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:
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 (Fg), friction including air resistance (Ff), normal force supplied by a surface (FN), and applied force (Fa).
Components of forces are often labeled (Fx) or (Fy).