Answer:
C. always equal to zero
Explanation:
According to newton's third law of motion, every action is always associated with equal and opposite reactions.
If object A applies a force to object B, then object B imparts a force equal to that of A towards it. So, the direction of force is opposite to each other.
The action force is called 'Fa' and the reaction force is '-Fr'
Due to the reversal of direction, the magnitude of the reaction force has a negative sign.
Fa = - Fr
∴ Fa + (-Fr) = 0
Hence, sum of these two forces is equal to zero.
