Moments: why door handles live far from the hinge

By the end of this lesson

By the end you can define the moment of a force, calculate it with its unit, and say whether it turns things clockwise or anticlockwise.

Same push, different result

Push a door right next to its hinges and it barely budges. Push just as hard at the handle and it swings open.

Same force, very different turning effect. Physicists call that turning effect the moment of the force.

Two things set its size:

how big the force is;
how far the force's line of action is from the pivot.

Double either one and the moment doubles.

Definition

Moment of a force

The moment of a force about a point is the product of the force and the perpendicular distance from the point to the line of action of the force.

$\text{moment} = F \times d$

Unit: N m (newton metre).

What's a line of action?

The line of action of a force is the straight line along the force, extended as far as you like in both directions.

The distance $d$ is measured from the pivot to that line, at right angles to it. When the force is at right angles to a lever, $d$ is simply the distance along the lever from the pivot to where the force acts.

Worked example: Easy one first: a spanner

You push with $40\ \text{N}$ at right angles to a spanner, $0.25\ \text{m}$ from the centre of the nut. What is the moment about the nut?

Have a go first?

Quick check

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