The question and solution is here http://i359.photobucket.com/albums/oo31/tanzl/Set4Problem3.jpg The solution is quite long. Is there any way I can use conservation of momentum (translational or rotation) to calculate this question? such as initial momentum of ball = final momentum of ball + final momentum of plank Since the plank does not have any translational motion at its center of mass, then its momentum is zero. But, I think it wont works because pivot actually exerts force on the plank to constraint its translational motion. So, the system is not closed.