A free-body diagram shows all the forces acting upon a object. Use a box to represent the object and arrows to show the direction of the various forces. The relative size of the arrows can be used to indicate the magnitude of each force.
Determine all the forces acting upon an object
In a free-body diagram, gravity is shown as a downward arrow extending from the bottom edge of the box. If the object is sitting on a surface, the force of the surface pressing upward is called the normal force. The normal force is shown as an arrow pointing upward from the top edge of the box. If the object isn't moving, these two forces are usually the only ones shown.
Diagram the force of friction
If an object is in motion, friction usually becomes a factor. Friction is the force exerted by a surface as an object moves across it. Friction typically opposes forward motion, so it is shown as an arrow pointing in the opposite direction of motion. Air resistance is also a form of friction, but its effect is often negligible unless an object is in free fall or has a high surface area-to-mass ratio. Air resistance is usually shown as an arrow pointing upward from the top of a falling object.
Diagram any applied forces
Applied forces come in many forms. A person or a machine pushing an object in a particular direction is an example of an applied force. An applied force is usually shown by an arrow pointing in the direction of motion. Two opposing applied forces are shown as arrows on either side of the object pointing in opposite directions.
Diagram the force of tension
Tension is the force transmitted to an object through the pull of a string, a rope or a wire attached to it and pulled taut. If an object is hanging from a string, the force of tension is shown as an arrow pointing upward.