Action and reaction forces
Sir Isaac Newton pointed out that, whenever a force acts on a body~here must be an equal and opposite force or reaction acting on some other body. This is called the third law of motion and may be expressed in the simple form: “To every action there an equal and opposite reaction.”
To take an example, a book presses on a table with a force equal to its weight. The table exerts an equal upward reaction force on the book. We shall meet similar examples in the chapters which follow.
Before leaving the subject, we must warn the reader not to confuse action and reaction forces with the forces which are keeping the book in equilibrium, or at rest, with respect to the table. The book is at rest under the force of gravity on it which is balanced by the upward reaction on it from the table. Both forces are acting on a single body, i.e., the book and hence cannot be called action and reaction forces (Fig. 2.2). For clarity, and to show their points of application, the forces in Fig. 2.2 have been displaced parallel to one another. They do, of course, act in the same straight line.