A metre stick of mass 400 gm is pivoted at one end and it is hanging freely .If the rod is  displaced through an angle 60o. The increase in its potential energy is

When the rod is displaced through an angle θ its center of gravity is increased to a height  BA as shown in fig.(7) Le OA=x. From ΔOAC, we get  x(1/2)=cos⁡60∘  or  2x=12  or  x=14m  From figure OB=12m ∴ BA=12m−x=12m−14m=14m  Change in potential energy =mg×(BA) =0⋅4×10×(1/4)=1 joule