A free-body diagram (abbreviated as FBD, also called force diagram) is a diagram used to show the magnitude and direction of all applied forces, moments, and reaction and constraint forces acting on a body. They are important and necessary in solving complex problems in mechanics.

What is and is not included in a free-body diagram is important. Every free-body diagram should have the following:

- The body represented as a dot if it is a point mass, and the body itself if it is a rigid body.
- The external forces/moments. The force vector should indicate: relative magnitude, point of application, and the direction.
- A properly defined coordinate system

A free-body diagram should not include the following:

- Bodies other than the body we are interested in.
- Forces applied
*by*the body - Internal forces depending on the chosen system. For example, a free-body diagram on a truss should not include the forces between individual truss members.
- Kinematic quantities (velocity and acceleration).

Warning!

Always assume the direction of forces/moments to be positive according to the appropriate coordinate system. The calculations from Newton/Euler equations will provide you with the correct direction of those forces/moments. Things that should not follow this are:

- Gravity
- Tension
- Friction if the velocity \( \vec{v} \) is provided

Warning!

If forces/moments are present, always begin with a free-body diagram. Do not write down equations before drawing the FBD as those are often simple kinematic equations, or Newton/Euler equations.