Energy

Energy in concept is fundamental to mechanics and physics. Energy has three common forms: physical (including sound waves), thermodynamic/heat transfer, and mass transfer including radiation. Energy is essential for any useful work; any movement that overcomes inertia. Physicists classify energy as potential (such as gravity or stretched elastic) or kinetic, which is any object or mass in motion. Common measurements are ergs, calories, British Thermal Units, kilowatt hours, and joules.