→ The energy for the ‘life processes’ comes from food. Activities like playing, singing, reading, w

thinking, jumping, cycling and running requires energy.


Work done on an object is defined as the product of the magnitude of the force acting on the body and the displacement in the direction of the force.

W = F.s

If a force acting on a body causes no displacement, the work done is 0. For example, pushing a wall.

Two conditions need to be satisfied for work to be done:
(i) A force should act on object
(a) The object must be displaced
Work = Force x Displacement

Unit of workdone = Joule = Newton x metre

1 Joule work is said to be done when 1 Newton force is applied on an object and it shows the displacement by 1 meter.


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

SI unit of energy or work = Joule (Nm)

Energy has different forms: Light, heat, chemical, electrical or mechanical.

Mechanical energy is the sum of:
(i) Kinetic energy (K.E)
(ii) Potential energy (P.E)


The rate of doing work or the rate of transfer of energy is called power. It is denoted by P

P = Wt

SI unit is Watt (J/s).

Average power = Total energy consumed / Total time taken

The commercial unit of power is kWh i.e. energy used in 1 hour at 1000 Joules/second.
1KWh = 3.6×106 J