问题 选择题

物体在运动过程中加速度不为零,那么以下结论正确的是(   )

A.物体的速度大小一定随时间变化

B.物体的速度方向一定随时间变化

C.物体的动能一定随时间变化

D.物体的动量一定随时间变化

答案

答案:D

加速度不为零说明物体的速度是改变的,速度改变包括大小的变化或方向的改变,故选项A、B不正确.由于动能是标量,动量是矢量,速度改变动量一定改变,动能则不一定改变,故选项C是错误的,D是正确的.

阅读理解

阅读短文,选择正确答案。

      On Sunday, April 22 nd, people all over the world take time out to appreciate the earth that we all share. It's

Earth Day, a special day to learn about our planet and how to take care of it.

      On Earth Day, some people listen to speeches about the environment. Others help clean up their offices or

save water and electricity at home. Your parents may even decide to take a day off from driving their cars.

      The idea for Earth Day came from a U.S. Senator (参议员). He was worried about pollution and the health

of plants and animals. And the idea quickly caught on (流行起来). In 1970, the first Earth Day was celebrated.

More than 20 million people in cities all over the U.S. took part in it. Since then, Earth Day has been used to

educate people about their roles in taking care of our planet.

      Now, Earth Day has become a worldwide celebration. In 1990, two hundred million people from 140

countries took part in clean-ups, tree planting and other environmental events on Earth Day.

1. Earth Day is a special day to learn about our planet and             .[ ]

A. how to pay attention to the earth

B. how to learn English

C. how to go to the moon

D. how to make use of time

2. On Earth Day, we are told to             . [ ]

A. go shopping

B. save water and electricity at home

C. listen to music

D. drive our cars

3. The idea for Earth Day was given by a from America             . [ ]

A. farmer

B. teacher

C. scientist

D. senator

4. According to the passage, the first Earth Day was celebrated in              in            . [ ]

A. 1990, the U.S.A.

B. 1970, Beijing

C. 1970, America

D. 1990, America

5. Why has Earth Day become a worldwide celebration ? Because            . [ ]

A. Earth Day is on Sundays

B. we can sing songs on Earth Day

C. there are so many people on Earth Day

D. Earth Day can teach people how to take care of our planet

名词解释