题目

．如图所示，质量为1kg的物体放于倾角θ为37º的足够长的固定斜面底端，受到30N的水平拉力作用而由静止开始向上运动，物体与斜面的动摩擦因数为0.5，2s后将水平拉力撤去（1）求物体向上运动的最高点的位置？（2）水平拉力撤去后还要经过多少时间物体再次回到斜面底端？（3）定性画出物体在斜面上运动全过程的速度——时间图线，以沿斜面向上为速度正方向(不要求说明理由)。   20、    Some day，little cars may take the place of today’s cars．If everyone  1   such a little ear in the future，there will be less pollution in the air；there will be more parking space in cities；the streets will be  2  crowded．The space now for one car of the usual size can hold  3  one little car．     The little car will  4  much less to own and to drive．Driving will be  5  too，because these little ears can go only 50 kilometers an hour．     The cars of the future will be fine for going  6  the city，but they will not be useful for going far away．The little car needs only two batteries(电池)一one battery for the motor，and  7  for the horn and the lights．     If we want the big cars to  8  along with the small ones，we will need to  9   two kinds of roads．Some roads will be used  10   the big fast cars，and other roads will be needed for the small，slower ones． 1．A．drives                    B．sits                             C．makes 2．A．more                     B．1ess                           C．fewer 3．A．fewer than             B．1ess than                      C．more than 4．A．pay                       B．spend                         C．cost 5．A．faster                    B．harder                         C．safer 6．A．over                      B．around                        C．from 7．A．another                  B．the others                    C．the other 8．A．make                     B．get                             C．let 9．A．build                     B．put                             C．find 10．A．to                        B．as                               C．for