hey look...........

himanshu has done a fine job.......

 

but i m just adding something more.................

 

as U = Mgh

 

M= m/3.........

 

g= g..........

h= l/6..............( because we take effective height ..............

                           that is height of the centre of mass of the hanging part.........)

 

therefore...........

 

U= mgl/18.............ans

 

thanks...........

i know u have got the answer but just .........giving the information why himanshu has taken

l/6

 

hope u understands........