The spring is unstretched at the moment a man starts pulling on the cord. The block has a mass M . If the man applies a constant force F , calculate the energy stored in the spring when the block passes through its equilibrium position.

Step-by-step solution

Let the equilibrium position of the block be at a displacement x from its initial unstretched position. At the equilibrium position, the net force acting on the block is zero. The constant force F applied by the man is balanced by the restoring force of the spring. F = kx x = F k The energy stored in the spring at this position is the elastic potential energy, given by: U = 1 2 kx^2 Substituting the value of x : U = 1 2 k ( F k )^2 U = 1 2 k ( F^2 k^2 ) U = F^2 2k