1. The problem statement, all variables and given/known data A pendulum, initially at equilibrium, is set into motion by a spring-loaded launcher (compressed a distance of 0.0150 m) which fires horizontally. If the mass of the pendulum bob is 0.340 kg and it rises to a maximum height 0.120 m (relative to equilibrium), what is the spring constant of the spring? 2. Relevant equations Gravitational energy: E = mgh Elastic energy: E = (0.5) (k) (x^2), where k is the spring constant and x is the displacement from equilibrium 3. The attempt at a solution My initial examination of this problem was to state that the gravitational energy at the point where the pendulum is at it's maximum height (and it is instantaneously at rest) was equal to the elastic energy input into the system. Therefore, mgh = (0.5) (k) (x^2). This resulted in a value of k that is not equal to 3550 N / m (the accepted answer in the textbook). As well, as change in energy is work, and the work done by the spring onto the pendulum wasn't equal to it's elastic potential energy (as we don't know how long the spring was in contact with the pendulum), this answer makes even less sense. I am at a loss as to how to further analyze the question.