Rigid body — where it appears
Named by 3 essays across one field — each of them below, with the objects they name alongside it.
Everything adds to nothing, and that is the whole of statics
A structure that stays put obeys two statements — the forces on it sum to zero, and so do the moments. Every number in the subject comes out of those two sentences.
The equation that is not new, and the three that are
A plane free body yields exactly three independent equations. Most attempts at a fourth are one of the first three wearing different clothes — and on a beam under vertical load, one of the three is already saying nothing.
Moving a force, and what it costs
Every free body on this site begins by putting a force somewhere convenient. That move is free along the force's own line, costs a couple across it, and in three dimensions leaves behind something no choice of point can remove.
Named alongside it
The objects these essays reach for when they reach for this one.
Free body diagramMoment equilibriumForce equilibriumLine of actionMoment centreBolt groupDeterminacyEccentricityIdealisationIndependent equationsLever armMoment about an axis