Free-body diagrams and system boundaries
List forces on the selected body once.
Key knowledge
A free-body diagram represents all external forces acting on the selected body or system. Forces the system exerts on its surroundings belong on another system’s diagram. A force and its components are alternative representations, and the net force is their sum rather than an extra force. For an extended body, force locations are also needed to determine torque.
Conditions and scope
- Choose the body/system and reference frame before listing forces.
- Third-law partners act on different systems; forces internal to a combined system are not external forces in that combined diagram.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Samuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)2 sources
- University Physics Volume 1, §5.7: drawing free body diagramsSamuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)Textbook chapter
- University Physics Volume 1, §12.1: conditions for static equilibriumSamuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)Textbook chapter
Evidence and review
Read §5.7 Constructing Free-Body Diagrams rules and §12.1 force/torque equilibrium. Torque application points are supported by the extended-body statics discussion; no new third-law cancellation on a single body.
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