The Mangonel works by pulling a long arm with a bucket attached down form its 90o angle of equilibrium. By doing this we store the potential energy of the catapult in the tension in the ropes and the arm. The tension is stored much like in a spring, therefore the equation of potential energy is the same as the for the Ballista. The k constant must be determined experimentally by find how much force is required to move the arm back a certain distance F=kx. The x is how far back the arm is stretched.
Unlike the Ballista the Mangonel does not release its energy in a linear fashion. The arm makes an arc (part of a circle) with radius equal the arm length. Therefore the Potential Energy is transferred in the Rotational Kinetic Energy. The w stands for the angular velocity and r for the length of the arm. I is the moment of inertial as the arm is a rod and its rotating on its end, I=1/3mr2