The SI unit of energy is the joule, denoted as $J$.

Energy is a scalar physical quantity that represents the capacity to perform work. In the International System of Units (SI), it is measured in joules ($J$). Specifically, one joule is defined as the amount of energy required to move an object with a force of one newton ($N$) through a distance of one metre ($m$) in the direction of that force.

Energy manifests in various forms, including kinetic energy, potential energy, thermal energy, electromagnetic energy, and chemical energy. Kinetic energy refers to the energy of motion, whereas potential energy is the energy stored in an object due to its position or configuration. Thermal energy relates to the temperature of an object, while electromagnetic energy is associated with the movement of charged particles. Chemical energy is the energy stored within chemical bonds.

Energy is a conserved quantity, meaning it cannot be created or destroyed; it can only be transformed from one form to another. This principle is known as the law of conservation of energy. It is a fundamental concept in physics and is essential for analyzing a wide range of physical phenomena, from the motion of particles to the behavior of entire systems.