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Work and Energy: Key Points

(1)     W = F × s


W = Work done on an object

F = Force applied

s = Displacement of the object


(2)     Unit of work:

Newton-metre (Nm) or Joule (J).


(3)     Work done, W could be:

(i) W = 0 (Zero, as the direction of force and displacement are perpendicular to each other or displacement is zero)

(ii) W = F × s (Positive, as force and displacement are in the same direction)

(iii) W = - F × s (Negative, as force and displacement are in the opposite direction)



(4)     Work done by gravity - negative.

Work done against gravity - positive.


(5)      Oblique Displacement:

W = F × s cos θ

Where, θ = angle between the force and displacement


(6)      More energy, more work:

Energy is ability to do work.


(7)     Unit of Energy:  

Joule (J) – SI unit

Kilojoule (kJ)

1 kJ = 1000 J


(8)     Types of Energies:

(a)       Kinetic Energy

(b)       Potential Energy

(c)        Chemical Energy

(d)       Sound Energy

(e)       Light Energy

(f)        Heat Energy

(g)       Electrical Energy


(9)     Kinetic energy:

K.E. = {1 \over 2}m{v^2}


M = mass of the object

v = velocity of the object


(10)   Work – Energy Theorem

Work = Final K.E. – Initial K.E.

Work = - Ve (stopping a moving object)


(11)   Potential Energy:

P.E. = mgh


m = mass of the object

g = acceleration due to gravity

h = height of the object from ground


(12)   Types of Potential Energy:

(a)       Gravitational Energy

(b)       Elastic Potential Energy


(13)   Mechanical Energy, (M.E.) = K.E. + P.E.

{1 \over 2}m{v^2} + mgh


(14)   Energy transformation:

(a)       Fan Electrical energy →  Kinetic energy

(b)       Bulb Electrical energy → Heat energy + Light Energy

(c)       Moving bicycle Chemical energy → Muscular Energy → Kinetic energy

(d)       Photosynthesis Light Energy → Chemical energy

(e)       Rain Potential energy → Kinetic energy

(f)        Striking of clouds Kinetic energy →  Light + Sound


(15)   Law of Conservation of Energy:

Energy can neither be created nor be destroyed, but can be converted from one form to another.


(16)   Power,

P = {{Work\left( W \right)} \over {time\left( t \right)}} or {{Energy\left( E \right)} \over {time\left( t \right)}}


(17)   Unit of power = J s-1 or W or kW

1 k W = 1000 W


(18)   Commercial unit of Energy:

1 unit = 1 kWh

1 kWh = 3.6 × 106 J


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