问题 问答题

如图所示,一个半径为R的绝缘光滑半圆环,竖直放在场强为E的匀强电场中,电场方向竖直向下.

在环壁边缘处有一质量为m,带有正电荷q的小球,由静止开始下滑,求小球经过最低点时对环底的压力.

答案

小球由静止开始下滑的过程中,由动能定理得

    mgR+qER=

1
2
mv2 ①

小球经过最低点时,由重力、电场力和轨道的支持力的合力提供向心力,则有

   N-mg-qE=m

v2
R
  ②

联立①②得,轨道对小球的支持力N=3(mg+qE),则由牛顿第三定律知,小球经过最低点时对环底的压力N′=N=3(mg+qE).

答:小球经过最低点时对环底的压力为3(mg+qE).

阅读理解

阅读理解:

      The elephant is the largest of all animals that live on land. An elephant's trunk is rather like an arm and a

hand. It is used to pick up and pull things in its mouth. It is also used to pull up trees when the elephant wants

to make path through the forests.

      The African elephant is usually larger than the Indian elephant. It often stands 10 or 11 feet high, and weighs

over 6 tons. Both males and females grow trunks. But only male Indian elephants have tusks.

      The elephant lives mostly on grass, leaves and fruits. It has been easy for men to train elephants. They are

used to carry heavy things. Elephants can live for about fifty years or more.

1. Elephants are _______. [ ]

A. not found in the forests

B. only found in Africa

C. larger than any other animal on land

D. the largest animal in the world

2.  Elephants use their _______.[ ]

A. trunks to do things as men use their hands to

B. trunks to pull up trees only when they want to make paths through the woods

C. tusks to do what men use their hands to

D. tusks to eat up trees 

3.  Which of the following is NOT right?[ ]

A. Female Indian elephants have tusks.

B. All African elephants have tusks.

C. The Indian elephant is usually smaller than the African elephant.

D. The African elephant usually weighs more than 6,000 kilograms.

4. The food for the elephant is _______.[ ]

A. grass

B. leaves

C. fruits

D. all of the above 

5.  From the text we know _______. 

 [ ]

A. an elephant is very clever and a good friend of men's

B. we can hunt elephants as we like

C. we should not train the elephant

D. there are elephants everywhere in the world

问答题 简答题