问题 选择题

如图5-6-3所示,质量为m的小球在竖直平面内的光滑圆环轨道上做圆周运动.圆半径为R,小球经过圆环最高点时刚好不脱离圆环.则其通过最高点时(    )

图5-6-3

A.小球对圆环的压力大小等于mg

B.小球受到的向心力等于重力mg

C.小球的线速度大小等于

D.小球的向心加速度大小等于g

答案

BCD

小球在最高点刚好不脱离圆环,其临界情况为圆环对小球无作用力,只有重力提供向心力.则mg==ma,所以v=,a=g.

单项选择题 A1/A2型题
单项选择题

The Man of Many Secrets—Harry Houdini—was one of the greatest American entertainers in the theater next century. He was a man famous for his escapes—from prison cells, from wooden boxes floating in rivers, from locked tanks full of water. He appeared in theaters all over Europe and America. Crowds came to see the great Houdini and his "magic" tricks.

Of course, his secret was not magic, or supernatural powers. It was simply strength. He had the ability to move his toes as well as he moved his fingers. He could move his body into almost any position he wanted.

Houdini started working in the entertainment world when he was 17, in 1891. He and his brother Theo performed card tricks in club in New York. They called themselves the Houdini Brothers. When Harry married in 1894, he and his wife Bess worked together as magician and assistant. But for a long time they were not very successful. Then Harry performed his first prison escape, in Chicago in 1898. Harry persuaded a detective to let him try to escape from the prison, and he invited the local newspapermen to watch.

It was the publicity that came from this that started Harry Houdini’s success. Harry had fingers trained to escape from handcuffs and toes trained to escape ankle chins. But his biggest secret was how he unlocked the prison doors. Every time he went into the prison cell, Bess gave him a kiss for good luck—and a small skeleton key, which is a key that fits many locks, pass quickly from her mouth to his.

Harry used these prison escapes to build his fame. He arranged to escape from the local prison of every town he visited. In the afternoon, the people of the town would read about it in their local newspapers, and in the evening every seat in the local theater would be full. What was the result World-wild fame, and a name remembered today.

In the fourth paragraph, the underlined word "this" refers to ().

A. his first prison escape

B. the year 1898

C. the publicity

D. Harry Houdini’s success