In physics, energy is a property of objects which can be transferred to other objects or converted into different forms, but cannot be created or destroyed.[note 1] It is difficult to give a comprehensive definition of energy because of its many forms, but one common definition is that it is the ability of a system to perform work.

In SI units, energy is measured in joules, the energy transferred to an object by the mechanical work of moving it 1 metre against a force of 1 newton.

All of the many forms of energy are convertible to other kinds of energy, and obey the conservation of energy. Common energy forms include the kinetic energy of a moving object, the radiant energy carried by light, the potential energy stored by an object's position in a force field,(gravitational, electric or magnetic) elastic energy stored by stretching solid objects, chemical energy released when a fuel burns, and the thermal energy due to an object's temperature.