Energy

In physics, energy is a quantity that is understood as the ability to perform work.The quantity of energy can be assigned to any particle , object or system of objects as a consequence of its physical state.

There are different forms of energy. they are potential,kinetic, thermal, gravitational, sound and elastic.Most of the time the forms of energy are  named after a related electromagnetic energyforce. Here is provided a link on the definition of kinetic energy.

Any form of energy can be transformed  into another form. When energy is in any form other than thermal energy, it may be transformed with good or even perfect efficiency, to any other type of energy.However,in thermal energy,  there are often limits to the efficiency of the conversion to other forms of energy, due to the second law of thermodynamics.

See the given link on the law of conservation of energy.

Energy is a scalar quantity. In the International System of Units, energy is measured in joules, but in some fields other units such as kilowatt-hours and kilocalories are also used.