AQA 7408 A Level Physics · Mechanics and Materials
Vectors, Moments and Equilibrium
Resolve forces consistently, choose a useful pivot and apply both force and moment conditions for equilibrium.
Vectors and components
Choose axes before resolving. A force at angle θ to the horizontal has components F cos θ horizontally and F sin θ vertically. Draw the triangle so the trigonometric choice follows from the geometry rather than a memorised rule.
Moments and couples
Moment = force × perpendicular distance from the pivot to the force line of action. A couple consists of equal, opposite, parallel forces and produces a moment equal to one force multiplied by their perpendicular separation.
Equilibrium strategy
For static equilibrium, the resultant force is zero and the resultant moment about any point is zero. Take moments about a point that removes the largest number of unknown forces, then use horizontal and vertical force balance.