牛顿第三定律

观察和实验表明：两个物体之间的作用总是相互的．一个物体对另一个物体有力的作用，后一个物体一定同时对前一个物体有力的作用，力是物体与物体间的相互作用，物体间相互作用的这一对力，常常叫做作用力和反作用力．把相互作用的两个力分成为作用力和反作用力并不是绝对的．我们可以把其中一个力叫做作用力，另一个力就叫做反作用力．

牛顿第三定律  作用力和反作用力之间存在什么样的关系呢？

把两个弹簧秤A和B连接在一起（如图示）用手拉弹簧秤，可以看到两个弹簧秤的指针同时移动．弹簧秤B的示数指出弹簧秤A对它的作用力F的大小，而弹簧秤A的示数指出弹簧秤B对它的反作用力F′的大小．可以看出，两个弹簧秤的示数是相等的．改变手拉弹簧的力，弹簧秤的示数也随着改变，但两个示数总相等．这说明作用力和反作用力大小相等，方向相反．

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：

阅读理解。阅读下列短文，根据短文内容选择最佳答案。

    Would you believe that the first outstanding deaf teacher in America was a Frenchman? His name was Laurent Clerc. He became a friend of Thomas Hopkins Gallaudet and together they founded America's first school for the deaf.

    Laurent Clerc was born in a small village near Lyons, France, on December 26, 1785. When he was one year old, he fell into a fire, losing both his hearing and his sense of smell.

    At 12, Laurent entered the Royal Institution for the Deaf in Paris where he did well in his studies. After he graduated, the school asked him to stay on as an assistant teacher.

    Meanwhile, in America, Thomas Hopkins Gallaudet was studying to be a minister. He was very concerned about the lack of educational opportunities for the deaf. Therefore, in 1815, Gallaudet sailed to London, England to seek ideas on how to teach deaf people. While he was there, he met a French educator of the deaf who invited him to go to Paris to spend three months learning at the Royal Institution for the Deaf, the school where Laurent Clerc was teaching. Gallaudet accepted the offer. The two worked and studied well together. When the time came for Gallaudet to return, he asked Clerc to come with him. Clerc accepted on one condition: that he would stay in America only a short time.

    The two men set sail on June 18, 1816. The voyage across the Atlantic Ocean took 52 days; however, Clerc and Gallaudet put the time to good use. Clerc studied English, and Gallaudet studied sign language. They discussed the school for the deaf which they planned to open. On the long trip, they had many conversations about education and deafness. The year after they arrived, they founded a school for the deaf in Harford, Connecticut.

At the school, Clerc led a busy life. He taught signs to Principal Gallaudet; he taught the pupils; and he taught hearing men who came to the school to study deaf education.

    In 1819, Clerc married Eliza Crocker Boardman, one of his pupils. They had six children. He retired from teaching in 1858. Although he had intended to return to France, he never did. He died on July 18, 1869 in the United States.