4.2 Energy
The capacity of an object to do work is called energy. The M.K.S Unit of energy is also 1 Joule.
Kinetic Energy
The capacity of an object to work on account of its motion is called Kinetic energy (K.E).
For a particle K.E=
.....(2)
Potential Energy
The capacity to do work by a system due to its mutually interacting parts is called Potential Energy
(P.E). Potential energy expressions vary ascending to the type of mutual interaction.
e.g. i) For an object of mass `m' at height `h' from ground, the
gravitational P.E. = mgh provided that h is very small compared to the radius of the Earth.
ii) Electrostatic P. E of a pair of charges is 
iii) Elastic P.E. of a spring of force that is constant `K' and deformed in length by amount `x'
is 
iv) Mechanical Energy: The sum of K.E. and P.E is called mechanical Energy (E).
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