问题 问答题

相隔一定距离的A、B两球,质量均为m,假设它们之间存在恒定斥力作用,原来两球被按住,处于静止状态.现突然松开两球,同时给A球以速度v0,使之沿两球连线射向B球,而B球初速为零.设轨道光滑,若两球间的距离从最小值(两球未接触)到刚恢复到原始值所经历的时间为t,求两球间的斥力.

答案

小球运动过程示意图如图所示:

当A、B相距最近时,二者速度应相等,设为v,当二者距离恢复原始值时,设A、B的速度分别为v1、v2,整个过程经历的时间为t′.ϖ

对B球,由动量定理得:Ft=mv2-mv ①

以A、B两球组成的系统为研究对象,系统动量守恒,以A的初速度方向为正方向,

由动量守恒定律得:

从开始到距离最小过程中:mv0=2mv ②

从距离最小到恢复到原始值过程中,mv0=mv1+mv2

整个过程中A、B两球对地的位移相等,则:

v0+v1
2
t′=
v2
2
t′④

联立①~④式解得:F=

mv0
2t
⑤;

答:两球间的斥力为

mv0
2t

阅读理解

My parents divorced when I was 8 years old. Their marriage was a mistake of World War II. And about 50 years ago, I was the only child I knew who didn't have a father at home on Father's Day. Divorce was very uncommon back then.

So when my friends were in town shopping for presents for their fathers, I decided since my mother was both Mother and Father, no one else was like her, who better deserved(be worth) a present on Father's Day than her?

I bought her a little set of tools, just a few things, hammer, pliers(钳子) and screwdriver(螺丝起子) and a box to put all these things in. I got some ideas from my grandfather and named it Mother's Tool Box. She was excited, more at the thought of being cared for on Father's Day than of the present. She always said she liked her Father's Day memories better than the Mother's Day ones. It made her feel special — and she was. I have noticed in the past few years, greeting cards are even made for "Mother on Father's Day". Ever since then, my mother has got cards on Father’s Day. By the way, she knew just how to use those tools and others she had ever used over the years.

小题1:Which sentence shows that divorce was very uncommon at that time?

A.My parents divorced when I was 8 years old.

B.My parents’ marriage was a mistake of World War II.

C.My mother has got cards on Father’s Day.

D.I was the only child who didn’t have a father at home on Father’s Day.小题2:What did the writer decide to do when he saw friends shopping for Father’s Day presents?

A.Go shopping with his friends.

B.Buy a present for his mother.

C.Help his mother.

D.Ask his grandfather for help.小题3:“Hammer, pliers and screwdriver” are probably things __________.

A.more often used by men

B.that cost a lot of money

C.very popular at that time

D.that his mother liked most小题4:Mother was excited because __________.

A.she was very interested in the tool box

B.she was deeply loved by her child

C.she saw the name on the tool box

D.she knew how to use the things小题5:Why did Mother feel special?

A.She had more Father’s Day memories.

B.She had only one child.

C.She was both Mother and Father.

D.She was the happiest one on Father’s Day

单项选择题