Energy is a fundamental concept in physics that refers to the ability of a system to do work. It is a scalar quantity, meaning it has magnitude but no direction. In simpler terms, energy is what allows things to happen and change in the universe.
Energy is measured in joules (J) in the International System of Units (SI). Other common units of energy include calories (cal) and kilowatt-hours (kWh). In certain contexts, other units such as electron volts (eV) or foot-pounds (ft-lb) may also be used.
There are many different types of energy, but they can be broadly categorized into two main forms: potential energy and kinetic energy. Potential energy is stored energy that an object has due to its position or condition, while kinetic energy is the energy an object possesses due to its motion.
Energy can be transferred from one form to another or converted from one type to another. For example, when a ball is thrown, the potential energy in the person's arm is converted into kinetic energy as the ball moves through the air. Energy can also be transferred from one object to another through processes such as heat transfer or electromagnetic radiation.
In physics, energy and mass are considered to be two forms of the same underlying entity, known as mass-energy. This is described by Einstein's famous equation, E=mc², where E represents energy, m represents mass, and c represents the speed of light. This means that mass can be converted into energy and vice versa.