Work, Energy and Power


If a body gets displaced when a force acts on it work is said to be done, Work is measured by the product of force and displacement of the body along the direction of force.
If a body gets displaced by S when a force F acts on it then the work, W = FS cos θ
where, &theta = angle between force and displacement
» It both force and displacement are in the same direction, then W = FS
» If force and displacement are perpendicular to each other.then W = O as cos θ = 0.
For example, in case of uniform circular motion work done by the centripetal force is zero.
Work is a scalar quantity and its SI unit is joule.


Capacity of doing work by a body is called its energy
» Energy is a scalar quantity and its SI unit is joule.
» Energy developed in a body due to work done on it is called mechanical energy.
» Mechanical energy of two types - Potential Energy and kinetic Energy

Potential Energy

The capacity of doing work developed in a body due to its position or configuration is called its potential energy.
PE of a body in the gravitational field is mgh.
Where, m = mass, g = acceleration due to gravity, h = height of the body from surface of the earth

Kinetic Energy

Energy possessed by a body due to its motion is called Kinetic Energy of the body.
If a body of mass m is moving with the sped v , then the kinetic energy of the body is 1/2mv2.

Principle of Conservation of Energy

Energy can neither be created nor can be destroyed. Only energy can be transformed from one form to another form. Whenever energy is utilized in one form, equal amount of energy is produced in other form. Hence total energy of the universe always remains the same. This is called the principle of conservation of energy.


Rate of doing work is called power.
» It an agent does W work in time t, then power of agent = W/t
» SI unit of power is watt named as a respect to the scientist James Watt.
      watt = joule/sec
      1 kW = 103 watt
      1 MW = 106 watt

» Horse power is a practical unit of power.
      1 H.P. = 746 watt.
      1 watt second = 1 watt x 1 second = 1 joule.
      1 watt hour (Wh) = 3600 joule
      1 kilowatt hour (kWh) = 3.6 x 106 joule.