题目

根据对短文理解，选择答案。 Americanschool students usually go to school on foot, or on the school bus. Some studentsride their bikes to school. In themiddle school, students usually take classes for six hours a day. They study English,math, history, science, PE, music and art. Schoolbegins at half past eight in the morning. At noon, students have lunch at school.After lunch, they go to their classes. School is over at about 4 o'clock. Most studentsoften watch TV and do homework after supper. On weekends,American school students go to the church, do shopping, visit grandparents or 质量为M=5.0 kg的平板车静止在光滑的水平面上，如图所示，当t=0时，两个质量分别为ma=3.0 kg、mb=2.0 kg的小物体A、B都以大小为v0=12 m/s的速度同时从小车板面上的左右两端相向滑上平板车，到它们在小车上停止滑动时，始终没有相碰。A、B与车间的动摩擦因数均为μ=0.2，取g=10 m/s2。问：（1）A在车上刚停止滑动时，车的速度为多大？（2）从A、B刚滑上车到A、B在车上都停止滑动时共经历多长时间？此时车的速度为多大？（3）在给出的坐标系中画出小车运动的速度—时间图象。