题干

如图,手拉着木块在粗糙程度不变的水平面上做加速运动,则木块受到的摩擦力大小(  )

 

A:逐渐增大

B:逐渐减小

C:保持不变       

D:总是等于拉力的大小

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答案(点此获取答案解析)

C

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完形填空

    Are you a man or a mouse? When people ask this question they want to know whether you think you are a brave person or a coward(懦夫). But you will never really know the answer to this question 1you are tested in real life. Some people 2they are brave but when they come face to face with real danger, they act like cowards. Others consider themselves as cowards, but when they meet danger, they act like3.

    Lenny had always thought of himself as a 4person. He got worried before examinations. He worried about his job and his health. All he wanted in life was to be safe and healthy. Then, on January 15th, 2002, a plane crashed into the Potomac River in Washington. Lenny went to the river to see what was happening. He saw a woman in the ice-cold water. Lenny did not feel afraid. He kept very 5and did a very adventurous thing. He jumped into Potomac, swam to the woman, and kept her head above the water. Seventy-eight people died that day. Thanks to Lenny, it was not 6.

    When you are in a very dangerous situation and feel afraid, the body automatically (自动地) produces a chemical in the blood. This chemical is called adrenalin(肾上激素). With adrenalin in the blood system, you actually feel stronger and stronger and are ready to fight or7 However, when you are absolutely terrified, the body can produce too much adrenalin. When this 8 the muscles become very hard and you find you can't move at all. You are paralyzed(使无能为力) with fear. This is9 when we are very frightened, we sometimes say we are “petrified”. This word comes from a Greek word “petros”, which means “stone”. We are10 frightened that we become stonelike.