Free body diagrams are used to gain an understanding of the forces (or sometimes the fields) acting on an object. This is a good first step when solving problems involving two or more forces, or forces that are not in line with the motion of an object.

To draw a free body diagram show the relative magnitude and direction of all forces acting on a body:

1. Draw a dot to represent the center of gravity of the object.
2. Draw and label arrows extending outward from the dot to represent the forces acting on the object. Drawing the arrows at least approximately to scale and correct angles helps.
3. Draw a reference frame (x and y axes) with the motion along one of the axes.
4. Dotted lines are drawn along x and y directions to represent the orthogonal components of the forces if necessary.

The relative lengths of the arrows in the diagram imply the direction of net force (and therefore, acceleration). If the acceleration of the object is zero, all forces cancel; if the object is accelerating, the net force is in the direction of the acceleration.

Commonly used forces in free body diagrams are weight (F_g), friction including air resistance (F_f), normal force supplied by a surface (F_N), and applied force (F_a).

Components of forces are often labeled (F_x) or (F_y).