A moment is the turning effect of a force
Definition
Moment of a force
A measure of the turning effect of a force about a pivot, the fixed point about which an object can turn.
moment=F×d \text{moment} = F \times d moment=F×dwhere FFF is the force in N\text{N}N and ddd is the perpendicular distance from the pivot to the line of the force in m\text{m}m, giving the moment in newton metres (N m\text{N m}N m).
- Everyday turning effects show the idea: a door opens most easily when you push far from the hinge, and a long spanner undoes a stiff nut more easily than a short one.
- The moment gets bigger if you use a larger force or apply it at a greater perpendicular distance from the pivot.
- The distance ddd is measured at right angles to the line of the force, not along the arm if the force is at an angle.
- A moment is clockwise or anticlockwise, so you always state which way it would turn the object.
Common Mistake
- Use the perpendicular distance from the pivot to the line of the force, not the length along a tilted arm.
- The unit of a moment is the newton metre (N m\text{N m}N m), which is not the same as the newton per metre used for a spring constant.
- Measure the distance from the pivot, so always identify the pivot before you calculate.
Exam technique
- Write moment=F×d\text{moment} = F \times dmoment=F×d, then check that the distance is in metres and perpendicular to the force before multiplying.
- State the direction (clockwise or anticlockwise) whenever a question asks for the turning effect, not just the size.
Self review
- What is meant by the moment of a force?
- Give the equation for a moment and the unit it is measured in.
- Which distance is used in the moment equation?
- A 30 N30\ \text{N}30 N force acts 0.40 m0.40\ \text{m}0.40 m from a pivot, at right angles to the arm. Find the moment.
- Why is a long spanner easier to use than a short one?
