问题 多选题

某物体沿直线运动的v-t关系如图所示,已知在第1s内合外力对物体做的功为W,则(  )

A.从第1s末到第3s末合外力做功为4W

B.从第3s末到第5s末合外力做功为-2W

C.从第5s末到第7s末合外力做功为W

D.从第3s末到第4s末合外力做功为-0.75W

答案

A、物体在第1秒末到第3秒末做匀速直线运动,合力为零,做功为零.故A错误.

B、从第3秒末到第5秒末动能的变化量与第1秒内动能的变化量相反,合力的功相反,等于-W.故B错误.

C、从第5秒末到第7秒末动能的变化量与第1秒内动能的变化量相同,合力做功相同,即为W.故C正确.

D、从第3秒末到第4秒末动能变化量是负值,大小等于第1秒内动能的变化量的

3
4
,则合力做功为-0.75W.故D正确.

故选CD

阅读理解

阅读短文,选择正确答案。

       Xie Qiang is a student in Grade7. Mother's Day is coming. The teacher wants to hold a party in the

classroom and asks him to organize the party. Now he is making the plan.

       First, he will write some invitations to students' mothers. He will ask the mothers to come to the classroom.

The students are going to join the party with their mothers together.

       Second, he will ask his classmates to perform (表演) at the party. Zhang Yanling will sing English songs.

Liu Yanhua will play the piano. Li Bingpeng will tell stories at the party. Wang Xiaolong will play games. 

       Third, Xie Qiang will buy some Coke, juice and some milk for the mothers. He will also ask the students to

prepare some presents for the mothers. They will let their mothers have a good time.

       Fourth, he will ask some mothers to tell some stories and play games with the students. He will also ask

some mothers to tell something interesting to children.

       He hopes mothers will have a lot of fun at the party. The students can't wait for Mother's Day.

1. What is Liu Yanhua going to do at the party? [ ]

A. Sing English songs.

B. Play the piano.

C. Tell stories.

D. Play games.

2. What can't mothers drink at the party? [ ]

A. Coke.

B. Juice.

C. Coffee.

D. Milk.

3. Which grade is Wang Xiaolong in? [ ]

A. Grade5.

B. Grade6.

C. Grade7.

D. Grade8.

4. Who is going to organize the party? [ ]

A. Xie Qiang's mother.

B. Zhang Yanling.

C. Xie Qiang's teacher.

D. Xie Qiang

5. Where does the teacher want to hold the party? [ ]

A. At Xie Qiang's home.

B. In the classroom.

C. In the park.

D. On the playground.

单项选择题