问题 问答题

如图所示,在光滑绝缘水平面上,不带电的绝缘小球P2静止在O点.带正电的小球P1以速度v0从A点进入AB区域.随后与P2发生正碰后反弹,反弹速度为

2
3
v0.从碰撞时刻起在AB区域内加上一个水平向右,电场强度为E0的匀强电场,并且区域外始终不存在电场.P1的质量为m1,带电量为q,P2的质量为m2=5m1,A、O间距为L0,O、B间距为L=
4L0
3
,已知
qE0
m1
=
4v02
3L0

(1)求碰撞后小球P1向左运动的最大距离及所需时间.

(2)判断两球能否在OB区间内再次发生碰撞.

答案

(1)碰撞后P1以速度v1=

2
3
v0向左做匀减速直线运动,设最大距离为s,由运动学公式有

v12=2as

v1=at②

由牛顿第二定律有

qE0=m1a③

qE0
m1
=
4v02
3L0

联立解得

s=

1
6
L0

所需时间t=

L0
2v0

(2)设碰后P2速度为v2,以v0方向为正方向,由动量守恒:

m1v0=m1(-

2
3
v0)+m2v2

设P1、P2碰撞后又经△t时间在OB区间内能再次发生碰撞,

P1位移为s1,P2位移为s2,由运动学公式,有

s1=-

2
3
v0△t+
1
2
a△t2

s2=v2△t⑨

s1=s2

联立解得

s2=

L0
2
<L=
4L0
3
两球能在OB区间内再次发生碰撞.

答:(1)碰撞后小球P1向左运动的最大距离为

L0
6
.所需时间为
L0
2v0

(2)两球能在OB区间内再次发生碰撞.

阅读理解

阅读理解。

     Shopping in the United States changes a lot. About ninety years ago most people shopped in small stores

that were owned by one person or a family. Women went from the bakery to the butcher's to the grocer and

on to the fruit and vegetable seller in order to get their food for the week.

     Then about sixty years ago, supermarkets were born. In a supermarket, people could get all the different

kinds of food they needed without going to different stores.

     The next big change in shopping in the United States was the shopping mall. A shopping mall is a group of

stores under one roof. Because malls allowed people to shop without worrying about the weather, they soon

became very popular. The mall became a place for people to socialize in addition to shopping. If you walk

through a mall, you will see older people sitting, chatting and drinking coffee. Malls are places for teenagers to

hang out. Many teens will often just "go to the mall" and spend time with their friends.

     The recent change in American shopping was the superstore. Large chain stores such as Wal-Mart, Office

Depot and Toys "R" Us have been built all across the United States. Because they are so large, they can buy

goods at a great discount and sell them much cheaper than smaller stores.

     Sometimes, when they are built near small towns, many of the small town stores have to close. They just

cannot compete with their giant neighbors.

     And now, online shopping is becoming more and more popular all over the States. People are too busy to

go to the physical stores, so they go shopping over the Internet. Online shopping has lots of advantages. For

instance, online stores are usually available 24 hours a day. Searching or browsing online shops can be faster

than browsing the physical stores. While, online shopping also has its disadvantages. People are at higher risk

of being cheated on the part of the merchant than in a physical store. And privacy of personal information

may be let out.

1. Which of the following shows the right order of shopping development in the United States?

    ①small stores ②superstores ③shopping malls ④shopping online ⑤supermarkets

A. ①②③④⑤

B. ①⑤②③④

C. ①⑤③②④

D. ②③④①⑤

2. Which is the place for people to spend time with others according to the passage?

A. Shopping malls.

B. Small stores.

C. Supermarkets.

D. Superstores.

3. Why can the superstores sell products at much lower prices?

A. Because they are built near small towns.

B. Because they are across the United States.

C. Because they sell all kinds of products people need.

D. Because they can buy goods at a reduction in the prices.

4. What's the disadvantage of online shopping according to the passage?

A. Wasting time.

B. Leaking personal information.

C. Fixed prices.

D. No chance to do physical activities.

单项选择题