问题 问答题

如图所示,石拱桥的正中央有一质量为m的对称楔形石块,侧面与竖直方向的夹角为α,重力加速度为g.若接触面间的摩擦力忽略不计,求石块侧面所受弹力的大小为多少?

答案

以楔形石块为研究对象,受到重力mg,两侧拱桥的弹力F,受力如图,由平衡条件根据力的合成法可得:

    mg=2Fcos(90°-α),

所以F=

mg
2cos(900-α)
=
mg
2sinα

答:石块侧面所受弹力的大小为

mg
2sinα

单项选择题
单项选择题

Our ape-men forefathers had no obvious natural weapons in the struggle for (1) in the open. They had neither the powerful teeth nor the p claws of the big cats. They could not (2) with the bear, whose strength, speed and claws (3) an impressive " small fire" weaponry. They could not even defend themselves (4) running swiftly like the horses, zebras or small animals. If the ape-men had attempted to compete on those terms in the open, they would have been (5) to failure and extinction. But they were (6) with enormous concealed advantages of a kind not possessed by any of their competitors.

In the search (7) the pickings of the forest, the ape-men had (8) efficient stereoscopic vision and a sense of color that the animals of the grasslands did not (9) . The ability to see clearly at close range permitted the ape-men to study practical problems in a way that lay far (10) the reach of the original inhabitants of the grassland. Good long-distance sight was (11) another matter. Lack of long-distance vision had not been a problem for forest-dwelling apes and monkeys because the (12) the viewpoint, the greater the range of sight-so (13) they had had to do was climb a tree. Out in the open, however, this simple solution was not (14) . Climbing a hill would have helped, but in many places the ground was flat. The ape-men (15) the only possible solution. They reared up as high as possible on their hind limbs and began to walk (16) . This vital change of physical position brought about considerable disadvantages. It was extremely unstable and it meant that the already slow ape-men became (17) still. (18) , they persevered and their bone structure gradually became (19) to the new, unstable position that (20) them the name Homo erectus, upright man.

12()

A.lower

B.further

C.clearer

D.higher