Energy is measured as the capacity to do a work. It is a scalar quantity having SI unit same as the work i.e., joule (J) and CGS unit is erg. A larger unit of energy is kilojoule.

An object having energy can exert a force (and hence do a work) on another object. Where it happens, energy is transferred from former object to latter object.

The energy and power do not mean the same, these are different. Energy refers to the total amount of work done while power determines the rate of doing work.

Different Forms of Energy

Energy can manifest in several forms. 

  1. Heat Energy. The energy possessed by a body due to its temperature
  2. Internal Energy. The total energy possessed by a body by virtue of particular configuration of its molecules and also their random motion
  3. Electrical Energy. The energy required to maintain the flow of current in an electric appliance
  4. Chemical Energy. The energy absorbed or released during a chemical reaction
  5. Nuclear Energy. The energy absorbed or released during a nuclear reaction (nuclear fission or nuclear fusion)

Kinetic Energy

The energy possed by a body due to its motion, is called its kinetic energy and is given by


v = velocity of a body at any time, m= mass of the body

and p=linear momentum

Thus, kinetic energy of an object increses with its speed.


  • When a fast moving cricket ball hits a stationary wicket, the wicket is thrown away.
  • When a raised hammer falls on a nail placed on a piece of wood, it drives the nail into the wood.

Potential Energy

The energy possessed by a body due to change in its position or shape, is callled potential energy.


  • Water stored in dam has potential energy due to its position at the height.
  • A stone lying on the roof of the building has potential energy due to its height.
  • A wound spring of a watch has potential energy due to its shape.

The sum of potential energy and kinetic energy of an object, is known as mechanical energy.

Gravitational Potential Energy

When an object is raised through a certain height above the ground, its energy increses. This is because work is done on it against gravity while it is being raised. The energy present in such an object is the gravitational potential energy.

The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against the gravity.

Gravitational potential energy U = mgh

g= acceleration due to gravity,

h= height above the surface, m = mass of the body.

Work-Energy Theorem

According to this theorem, work done by all the forces acting on a body is equal to the change in kinetic energy of the body

Work done = Change in kinetic energy

Law of Conservation of Energy

According to the law of conservation, energy can only be transformed from one form to another. It can neither be created nor be destroyed. e.g., when an object is dropped from the height, its potential energy continuously converts into kinetic energy.

When an object is through upwards, its kinetc energy continuously converts into potential energy. The total energy before and after transformation always remains constant.

PE + KE = constant or mgh+1/2mv²=constant

Transformation of Energy

The conversion of energy from one form to other is known as the transformation of energy. The phenomenon of transformation of energy from useful to useless form is known as dissipation of energy.

  • Green plants prepare their own food (chemical energy) using solar energy through the process of photosynthesis.
  • When we throw a ball, the muscular energy which is stored in our body gets converted into kinetic energy of the ball.
  • When an athlete runs, the body’s internal energy is converted into kinetic energy.

Einstein’s Mass-Energy Equivalence

According to Einstein, neither mass nor energy of the universe is conserved but they are interconvertible. The conversion is expressed by the equation E=mc²

c=3×108m/s is the speed of light

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