In the figure shown, stiffness of spring is k and mass of the block is m. The pulley is fixed. Initially the spring is relaxed. Now the system is released from rest. The
maximum elongation in the spring is
When the spring has maximum elongation, the mass is momentarily at rest. Now, decrease in gravitational P.E. = Increase in spring P.E.
∴mg xm=12kx2m⇒xm=2mgk