Static equilibrium and torque balance
Zero net force alone does not prevent rotation.
Key knowledge
A rigid body initially at rest in an inertial frame remains in static equilibrium when both net external force and net external torque vanish. Torque must include lever arms and applied couples. Once net force is zero, changing the torque origin does not change the net torque; a pair of opposite forces at different locations can still create a couple.
Values and equations
ΣF_external=0; Στ_external,O=0
- F_external
- External forces N
- τ_external,O
- External torques about the stated origin, including couples N m
- O
- Torque origin
Conditions and scope
- Rigid-body, classical, fixed-mass model, with all external forces and couples included.
- Equilibrium does not establish stability. A force-balanced body initially rotating is not thereby at rest; constraints and perturbations require separate analysis.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read §12.1 both equilibrium conditions12.2/12.5 and extended-body force/torque treatment. Origin independence at zero net force and couple interpretation are disclosed consequences of torque r×F.
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Entry details
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- foundation-physics-static-equilibrium
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- foundations-refresh.1
- Structured reference record
- Foundational reference additions