At any instant, the total energy of a simple pendulum is the sum of kinetic energy $$\frac12mv^2$$ and potential energy $$\frac12kx^2$$. The amplitude is $$10\text{ cm}$$ and total energy is $$4\text{ mJ}$$. Find k.

At any instant of time, the total energy (E) of a simple pendulum is equal to the sum of its kinetic energy $$(\frac12mv^2)$$ and potential energy ($$\frac12kx^2$$), where, m is the mass, v is the velocity, x is the displacement of the bob and k is a constant for the pendulum. The amplitude of oscillation of the pendulum is 10 cm and its total energy is 4 mJ. Find k.