Statement : During interaction between two whatever amount of force is applied by first object on second object, similar force is applied by second object on first object. Direction of these forces is in opposite direction.

OR

Action and reaction are force of equal magnitude but act in opposite direction. Here, any one force can be taking action force and another force as force of reaction. From this law it is clear that an isolated force do not exist. Forces are always in pair. But, these force act on different object and in mutually opposite direction.

Example : When a spring is compressed, it is compressed due to force applied by hand. Compressed spring provide restoring force on hand which can be experienced on the hand.

$(1)$ In this law action and reaction both are force which indicate that forces are always in pair. Force applied by B on A is equal to force applied by A on B but in opposite direction.

$(2)$ Isolated force do not exist because forces are always in pair.

$(3)$ Action and reaction force are equal in magnitude but act on different object. Thus, action is cause and reaction is effect.

$(4)$ Action and reaction are force which act on different object. Let A and B are object of given pair.

By Newton's third law of motion, $\overrightarrow{\mathrm{F}}_{\mathrm{AB}}=-\overrightarrow{\mathrm{F}}_{\mathrm{BA}}$

(Force applied by A on B) $=-$ (Force applied by $B$ on $A$)

When we consider motion of any one object ($A$ and $B$) then we have to consider force on one object only.

It is not correct to consider resultant force as zero.

When we consider system of two particle $\mathrm{A}$ and $\mathrm{B}$ then $\overrightarrow{\mathrm{F}}_{\mathrm{AB}}$ and $\overrightarrow{\mathrm{F}}_{\mathrm{BA}}$ are internal forces of system (A and B) resultant of internal forces is zero. Hence, we can use Newton's second law for system of particles also.