A free-body diagram is used in physics to show the direction and relative magnitude of all forces acting upon an object. Typically, the object is represented by a box or a dot. Arrows are used to show the direction of each force acting on the object.
In the case of a book sitting on a desk, the force of gravity is shown as an arrow pointing down from the bottom edge of the box. The force of the desk pushing upward on the book, called the normal force, is shown by an arrow pointing up from the top edge of the box. For an object in motion, the applied force is shown as an arrow pointing in the direction of motion, and friction is shown as an arrow pointing in the opposite direction from the back edge of the box. Arrows of different lengths are used to indicate the relative magnitude of the various forces.