Energy is defined as the ability to do work. Its unit is the same as that of work. Energy is a scalar quantity.

SI unit of energy or work = Joule (Nm) or Kg-m2 s-2.

Different Forms of Energy

Energy has different forms: Light, heat, chemical, electrical or mechanical.
Mechanical energy is the sum of these two energy:-
(i) Kinetic energy (K.E)
(ii) Potential energy (P.E)

 Kinetic Energy:

Every moving object possesses some energy called Kinetic Energy. As the speed of the object increases so is its kinetic energy.


Derivation for Kinetic Energy:


Work-Energy Theorem

The work-energy theorem states that the net work done by a moving body can be calculated by finding the change in KE.

W net = KEfinal − KE initial

 W net= ½ m[v2−u2]

Potential Energy:

Every object possesses some energy called Potential Energy. An object when gains energy may store it in itself as potential energy.

Energy can get stored in an object when work is done on it.

For example, stretching a rubber string. The energy that is possessed by a body by virtue of its configuration or change in position is known as Potential Energy.


 It is defined as the amount of work done required in raising an object above the ground up to a certain point against the gravity. An object ‘A’ having mass ‘m’ is lifted by height ‘h’ above the ground. Let us calculate the potential energy of object A at height ‘h’:


Work is done against gravity to change its position. This energy is stored as Potential Energy.

W = F.s

F = ma
In the case of increasing the height, F = mg
Therefore, W (P.E) = mgh
ΔPE=mg (hfinal –hinitia)

Gravitational potential is path independent.

Law of conservation of Energy:

“Energy can neither be created nor be destroyed .One form of energy can be transformed into other forms of energy"

According to the law of conservation of energy, the total amount of energy before and after transformation remains the same.

Example:- Light Bulb

When the switch connected to the light bulb is pushed or is turned on, the electrical connection between the supply unit and the bulb gets established. The current begins to flow in the closed electrical circuit causing the bulb to glow. Here, the electrical energy gets converted into light energy after entering the internal circuitry of the light bulb.