题目

       Beijing plans to build huge free or low-cost parking lots beyond the Fourth and Fifth Ring Roads to encourage more car owners to take buses or subways to the downtown area．        The plan is just one of the many measures the city plans to take to reduce its traffic jams．Low or no parking fees would be used as economic leverage(杠杆作用) to reduce growing parking demands from urban areas．        Car owners living in the suburbs will be encouraged to park their cars beyond the Fourth and Fifth Ring Roads and take buses or subways to the downtown area．Statist如图所示，一质量m2＝0．20 kg的平顶小车，车顶右端放一质量m3＝0．25kg的小物体，小物体可视为质点，与车顶之间的动摩擦因数μ＝0．4，小车静止在光滑的水平轨道上．现有一质量m1＝0．05kg的子弹以水平速度v0＝12 m/s射中小车左端，并留在车中．子弹与车相互作用时间很短．若使小物体不从车顶上滑落，g取10 m/s2．求：（1）最后小物体与小车的共同速度为多少？小车的最小长度应为多少？（2）小物体在小车上相对小车滑行的时间．