问题 选择题

如图所示,一轻绳一端固定质量为m的小球,以另一端O为圆心使小球在竖直平面内作半径为R的圆周运动,重力加速度为g,则下列说法中正确的是(  )

A.小球的动能始终保持不变

B.小球在最高点的速度为

gR

C.小球由最高点运动到最低点的过程中,动能的增加量为2mgR

D.小球运动到最高点时,如果轻绳突然断开,小球将做匀速直线运动

答案

A、轻杆拉着小球在竖直平面内做圆周运动,只有重力做功,机械能守恒,势能变化,动能也在变化,故A错误;

B、小球在最高点的最小速度为

gR
,故B错误;

C、根据机械能守恒定律可知,△Ek=mgh=mg2R,故C正确;

D、小球运动到最高点时,如果轻绳突然断开,小球具有水平方向的初速度,只受重力,做平抛运动,故D错误;

故选C

阅读理解

阅读理解。

     More and more people around the world are joining in dangerous sports. Some people climbed the

highest mountains; some traveled into unknown parts of the world; some sailed small boats across the

largest sea. Now some people begin to look for new excitement.

     Bungee jumping (蹦极) and motorcycle racing  (摩托车赛) are  quite dangerous sports. Bungee

jumping only lasts a few minutes or even seconds. You jump from a high place, 200 meters above the

ground, and there is a  rubber  band (橡胶带子) tied to your legs. When you jump down, the rubber

band pulls you up. About 2,000,000 people around the world have tried bungee jumping.

     Why do people join in these dangerous sports? Some scientists say that it is because modern life

has become safe and it is not interesting. In the past, people lived in danger. They had to go out and

look for food, and life was like a fight but was interesting.

     Many people think that there is little excitement in life. They live and work in safe places, buy food

in shops, and there are doctors and hospitals to look after them if they become ill.

1.People like to join in dangerous sports because _______.

A.they have a lot of free time

B.they can go to the hospital if they are hurt

C.they need excitement

D.they don't need to look for food

2._______ is not a dangerous sport.

A.Climbing the highest mountains

B.Riding on the road

C.Traveling into unknown parts of the world

D.Sailing small boats across the largest sea

3.In bungee jumping, you _______.

A.jump up as high as you can

B.jump down with a rubber band tied to your legs

C.jump down without a rubber band

D.jump to the ground

4.In the past, people lived in danger because _______.

A.the living condition is poor

B.there was no doctor and hospital

C.there were many dangerous animals

D.all of above

5.Which is not TRUE?

A.Everyone's life is interesting.

B.Many people live and work in safe places.

C.Many people buy food in shops.

D.People can go to see the doctors when they become ill.

单项选择题