WORK, ENERGY AND POWER
 

4.1 Work

When an object undergoes displacement under force then work is said to be done by the force and the amount of work (W) is defined as

Note: (i) if then F S 1 is positive and then force is said to do work on the object.

(ii) if then W = 0, no work is done by force.

(iii) if in opposite directions) then is negative and then the object is said to do work against the force.

The M.K.S unit of W is 1 Joule = 1 Newton 1 meter

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). 

Law of Conservation of Energy

For a particle,

Therefore E = K.E + P.E. = Constant. The equation (3) is a specific case of a more general law of the Conservation of Energy: "Energy can neither be created nor destroyed but can only be transformed from one form to another."

Power (P)

The rate at which work can be done is called Power.

Its M.K.S unit is 1 Joule/sec = 1 Watt.

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