问题 选择题

如图,跳伞运动员在空中匀速直线下降的过程中,下列说法正确的是

A.人和伞的总重力大于空气的阻力

B.人和伞的总重力等于空气的阻力

C.人和伞的重力势能在增大

D.人和伞的动能在减小

答案

答案:B

题目分析:跳伞运动员在空中匀速直线下降的过程中,处于平衡状态,受到的人和伞的总重力与空气阻力是一对平衡力,大小相等。所以A选项错误,B选项正确。在下降的过程中,人和伞的重力高度减小,重力势能减小,C选项错误。人和伞匀速下降,速度大小不变,人和伞的动能不变,D选项错误。正确的选项是B。

阅读理解

阅读理解。

     One day when he was crossing the road, Tom was hit by a car. He fell down and broke his arm.

     An ambulance (救护车) soon arrived and took Tom to a hospital. A doctor operated on (动手术)

his arm and put the arm in a plaster cast(上石膏). Tom could not move it at all.

     "You must keep your arm in a cast for six weeks." the doctor said, "That would give the broken arm

a chance to heal (愈合)."

     At the end of the six weeks, Tom went back to the hospital. The doctor cut the cast with a large

saw (锯) and took the arm out of the cast.

    "Can you move your arm, Tom?" he asked.

    Tom tried to move his arm. At first it was difficult, but soon he was able to move it a little.

    "Yes, it's fine." he said, "Thank you very much, doctor."

    "In a few days it will be as good as usual." the doctor said, "Just exercise it a little."

    "Will I be able to play the piano?"Tom asked.

     The doctor smiled. "Of course, you will." he said, "You'll have no problem." "That is good news."

Tom said, "Because I couldn't play it at all before."

1. How did Tom break his arm?

A. He fell off a tree.

B. He broke it while playing football.

C. He was hit by a car on the road.

2. How long did he have to keep the arm in the plaster cast?

A. About 40 days.

B. Only for a few days.

C. Almost half a year.

3. What did the doctor tell Tom to do after the cast was taken off?

A. He asked him not to move his arm.

B. He asked him to exercise the arm.

C. He asked him to play the piano.

4. Why was Tom pleased?

A. He thought he would be able to play the piano.

B. The doctor would tell him how to play the piano.

C. The driver would give him a piano as a present.

5. Which of the following is NOT true?

A. The cast was so hard that the doctor had to cut it open with a special tool.

B. Tom liked playing the piano but he didn't know how to play it?

C. An operation on Tom's arm made him be able to play the piano.

单项选择题