What is a Moment?
When you push open a door, tighten a bolt with a spanner, or use a seesaw, you are making something rotate. The turning effect of a force is called a moment.
The turning effect of a force about a fixed point (called the pivot). It is calculated as the product of the force and the perpendicular distance from the pivot to the line of action of the force.
The formula for a moment is:
Where:
- = moment (in Newton-metres, N m)
- = force applied (in Newtons, N)
- = perpendicular distance from the pivot to the line of action of the force (in metres, m)
The distance must be the perpendicular distance — that is, measured at a right angle from the pivot to the line along which the force acts. If the force is applied at an angle, you must use the perpendicular component of distance (or the perpendicular component of the force).
The unit of moment is the Newton-metre (N m). Note that this is the same unit as energy (Joules), but moments and energy are different physical quantities — don't confuse them!
The Pivot (Fulcrum)
Every rotation happens around a fixed point. This point is called the pivot or fulcrum.
The fixed point about which an object rotates or tends to rotate when a force is applied.
Examples of pivots in everyday life:
- The hinge of a door
- The axle of a wheel
- The balance point of a seesaw
- The joint of a pair of scissors
- The bolt at the end of a spanner
Think of the pivot like the centre of a merry-go-round. No matter how hard you push on the edge, the centre stays still — everything spins around it. The further from the centre you push, the easier it is to make it spin.
The position of the pivot is crucial: the further the force is from the pivot, the larger the moment it produces — even if the force itself stays the same. This is why door handles are placed at the far edge of the door, not near the hinge.
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