The Newton's third law of motion states that for every action, there is an equal and opposite reaction. The force exerted by the second body on the first body is called as the reaction. The action and reaction are equal and opposite. Given here is an online Newton's third law calculator tool for the calculation of Newton's 3rd law of motion of an object. The Newtons third law of motion calculator states that all forces between two objects exist in equal magnitude and opposite direction.