The unit of measurement of **energy** is the same as that of work. That is, the S.I. unit for the measurement of energy is joule, which is represented by the letter J. One joule of energy is approximately equal to the energy required to lift a mass of 100 grams to a height of 1 metre above the surface of earth. Joule is actually a small unit of energy, so to express bigger values of energy, the bigger units of energy like kilojoule (kJ) and megajoule (MJ) are used.

1 kilojoule = 10^{3} joules

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or 1 kJ = 10^{3} J

and 1 mega joule = 10^{6} joules

or 1 MJ=10^{6}J

The commercial unit (or trade unit) of energy is kilowatt-hour (kWh) which is related to joule as follows:

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1 kilowatt-hour = 3.6 x 10^{6} joules

or 1 kWh = 3.6x 10^{6}J

Just to give you an idea of the magnitude of energy represented by the various units of energy we would like to state that the energy required by the heat to pump the blood is about 1 joule per heart beat; the energy required to lift a mass of 1 kilogram to a height of 1 metre is about 10 joules; the energy required for making a cup of tea is approximately 75 kilojoules, whereas the energy provided by 1 litre of petrol on burning is around 37 mega joules. Heat is a form of energy. The old unit of heat energy is calorie. One calorie is that amount of heat energy which raises the temperature of 1 gram of water by 1°C.

The ability of a body to do work may be due to its higher position above the ground or due to its motion. In other words, the energy of a body may be due to its higher position above the earth or due to its motion. This gives rise to two types of mechanical energy : potential energy and kinetic energy.