题干

如图所示,物体P左边用一根轻弹簧和竖直墙相连,放在粗糙水平面上,静止时弹簧的长度大于原长。若再用一个从零开始逐渐增大的水平力F向右拉只直到拉动,那么在P被拉动之前的过程中,弹簧对P的弹力丁的大小和地面对P的摩擦力的大小变化情况是                                                                                      (   )

A:T始终增大,始终减小

B:T先不变后增大,先减小后增大

C:T保持不变,始终减小

D:T保持不变,先减小后增大

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D

同类题5

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    Most people agree that honesty is a goodthing. But does Mother Nature agree? Animals can't talk, but can they lie in other ways? Can they lie with their bodies and behavior? Animal experts may not call it lying, but they do agree that many animals, from birds to chimpanzees, behave dishonestly to fool other animals. Why? Dishonesty often helps them survive.

    Many kinds of birds are very successful at fooling other animals. For example, a bird called the plover sometimes pretends to be hurt in order to protect its young. When a predator(猎食动物)gets close to its nest, the plover leads the predator away from the nest. How? It pretends to have a broken wing. The predator follows the "hurt" adult, leaving the baby birds safe in the nest.

    Another kind of bird, the scrub jay, buries its food so it always has something to eat. Scrub jays are also thieves.They watch where others bury their food and steal it. But clever scrub jays seem to know when a thief is watching them. So they go back later, unbury the food, and bury it again somewhere else.

    Birds called cuckoos have found a way to have babies without doing much work. How? They don't make nests(鸟巢). Instead, they get into other birds' nests secretly. Then they lay their eggs and fly away. When the baby birds come out, their adoptive parents feed them.

    Chimpanzees, or chimps, can also be sneaky. After a fight, the losing chimp will give its hand to the other. When the winning chimp puts out its hand, too, the chimps are friendly again. But an animal expert once saw a losing chimp take the winner's hand and start fighting again.

    Chimps are sneaky in other ways, too. When chimps find food that they love, such as bananas, it is natural for them to cry out. Then other chimps come running. But some clever chimps learn to cryvery softly when they find food. That way, other chimps don't hear them,  and they don't need to share their food.

    As children, many of us learn the saying "You can't fool Mother Nature." But maybe you can't trust her,either.