问题 计算题

如图甲所示,一固定的粗糙斜面的倾角为37°,一物块m=5kg在斜面上,若用F=50N的力沿斜面向上推物块,物块能沿斜面匀速上升,g=10m/s2,sin37°=0.6,cos37°=0.8.

(1)求物块与斜面间的动摩擦因数μ;

(2)若将F改为水平向右推力(如图乙),则为多大时才能使物块沿斜面匀速运动。(设最大静摩擦力等于滑动摩擦力)

答案

(1)     (2)9.1N 

题目分析:(1)取平行于斜面为x轴方向,垂直斜面为y轴方向,根据平衡条件有:

           ①(2分)

            ②(2分)

              ③(1分)

联立①②③解得            ④(1分)

(2)当物体匀速上滑时,根据平衡条件有:

          ⑤(1分)

          ⑥(1分)

             ⑦(1分)

联立④⑤⑥⑦解得 N           (1分)

当物体匀速下滑时,⑤式改写为:

          ⑧(1分)

联立④⑥⑦⑧解得 N9.1N         (1分)

单项选择题
单项选择题

In bringing up children, every parent watches eagerly the child’s acquisition of each new skill—the first spoken words, the first independent steps, or the beginning of reading and writing. It is often tempting to hurry the child beyond his natural learning rate, but this can setup dangerous feelings of failure and states of worry in the child. This might happen at any stage. A baby might be forced to use a toilet too early; a young child might be encouraged to learn to read before he knows the meaning of the words he reads. On the other hand, though, if a child is left alone too much, or without any learning opportunities, he loses his natural enthusiasm for life and his desire to find out new things for himself.

Parents vary greatly in their degree of strictness towards their children. Some may be especially strict in money matters. Others are strict over times of coming home at night or punctuality tot meals. In general, the controls imposed represent the needs of the parents and the values of the community as much as the child’s own happiness.

As regards the development of moral standards in the growing child, consistency is very important in parental teaching. To forbid a thing one day and excuse it the next is no foundation for morality. Also, parents should realize that "example is better than precept". If they are not sincere and do not practise what they preach, their children may grow confused, and emotionally insecure when they grow old enough to think for themselves, and realize they have been to some extent fooled.

A sudden awareness of a marked difference between their parents’ principles and their morals can be a dangerous disappointment.

Eagerly watching the child’s acquisition of new skills ().

A.should be avoided

B.is universal among parents

C.sets up dangerous states of worry in the child

D.will make him lose interest in learning new things