Energy is the ability to do [[Work|work]] or cause change. It exists in many forms: - [[Kinetic Energy|Kinetic energy]] (motion) - [[Potential Energy|Potential energy]] (stored energy) - [[Heat|Thermal energy]] (heat) - Chemical energy (fuels) Energy is measured in **Joules (J)** in the [[SI System|SI system]]. One of the most fundamental principles in physics is the [[First Law of Thermodynamics|Law of Conservation of Energy]], which states that energy cannot be created or destroyed, only transformed from one form to another. For example, when a ball falls, its **potential energy** converts into **kinetic energy**.