If only conservative forces apply on the system, the work and energy equation can be simplified to conservation of energy equation. All the problems that can be solved using conservation of energy can also be solved using work and energy method but not the other way round. We have two types of potential energy, elastic potential energy stored by an elastic spring and gravitational potential energy due to change in height. Problem Statement: Problem 18-46. The 12kg slender rod is attached to a spring, which has an unstretched length of 2 m. If the rod is released from rest when θ=30^0, determine the angular velocity of the rod the instant the spring becomes unstretched.