问题 实验题

(6分)一个实验小组的同学设计了一个测量动摩擦因数的试验。如图甲所示,把弹簧测力计的一端固定在墙上,另一端与一物块A相连,用力F水平向左拉物块A下面的金属板B,金属板B向左运动,此时测力计的示数稳定(图乙中已把弹簧测力计的示数放大画出),则物块A与金属板B间的滑动摩擦力的大小是_________N。若用弹簧测力计测得物块A重13N,根据下面表格中给出的动摩擦因数,则可推算出物块A的材料是_______。

材料动摩擦因数
金属—金属0.25
橡胶—金属0.30
木头—金属0.20
皮革—金属0.28

答案

2.60   木头

题目分析:弹簧秤的示数即为物块A与金属板B间的滑动摩擦力的大小由图乙可知滑动摩擦力的大小是2.60N;由滑动摩擦定律有,则,由题表信息可知物块A的材料是木头

点评:难度较小,此题注意当弹簧示数稳定时物体A受力平衡,弹簧对A的拉力与B对A的摩擦力是一对平衡力;由于是水平桌面,则正压力与A物体的重力大小相等

阅读理解

阅读理解。

     The Read family come to a new big city, but Mrs Read's son, Bob, is not happy. He doesn't have

anyone to play with. "Don't worry!" says his mother. "You will soon make friends here."

     One morning, there is a knock at the door. Bob's mother, Mrs Read, opens it. There stands a woman

with brown hair. It is Mrs Miller. She comes to borrow two eggs. She wants to make cakes. Mrs Read

gives her two. In the afternoon there is another knock at the door. Mrs Read opens it and she sees a boy

standing there. "My name is Jack Miller," he says. "My mother sends you this cake and two eggs." "Well,

thank you, Jack," says Mrs Read. "Come in and meet my son Bob."

     Look! Bob and Jack are having the cake and milk.They are playing football. Now they are friends! Jack

says, "I'm glad you live next door." Bob says, "I must thank your mother for coming to borrow eggs." Jack

laughs and tells Bob, "My mother doesn't want the two eggs, but she wants to make friends with your

mother!" Bob says, "That's a funny way to make friends. It's an easy way. It can work!"

1. Bob feels sad when he is in the new city because _____. [ ]

A. his mother isn't in the city

B. he has no new friends

C. he thinks the city is too noisy

D. Jack doesn't like him

2. Mrs Miller is _____. [ ]

A. Jack's friend

B. Bob's mother

C. Mrs Read's old friend

D. Mrs Read's new neighbor

3. Mrs Mtller _____ one morning.[ ]

A. gives Mrs Read a piece of cake

B. asks Bob to play with her son

C. borrows eggs from Mrs Read

D. gives back the eggs to Mrs Read

4. In the afternoon, _____ have the cake and milk together.[ ]

A. Bob and Jack

B. Jack and his mother

C. Bob and his mother

D. Mrs Read and Mrs Miller

5. What does Mrs Miller really want to do by borrowing eggs?[ ]

A. To make friends with the Reads.

B. To make cakes with eggs.

C. To find out what Mrs Read does.

D. To see if her friend is in.

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