3. Third Law of Motion
According to this law, 'Every action has its equal and opposite reaction". When two bodies A and B exert force on each other, the force (action) of A on B (), is always equal and opposite to the force of B on A ()
Thus = –

(i) This law expresses the nature of force.
(ii) Action and reaction always acts on different bodies
= –
Impulse :
If a force acts on a body for a short duration &Detla;t, then impulse is defined as product of force and its time of action
i.e. Impulse = Force × Duration
⇒ Δ = ×Δt
By Newton's second law
=
Δt = Δ = –
When and are initial and final momenta of the body respectively, Thus impulse of force = Δt = Δ = –