Peelastic = kx2: the total mechanical energy of a mass-spring oscillator includes the possibilities of kinetic energy, gravitational potential energy, and elastic potential energy due to the spring: Emechanical = ke + pegravitational + peelastic: for an oscillating mass-spring system where mechanical energy is conserved, and the energy oscillated between the different forms during a cycle. Emechanical = ke + pegravitational + peelastic = constant. In other cases (e. g. a leg), the mass is distributed much more evenly across the entire system: this is called a physical pendulum, the period of a physical pendulum is: In real systems, there are forces acting on the system that either increase or decrease its mechanical energy. If a force acting on an oscillating system adds mechanical energy to it, the system is said to be driven: e. g. a child on a swing- a person standing behind and pushing it adds energy to the system.
