问题 选择题

关于功,下列说法中正确的是: ( )

A.因为功有正负,所以功是矢量

B.力对物体不做功,说明物体一定无位移

C.功的多少是由力的大小和物体在力的方向上的位移的大小确定的

D.力对物体做功少,说明物体的受力一定小

答案

答案:C

题目分析:A、功的正负表示物体对外做功还是克服外力做功,不代表方向,功是标量;错误

B、如果力的方向和运动方向垂直,即使位移不为零力对物体也不做功;错误

C、功等于力和物体在力的方向上的位移的乘积;正确

D、力对物体做功少,说明力小或物体在力的方向上的位移小;错误

故选C

点评:一个力所做功的多少,仅与力F、位移L和力F与位移L夹角的余弦cosα三者有关。

阅读理解

阅读理解。

     We all know the dangers of fires. It's good and necessary for a family to learn how to prepare for a fire.

Here are some suggestions. Put a smoke alarm (火警) in the house. Smoke from a fire causes the alarm to

go off. The alarm makes a loud sound. The sound tells everyone to leave the house at once.

     Make escape (逃脱) plans. We should know all the ways out of the house. If there is a fire, everyone

follows the plan to get out. Part of the plan is to check all the windows to make sure they can be opened

easily.

    Buy fire extinguishers (灭火器) in the house. Everyone in the family should know how to use them.

     Practise for a fire. We do fire practice because we teach children about fire safety. Everyone in the family

should know the following fire rules:

    ★ Don't open a hot door! The fire can grow more quickly if you open the door.

    ★ Stay close to the floor! Smoke can be more dangerous than fire. The best air is near the floor because

smoke rises.

    ★ What will you do if your hair or clothes start to burn? First, stop! Don't run! The fire burns faster

because of more air. Drop! Fall to the floor. Then roll! Turning over and over will make the fire go out. Put

a blanket (毯子) around you to keep air away from the fire that may still be on you.

    There are many possible causes for fires. A wise family is ready all the time. If there is a fire, don't forget

to call 119 for help.

1. When a smoke alarm rings at home, it means _________.

A. you have to get up

B. water is running to the floor

C. something is burning

D. someone breaks your window

2. The writer advises us to do the following to prepare for a fire except that _________.

A. we practise for a fire

B. we make escape plans

C. we buy fire extinguishers

D. we use electrical cookers

3. When a fire happens, _________ if you open the hot door.

A. the door will soon be on fire

B. the electricity will be cut off

C. the fire will grow more quickly

D. the house will fall down

4. What are the right steps you should take when your hair or clothes catch fire?

A. Stop, run, roll.

B. Stop, drop, roll.

C. Run, drop, roll.

D. Roll, drop, stop.

5. What is the best title for this passage?

A. Be Ready for a Fire.

B. The Causes of a Fire.

C. Learn to Use a Fire Extinguisher.

D. The Dangers of a Fire.

判断题