Moments
M = Fd. Moment of a force about a pivot = F × perpendicular distance.
Y13 Mechanics. For a rigid body in equilibrium: sum of forces = 0 AND sum of moments about any point = 0. Choose the pivot to eliminate unknowns.
Worked examples
3 m beam pivoted at centre; 5 kg at one end, 10 kg at other end 1.5 m from pivot.
Moments about A = sum of (force × distance from A).
Ladder against wall: friction at base prevents slipping.
Frequently asked questions
Anti-clockwise positive?
Convention. Pick a direction; stick with it.
Pick the pivot wisely?
Through an unknown force eliminates that force from the moment equation.