1.6 (a) Energy

If you own a car, it will not move without fuel. At present this fuel could be petrol, or alcohol or diesel fuel. In the past the fuel could, just possibly, have been coal; and in the future, it could be hydrogen or electricity stored in a battery. But whatever fuel you use, you are buying something with the ability to make that car move. This stored ability is known as potential energy.

A clock needs energy to make the hands move, and this energy can be stored in a spring that you wind up with a key, in an electrical battery, or in weights that are raised up.

Stored, or hidden, energy is called potential energy(P.E)

If the spring is released or the load moves back to the ground, the stored potential energy is transferred to movement energy, which is called kinetic energy(K.E)

In all of the examples above, the potential energy can be used to make an object move, and hence give it kinetic energy. Kinetic energy can also be turned into potential energy, and this can be seen most cearly in the action of a pendulum, where at each end of its swing the pendulum has a

maximum amount of gravitational potential energy, and at the middle of its swing where some of the potential energy has been used {the pendulum is lower down} and turned into kinetic energy (the pendulum is moving fastest).

Energy

The energy of a body is its capacity to do work. The unit of energy is same as that of work. The S.I unit is joule(J).

The bigger units of energy are watt hour(Wh) and kilowatt hour(kWh)

Heat energy is usually measured in calorie or kilocalorie.

The energy of atomic particles is very small, so it is measured in electronic volt(eV)

1 eV=1.6times 10^{-19} J

Potential energy(P.E)

The energy possessed by a body by virtue of its state of rest is called potential energy.

therefore P.E=mgh

where P.E – gravitational potential energy, m– mass in Kg , g– gravitational field strength, h– height

Kinetic Energy(K.E)

 The energy possessed by a body by virtue of its state of motion is called kinetic energy.

therefore K.E= frac{1}{2} mv^{2}

where K.E-kinetic energy,  m- mass in Kg,  v– velocity

Go here for Examples of energy in different forms