问题 多选题

如图所示,本盒内放置一小球,小球恰与木盒各面相接触,现给木盒一向上的初速度,下列说法正确的是(  )

A.若不考虑空气阻力,上升过程中,木盒底部对小球有弹力作用

B.若不考虑空气阻力,下落过程中,木盒顶部对小球有弹力作用

C.若考虑空气阻力,上升过程中,木盒顶部对小球有弹力作用

D.若考虑空气阻力,下落过程中,木盒底部对小球有弹力作用

答案

若不考虑空气阻力,设整体质量为M,对整体有:Mg=Ma   ①

假设木盒对小球作用力为F,所以有:F+mg=ma   ②故由①②解得F=0,即木盒与小球之间作用力为零,故AB错误;

当有空气阻力时,设阻力为f,

当上升时,方向向下,有:Mg+f=Ma   

对小球有:F+mg=ma    所以F向下,即木盒顶部对小球有弹力作用,C正确;

当下降时,方向向上,有:Mg-f=Ma   

对小球有:F+mg=ma,因此F为负,即方向向上,木盒底部对小球有弹力作用,故D正确.

故选CD.

阅读理解

阅读理解。

     The crowd stopped their talks as the runners came to the starting line. At this moment, all eyes were

on two people. Mark, the favorite for the gold medal, and Jim, a new face to all.

     Mark had won a lot of competitions in 3000 meters and was thought to be the finest runner in the country.

Until a few months before, Jim was almost unknown outside his hometown. But he had won one competition

after another, which was a big surprise. Many people thought Jim would also do very well this time. It was

going to be an exciting race.

     Bang! The race started. For the first half, Jim ran with Mark in the leading group. At 1700 meters, Jim

was about one meter behind Mark. Suddenly, the crowd stopped cheering. Mark fell and lay on the grass. He

tried to get up, but his hand had been injured and was out of the race.

     The crowd believed that Jim had tripped (绊倒) Mark and they began to shout at him. Jim looked back

and slowed down. Then _____. But the race was over for Jim, too. He was far behind others now. Jim was

very unhappy because of what had happened.

     Later, after the judges (裁判) had studied the video of the race, they decided that Jim had done nothing

wrong. Jim went to see Mark and said,"I'm sorry for..." "It's not your fault. I got injured myself in the leg.

Such a thing never happens to me. I've got to see the doctor."

     The two went on to become good friends and were often seen training together.

1. Why was the race going to be exciting?

A. Because it was a 3000-meter race.

B. Because there were only two runners in the competition.

C. Because both Mark and Jim were very good runners.

D. Because a large crowd of people came to watch the competition.

2. The crowd stopped cheering suddenly because ________.

A. it began to rain heavily

B. Mark fell and lay on the grass

C. Jim was about one meter behind Mark

D. Jim ran with Mark in the leading group

3. Which of the following can be put in the blank (空白处) in Paragraph 4?

A. he went on with the race

B. the doctors came to help

C. The judges told him to stop

D. he didn't know what had happened

单项选择题