质量为30kg的物体静止放在水平面上，物体和水平面的最大静摩擦力为31N，物体和水平面之间的动摩擦因数为0.1．当水平拉物体的力分别为28N和50N时，物体所受的摩擦

The Marches were a happy family. Poverty, hard work, and even the fact that Father March was away with the Union armies could not down the spirits of Meg, Jo, Amy, and Marmee, as the March girls called their mother.
The March sisters tried to be good but had their share of faults. Pretty Meg was often displeased with the schoolchildren she taught; boyish Jo was easy to become angry; golden-haired schoolgirl Amy liked to show up; but Beth, who kept the house, was loving and gentle always.
The happy days passed and darkness came when a telegram arrived for Mrs. March. “Your husband is very ill,” it said, “come at once.” The girl tried to be brave when their mother left for the front. They waited and prayed. Little Beth got scarlet fever (猩红热) when she was taking care of the sick neighbor. She became very ill but began to recover by the time Marmee was back. When Father came home from the front and at that joyful Christmas dinner they were once more all together.
Three years later the March girls had grown into young womanhood. Meg became Mrs. Brooke, and after a few family troubles got used to her new state happily. Jo had found pleasure in her literary efforts. Amy had grown into a young lady with a talent for design and an even greater one for society. But Beth had never fully regained her health, and her family watched her with love and anxiety.
Amy was asked to go and stay in Europe with a relative of the Marches. Jo went to New York and became successful in her writing and had the satisfaction of seeing her work published there. But at home the bitterest blow was yet to fall. Beth had known for some time that she couldn’t live much longer to be with the family and in the spring time she died.
News came from Europe that Amy and Laurie, the grandson of a wealthy neighbor, had planned to be married soon. Now Jo became ever more successful in her writing and got married to Professor Bhaer and soon afterwards founded a school for boys.
And so the little women had grown up and lived happily with their children, enjoying the harvest of love and goodness that they had devoted all their lives to.
【小题1】The members of the March family were Father March, Mrs. March and their . A．four daughtersB．five daughtersC．son and four daughters D．son and five daughters【小题2】Who was the most successful in career (事业) among the March girls? A．Jo B．BethC．AmyD．Meg【小题3】Which of the following would be the best title for the passage? A．The march Family B．The March ParentsC．The March GirlsD．The March Relatives【小题4】It can be inferred from the passage that the March family had . A．both happiness and sadnessB．wealthy neighborsC．more girls than boysD．a lot of rich relativesA

(答案→)A 解析：【小题1】细节题。从第二段“the March sisters”可知the Marches有Meg，Jo，Amy，Beth这4个女儿。【小题2】细节推断题。从第五段“Jo went to New York and became successful”和第六段“Now Jo became ever more successful”可知，Jo在姐妹中最成功。【小题3】主旨题。本文一直在讲March家的女儿们从小到大的情况，因此标题应该是The March Girls。【小题4】推理题。从文中可知，March家的女儿长大了，享受着爱的果实，过着幸福的生活；但是，Beth后来病死了。由此可知，March家有幸福也有悲伤。

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解：因为物体和水平面的最大静摩擦力为31N，当水平拉物体的力分别为28N，物体不动，所受的摩擦力为静摩擦力，根据平衡有：f1=F=28N．
当水平拉力为50N时，物体运动．根据f=μFN得，f2=0.1×300N=30N．
答：物体所受的摩擦力不等，分别为28N　30N．解析分析：当水平力拉不动物体时，物体所受的摩擦力为静摩擦力，根据平衡求出静摩擦力的大小；当拉得动时，物体所受的摩擦力为滑动摩擦力，根据滑动摩擦力公式求出摩擦力的大小．点评：解决本题的关键知道静摩擦力和滑动摩擦力的区别，以及掌握静摩擦力和滑动摩擦力的求法．

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