   when I get home．I am too tried, I just want to sleep."   He said.
"More and more kids (小孩) become unhappy    4  they have too many activities to do." said Mr. Wang,an
expert (专家)of child development．"4l％of the children aged 9-13 said they felt   5   most of the time,and more
than 75％0f them said that they wished they had more   6   to play."
     "Over-scheduling (过量的课程安排) is a growing problem(问题) for Chinese families." said Mr. Wang．It
is not good for both kids and their parents. More families eat dinner   7    or often eat fast food    8   their way to
football practice or music lessons. We all know it is important for kids to learn how to relax(放松). If they don't
learn that now, it will be more difficult to learn when they get  9     . Kids need time to relax. Parents should    10
about what is right for kids.

( )1. A. take
( )2. A. sports
( )3. A. riding
( )4. A. but
( )5. A. excited
( )6. A. great time
( )7. A. slowly
( )8. A. on
( )9. A. taller
( )10. A. hear

B. takes
B. business
B. flying
B. or
B. interested
B. full time
B. quickly
B. by
B. stronger
B. think

C. took
C. homework
C. walking
C. because
C. worried
C. right time
C. quietly
C. in
C. older
C. look

D. to take
D. housework
D. driving
D. so
D. dangerous(危险的)
D. free time
D. happily
D. across
D. better
D. tell

判断题

各种物质运动形式之间有着严格的界限，不能相互包含。( )