In the process of separating a body from its surroundings,we make use of a principle known as newton's third law,which states that action equals reaction. That is ,if a body a exert a force on body B, exerts a force on body a equal in magnitude,opposite in direction,and having the same line of action. Newton's third law cannot be proven mathematically. It agrees with intuition,and deduction from the third law agrees with experiment.