题干

如图所示,一辆小车静止在水平地面上,车内固定着一个倾角为60°的光滑斜面OA,光滑挡板OB可绕转轴O在竖直平面内转动.现将一重力为G的圆球放在斜面与挡板之间,挡板与水平面的夹角θ=60°.下列说法正确的是(   )

A:若保持挡板不动,则球对斜面的压力大小为G

B:若挡板从图示位置顺时针方向缓慢转动60°,则球对斜面的压力逐渐增大

C:若挡板从图示位置顺时针方向缓慢转动60°,则球对挡板的压力逐渐减小

D:若保持挡板不动,使小车水平向右做匀加速直线运动,则球对挡板的压力可能为零

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A,D

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    There was a time when no one knew the name Harry Potter. Now the adventures of this extraordinary student at Hogwarts School of Witchcraft and Wizardry are read in over 45 languages, including Russian, Thai, and even ancient Greek. No one can explain the Harry Potter phenomenon (现象) – not even J.K. Rowling, his creator.

    J.K. Rowling was born in England in 1965. From a young age, she knew she wanted to be a writer. When she was 6, she wrote her first story—about a rabbit that gets sick. At school, she used to make up stories to tell her friends.

    After graduation from college, she worked as a secretary. But she didn't give up her dream.She spent her lunch hour writing stories, mainly for adults. Then in 1990, on a train trip to London,she got the idea from the boy wizard. She says he just appeared in her head. She soon created many other characters to help Harry fight the forces of darkness.

    She kept working on the story while she was teaching English in Portugal, where she married, had her first child, and divorced (与……离婚) a year later. When she returned to England, she brought back a box of Harry Potter stories.

    After returning home, she was broke and living in a small house. She continued writing, and in 1995, finished the first book in the series, Harry Potter and the Sorcerer's Stone. It was published (出版) in 1997 and became an unexpected bestseller.

    Rowling's life has changed a lot. She has become internationally famous and now earns around $40 million a year. She remarried, had a second child and lives in Scotland now.