镜头—：于某年满18周岁，符合义务服兵役的条件，可他的父亲早已病逝，母亲身体不好，常年因病卧床，王某面临着一个非常困难的选择，是去服兵役还是在家照顾母亲？

镜头二：北京奥运会即将举办，大会组委会决定招募志愿者，北京奥运会志愿者总需求约为7万人，仅仅过了一个月，报名者就突破了70万。

运用教材知识问答问题：

如图所示，一物体从倾角为30°的斜面顶端由静止开始下滑，s₁段光滑，s₂段有摩擦，已知s₂=2s₁，物体到达底部的速度刚好为零，则s₂段的动摩擦因数μ为多少？

    We have a big playground in our school .We all like to 1 games 2 after school .It's Friday .We have only two classes 3 the afternoon . 4 school is over（结束的）,a lot of students go to the playground quickly.  The boys of Grade Two often 5 soccer. The girls like 6 them playing .Soccer is their favorite sport . Their teacher Mr Li also 7 football. Li Ming , Li Lei and Wei Fang are students of Grade One .They sometimes play 8 the students of Class Two. It’s half past five 9 . It's time 10dinner . My father is waiting for me at the school gate .