题干

观察和实验表明:两个物体之间的作用力和反作用力总是大小相等,方向相反,作用在一条直线上.这就是牛顿第三定律.

牛顿第三定律在生活和生产中应用很广泛.人走路时用脚蹬地,脚对地面施加一个作用力,地面同时给脚一个反作用力,使人前进.轮船的螺旋桨旋转时,用力向后推水,水同时给螺旋桨一个反作用力,推动轮船前进.摩托车的发动机驱动后轮转动,由于轮胎和地面间有摩擦,车轮向后推地面,地面给车轮一个向前的反作用力,使摩托车前进.摩托车的牵引力就是这样产生的.如果把后轮架空,不让它跟地面接触,这时让发动机驱动后轮转动,由于车轮不推地面,地面也不产生向前推车的力,摩托车就不能前进.

请同学们根据上述自学的知识,回答下列问题:

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    The road in front of my school is not very wide. Every afternoon when school is over, the road becomes very crowded as children, bicycles, cars and buses fill the road. Sometimes a policeman is there to help control the traffic. However, we still have to be careful enough to stop accidents from happening.

    I am always careful when I cross the road because I ever witnessed (目击) a terrible accident. It happened just after school .As usual, the road was full of traffic. Children were running across the road to get to their cars and buses. When the traffic lights turned red, I stopped to wait, but I saw a little boy running across the road .At this moment, a car came and hit him. The boy fell and lay on the ground.  Fortunately, the car was not moving very fast and the driver successfully stopped the car before it could run over the fallen boy.

    At that time all the traffic stopped. I ran to the boy and saw some blood on the road. His head was hurt . A woman came and examined the boy. Then she lifted him and carried him to her car. They hurried to the hospital. Many people crowded around the driver who looked perplexed (糊涂的),and his face turned white. Later a policeman came and calmed the situation downAs there was nothing I could do, I left the place where the accident happened and walked down the road carefully.

    It's terrible to witness an accident. I don't think I can forget it. Certainly, I would not like to get into one. So I have to be more careful in the future.