Mechanics · Moments
Chapter 1 · 3
The idea
Equilibrium of rigid bodies
A rigid body in equilibrium needs both zero resultant force and zero resultant moment — using these two conditions to find the reaction forces on a supported beam, and taking moments about a support to eliminate an unknown.
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Mechanics · Moments
Equilibrium of rigid bodies
A rigid body in equilibrium needs both zero resultant force and zero resultant moment — using these two conditions to find the reaction forces on a supported beam, and taking moments about a support to eliminate an unknown.
Why it works
Two conditions, not one
For a particle, equilibrium just means the forces balance: . But a rigid body — a beam, a plank, a see-saw — can also rotate, so balancing the forces isn't enough. A body in equilibrium must satisfy two conditions:- No resultant force: the forces balance in each direction (), so it doesn't move off.
- No resultant moment: the turning effects balance ( about any point), so it doesn't spin.
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