Definition: Energy is described as internal capacity of doing work. When we call that a body has energy we mean that it can do work.

Energy seems in many forms such as electrical, mechanical, chemical, optical, thermal, Acoustical (sound), nuclear, atomic, molecular, etc. and can converts from one form to the other.

Kinetic Energy

(i) The energy possessed by a object by virtue of its motion is called as kinetic energy (KE).

(ii) The KE of a moving body is same to the amount of work that must be given to bring a body from rest into the state of motion. Conversely, the value of work that we must do in order to collect a moving object to rest is same to the negative of the kinetic energy of the body, i.e., to call a moving object to rest is equal to the negative of the kinetic energy of the object. i.e., to bring a moving body to rest is same to the negative of the kinetic energy of the object, i.e.,

            KE ¾ work done to put the object into motion ¾ -work done to bring the body to a stop

Potential Energy

The energy possessed by a object by virtue of its position or its configuration is known its potential energy.

Concept of potential energy is valid only in conservative field. In conservative force field when a object moves on a round trip then its kinetic energy will not modify. But if we suppose a point B on the path, then at point B, its kinetic power will not be equal as at point A. (Let K.E_B < K.E_A), we may say that this modification in kinetic energy has stored in configuration of system and this kept energy is called as potential energy of system.

So potential energy saved in system when body moves from A to B

            ¾ - (K.E_B - K.E_A) ¾ - (Work done by net force)

We suppose our reference point as A where its P.E is zero.

So U_B - 0 = mgh    U_B = mgh

So potential energy of a object at a height h above the point A will be mgh and h height below the point A potential energy will be - mgh  

Electrostatic Potential Energy: Two point modifies q₁ and q₂ are placed at a distance r from one another then electrostatic potential energy of two phase charge system will be kq₁q₂/r. We consider P.E. of two charge system to be zero when they are placed at infinity.

Spring Potential Energy: A block of mass m is related by a spring of force constant k. If we elongate or compress the spring x₀ from natural length then work done by spring when object moves from A to B is-

Let suppose that the potential energy of system is zero when spring is in its natural size

The other forms of potential energy are elastic potential energy, magnetic potential energy etc.

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