题干

如图所示,固定在水平面上的光滑半球,球心O的正上方固定一个小定滑轮,细绳一端拴一小球,小球置于半球面上的A点,另一端绕过定滑轮,如图所示.今缓慢拉绳使小球从A点滑向半球顶点(未到顶点),则此过程中,小球对半球的压力大小N及细绳的拉力T大小的变化情况是

A:N变大,T变大

B:N变小,T变大

C:N不变,T变小

D:N变大,T变小

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答案(点此获取答案解析)

C

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阅读理解

    Imagine you are in a jumping contest. Animals and insects can also enter this contest. But they might just leave humans in the dust!

    The first event is the long jump. The human athlete is Mike Powell. In 1991, he jumped nearly 30 feet, which is the world record for the long jump. That is about five times his own height.

    His competitor in the long-jump contest is a frog named “Rosie the Ribbiter.” Rosie set the world record for frogs in 1986 by jumping more than 21 feet. Her record has never been broken by any other frog. Compared to Mike Powell, Rosie's jump is not that amazing. But wait! Rosie is only about 10 inches long when her legs are stretched to their full length. She can jump more than 25 times her size. Rosie, the frog, wins the long-jump event.

    The next event is the high jump. Javier Sotomayor, the world record holder, can jump a bar (横杆) 8 feet high. That is about as high as the ceiling in most homes.

    Looking around for someone to challenge his record, Javier might need a magnifying glass (放大镜). The next contestant is a tiny insect called the spittlebug (吹沫虫).

    The spittlebug can jump 28 inches into the air. It is only a quarter-inch long, less than the width of a pencil. If the spittlebug were the same size as Javier, it would be able to jump 600 feet into the air. That is like a human jumping over a building 55 stories high!

    Though the humans lose the “jumping contest,” their competitors would probably agree that Mike and Javier are still pretty excellent athletes. If only Rosie and the spittlebug could speak!