Drawing Free-body Diagrams: Rules

The first step in describing and analyzing most phenomena in physics involves the careful drawing of a free-body diagram. Free-body diagrams are useful in analyzing forces acting on an object or system, and are employed extensively in the study and application of Newton's laws of motion. The steps to draw a free-body diagram are listed below:

• • Draw the object under consideration. If treating the object as a particle, represent the object as a point. Place this point at the origin of a xy-coordinate system.
• • Include all forces that act on the object, representing these forces as vectors. However, do not include the net force on the object, or the forces that the object exerts on its environment.
• • Resolve all force vectors into x– and y-components.
• • Draw a separate free-body diagram for each object in the problem.

Once a free-body diagram is drawn, Newton's laws of motion can be applied to solve the problem. Newton's first law is applied if the body is in equilibrium, whereas Newton's second law is applied if the body is accelerating.

Note: If there is acceleration, it is not directly included in the free-body diagram; however, it may be helpful to indicate acceleration outside the free-body diagram. It may be labeled in a different color to indicate that it is separate from the free-body diagram.

This text is adapted from Openstax, University Physics Volume 1, Section 5.7 Drawing Free-Body Diagrams.