Free Body Diagram

A free body diagram (FBD) is the essential first tool for analysing motion with Newton's laws. It is built with the following systematic steps:

1. Identify every force acting on the chosen object (never a force the object itself exerts on something else).
2. Represent the object as a single point.
3. Draw each identified force as a vector arrow from that point, with its correct direction and (where relevant) relative magnitude.

For a book resting on a table, exactly two forces act on the book: its weight $mg$ (downward, from Earth's gravity) and the table's normal contact force $N$ (upward); since the book is in equilibrium, $N=mg$, and the two arrows are drawn equal in length, opposite in direction. By Newton's third law, the book in turn exerts a downward force on the table (equal to $N$) and an upward gravitational pull on the Earth (equal to $mg$) — but neither of these belongs in the book's free body diagram, since they are forces exerted by the book, not on it. …