一道大学物理题..关于转动惯量的..

一道大学物理题..关于转动惯量的..
将一个40cmX40cm的正方形油画用一个钉子固定它的住左下角.然后松手--------这个时候,油画会在重力的作用下,绕着钉子旋转..求：当油画的质心到达钉子的正下方时,油画转动的角速度.谁能帮个忙啊..我算了好久都没算对..正确答案是5.3 m/s,但是谁能告诉我这个答案是怎么算出来的- -....
附：原题(翻以前)是这样的:An enthusiastic physics student mounts on her wall a portrait of her favorite physicist.Her roommate needs tacks to put up photos of David Bowie,and when there is only one tack left,in the lower left corner,gravity causes the portrait to rotate freely around that remaining tack.Assume that the portrait is a square 40 cm on a side.(c) Use energy ideas to determine the velocity of the portrait's center when it has reached a position directly beneath the tack.