Exam tip 1
Paper 2 equilibrium questions reward a clear diagram first: mark distances, forces, and pivot. Then write ΣF = 0 and ΣM = 0 separately — do not mix them in one line.
9702 · 4.2
Equilibrium of Forces — common mistakes
Common exam mistakes on 9702 Equilibrium of Forces. Learn what loses marks, then practise the topic with Examiner’s Ink.
Why must I use perpendicular distance for moments?
Only the component of force perpendicular to the position vector from the pivot produces rotation. Force directed through the pivot line has zero moment.
Can an object have zero resultant force but still rotate?
Yes — a couple has zero resultant force but non-zero torque, so it spins without translating.
When is the triangle of forces method valid?
For three coplanar forces on a rigid body in translational equilibrium. It is a graphical check that the vector sum is zero.