大学物理题（）,A 15-g bullet is shot vertically into an 5-kg block.

大学物理题（）,
A 15-g bullet is shot vertically into an 5-kg block.The block lifts upward 3.0 mm (see the figure).The bullet penetrates the block and comes to rest in it in a time interval of 0.0010 s.Assume the force on the bullet is constant during penetration and that air resistance is negligible.
What is the initial kinetic energy of the bullet?

分两个阶段.子弹穿进物块,物块匀加速至速度v,然后两者相对静止.2.物块继续上升至3mm处.
第一阶段,耗时0.001s,子弹初动量mv0=(M+m)v+重力冲量（M+m）g*0.001s （1）平均速度v/2,上升距离v/2 *0.001s
第二阶段全部是匀加速运动,所以两个阶段平均速度都是v/2,第二阶段耗时v/g,所以上升距离v/2 *v/g=vv/2g.
总共上升v/2 *0.001s+vv/2g=3mm得到v=0.24m/s带入第一阶段（1）式,得到初始速度v0=83.6m/s