First Law of Motion
A body continue to be in its state of rest or of uniform motion along a straight line, unless it is acted upon
by some external force to change the state.
(1) If no net force acts on a body, then the velocity of the body cannot change i.e. the body cannot
(2) Newton’s first law defines inertia and is rightly called the law of inertia. Inertia are of three types :
Inertia of rest, Inertia of motion, Inertia of direction
(3) Inertia of rest : It is the inability of a body to change by itself, its state of rest. This means a body at
rest remains at rest and cannot start moving by its own.
Second Law of Motion
(1) The rate of change of linear momentum of a body is directly proportional to the external force applied
on the body and this change takes place always in the direction of the applied force.
(2) If a body of mass m, moves with velocity v then its linear momentum can be given by p= mv and if force is applied on a body, then Force = mass ? acceleration
Third Law of Motion
To every action, there is always an equal (in magnitude) and opposite (in direction) reaction.
(1) When a body exerts a force on any other body, the second body also exerts an equal and opposite force
on the first.
(2) Forces in nature always occurs in pairs. A single isolated force is not possible.
(3) Any agent, applying a force also experiences a force of equal magnitude but in opposite direction. The
force applied by the agent is called ‘Action’ and the counter force experienced by it is called ‘Reaction’.
(4) Action and reaction never act on the same body. If it were so the total force on a body would have
always been zero i.e. the body will always remain in equilibrium.
(5) If F (AB)= force exerted on body A by body B (Action) and
F(BA)= force exerted on body B by body A (Reaction) Then according to Newton’s third law of motion F (AB) = F(BA)
(6) Example : (i) A book lying on a table exerts a force on the table which is equal to the weight of the
book. This is the force of action.