when a spring is connected with a block if we compress the spring the potential energy of block changes. But if we take only the block as system then no internal conservative force is acting on system and hence by the relation change in potential energy = -(work done by conservative internal force) we get change in potential energy to be zero . Can't understand please help
The potential energy of the block does not change at all. Its the potential energy of the spring that changes. The spring potential energy U = k/2(x^2) which changes as you compress or elongate it. This potential energy does work on the block changing its kinetic energy when the spring is released. Taking the block as the system will definitely mean the change in potential energy is zero since the internal forces tend to cancel out.