Question

‘The action and reaction both act simultaneously.’ Is this statement true and what is the net force?
A) Yes, zero
B) No, zero
C) Yes, positive
D) No, negative

Answer 1

Hint: The third of Newton’s laws of motion of classical mechanics describes that all forces occur in pairs such that if one object exerts a force on another object, an equal and opposite force is exerted on the first object by the second object.

Complete step by step solution:
We all know the basic condition of equilibrium of a body. When the net force acting on the body is zero, the body is said to be in equilibrium. So how do we check if the net force is zero? If two equal forces are acting on a body such that their direction is opposite to each other, they are said to cancel each other out and the net force is said to be zero.
After reading this definition and the statement of the newton’s third law of motion, your first thought would be that since action and reaction have the same magnitude and are acting in opposite directions, their net or the resultant force would be zero.
But there is a slight catch here so wait a second. The definition of equilibrium mentions that the two equal and opposite forces need to be applied on the same body for their resultant force to be zero and Newton’s law describes that while action acts on the second body, the reaction acts on the first body. As such, even though the action-reaction pairs are equal and opposite and act simultaneously, their resultant is not zero.
Hence option (C) is the correct answer for the given question.
In conclusion, we can say that the net force on bodies depends on the system of reference. If we consider only one of the bodies as our system, the net force on the body will not be zero but positive in magnitude. In contrast, if we consider both the bodies as our system, then the net force will be zero.

Note: We can also make the correct choice of options in this question using mere common sense. We know that if equal and opposite forces act on a body, the body is in equilibrium and doesn’t change its state. We also know that action-reaction pairs act simultaneously. As such, no object should ever change its state of equilibrium but we witness it every day. Hence we can conclude that the net force acting on a body must be positive for a state change to be possible, even though action-reaction pairs are always present.