Engineering & design · University
Statics, free-body diagrams and design margins · revision notes
A free-body diagram isolates a body and displays external forces and moments. Internal forces cancel only for the correctly chosen whole system.
Key ideas and conditions
Forces
A free-body diagram isolates a body and displays external forces and moments. Internal forces cancel only for the correctly chosen whole system.
Equilibrium
In planar statics, net force in two independent directions and net moment vanish. A balanced force sum alone may leave an unbalanced couple.
Reactions
Support models define which motions are constrained and which reaction components can occur. Do not invent reactions inconsistent with the support.
Engineering limits
Classroom equilibrium calculations do not certify a real structure. Materials, dynamic loads, codes and qualified review are additional requirements.
Explain without looking
- Why can a support model change the solution of a statics problem?
- Two equal opposite 3 N forces separated by perpendicular distance 4 m form a couple. Find its moment magnitude.
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