Key Equations
Moments, Couples & Equilibrium — AQA A-Level Physics
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Moment of a force
$$M = F \times d$$
- Where:
- $M$ = moment (N m)
- $F$ = force (N)
- $d$ = perpendicular distance from pivot (m)
d must be the perpendicular distance to the line of action.
Moment when force is at an angle
$$M = F d \cos \theta$$
- Where:
- $M$ = moment (N m)
- $F$ = applied force (N)
- $d$ = distance from pivot (m)
- $θ$ = angle between force direction and perpendicular
Takes the perpendicular component of the distance.
Moment of a couple
$$M = F \times d$$
- Where:
- $M$ = moment of couple (N m)
- $F$ = magnitude of one force (N)
- $d$ = perpendicular distance between lines of action (m)
The moment of a couple is independent of any pivot point.