问题 问答题

学校的物理课外活动小组欲测滑块在斜面上下滑的加速度以及滑块与木板间的动摩擦因数,一位同学想出了一个巧妙的方案.如图所示,将一小钢球和滑块用细线连接,跨在木板上端的小定滑轮上,开始时小球和滑块均静止,剪断细线小球自由落下,滑块沿斜面下滑,可先后听到小球落地和滑块撞击挡板的声音.他反复调整挡板的位置,重复上述操作,直到能同时听到小球落地和滑块撞击挡板的声音,然后用刻度尺测量出H=1.25m,x=0.50m,h=0.30m.根据以上数据可得滑块在斜面上下滑的加速度a=______m/s2,滑块与木板间的动摩擦因数μ=______.(g取10m/s2

答案

对于小球做自由落体运动,则有H=

1
2
gt 2,得到t=
2H
g

对于滑块,沿斜面向下做匀加速运动,运动时间与小球自由下落时间相等,则有

   x=

1
2
at 2

联立以上两式,得:a=

xg
H
=
0.5×10
1.25
m/s2=4m/s2

设木板的倾角为α,根据牛顿第二定律得:mgsinα-μmgcosα=ma

由图看出,sinα=

h
x

联立解得:μ=0.25

故答案为:4,0.25

阅读理解

阅读理解。

     Most American families are smaller than the families in other countries. Most American families have one

or two parents and one or two children each.

     Children in the US will leave their parents' home when they grow up. They usually live far from their

parents because they want to find good jobs. They often write to their parents or telephone them. And they

often go to visit their parents on holiday.

     Parents usually let their children choose their own jobs. Americans think it important for young people to

decide on their lives by themselves.

     Children are asked to do some work around their house. And in many families, children are paid for doing

some housework so that they learn how to make money for their own use.

1. The size of most American families is ______ that of other countries. [ ]

A.larger than

B.smaller than

C.as big as

D.as small as

2. When children grow up,they leave their parents' home to ______. [ ]

A.get married

B.be free

C.find good jobs

D.go to university

3. They visit their parents ______. [ ]

A.on weekdays

B.on weekends

C.on holiday

D.very often

4. In America, children are allowed to ______.[ ]

A.choose their own jobs

B.get pay for doing house work

C.1ive with their parents when they grow up

D.Both A and B

5. Which of the following statements is WRONG? [ ]

A.Children have the freedom to choose their own job.

B.Parents don't ask their children to do the housework.

C.Parents think it important for children to make their own decision.

D.Children in America have chances to learn to make money at home.

单项选择题