9702 · 4.2
Equilibrium of Forces — FAQ
Frequently asked questions for 9702 Equilibrium of Forces. Direct answers first, then deeper explanation — then practise with marking.
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.