a uniform metal rod is suspended from one end. when it is displaced through 60o from the vertical, the change in potential energy is U, the change in potential energy when it is displaced further trough 60o is? (ans : 2U)

The P. E. when the rod is hanging vertically=U1=mglWhen the rod is deflected by 60° then P. E.=U2 mgh=mglcos60°=mg×l2Therefore change in P. E.=U =U1-U2=mgl-mgl2=mgl2When further the rod is deflected by another 60° then P. E.=U3=mglcos120°=-mgl2Therefore change in P. E. =U3-U2=-mgl2-mgl2=mgl=2U

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