江西省红色六校2014届高三第二次联考理综试题答案(7)

                                   a2=36m/s2··················2分
                       下落时间t2
                               v1-v0=a2t2
                                  t2=0.2s ····················1分
            下落总时间t总=t1+t2=1.2s     ······················2分
25.（19分）解析：（1）设B球碰后速度大小为vB
        因为 qE1=mg,  B球在第三象限内做匀速圆周运动
           qBvB=m     
                ·········································2分
     B球运动半周后再垂直电场方向进入第四象限内做类平抛运动，时间为t
      沿y轴正方向加速度maB=
                         aB=g ··································1分
         y轴正方向匀加速运动  2R= ·······················1分
         x轴正方向匀速直线运动L=vBt     ·······················1分
     联立以上方程解得vB=    ······················2分
则  2R=  ·············2分
      B球经过y轴时的坐标为（0，—）···················2分
（2）碰后物块A速度为vA，沿薄板水平匀减速运动到P点