题目

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项，选项中有两项为多余选项。 Some students prefer to learn by themselves. They think they can learn better in this way because they have more freedom, pressure and independence. There is some truth in their way. It is better to have a teacher, especially when they learn something more difficult. First, a teacher can act as a guide to lead you to success. When you learn, you need help and advice. A teacher will give you a hand when you face difficulties. What is more, a teacher puts real pressure on yo4．如图，一质量为mB=2kg的木板B静止在光滑的水平面上，其右端上表面紧靠一固定斜面轨道的底端（斜面底端与木板B右端的上表面之间有一段小圆弧平滑连接），轨道与水平面的夹角θ=37°．一质量也为mA=2kg的物块A由斜面轨道上距轨道底端x0=8m处静止释放，物块A刚好没有从木板B的左端滑出．已知物块A与斜面轨道间的动摩擦因数为μ1=0.25，与木板B上表面间的动摩擦因数为μ2=0.2，sin θ=0.6，cos θ=0.8，g取10m/s2，物块A可看作质点．请问：（1）物块A刚滑上木板B时的速度为多大？（2）物块A从刚滑上木板B到相对木板B静止共经历了多长时间？木板B有多长？