1.5.5 Turning Effect

The Moment of a Force

• A moment is the turning effect of a force
• Moments occur when forces cause objects to rotate about some pivot
• The size of the moment depends upon:
• The size of the force
• The distance between the force and the pivot
• The moment of a force is given by the equation:
`Moment = Force × perpendicular distance from the pivot`
• Moments have the units newton centimetres (N cm) or newton metres (N m), depending on whether the distance is measured in metres or centimetres

Diagram showing the moment of a force causing a block to topple

• Some other examples involving moments include:
• Using a crowbar to prize open something
• Turning a tap on or off
• Opening or closing a door

The Principle of Moments

• The principle of moments states that:
• For a system to be balanced, the sum of clockwise moments must be equal to the sum of anticlockwise moments

Diagram showing the moments acting on a balanced beam

• In the above diagram:
• Force F2 is supplying a clockwise moment;
• Forces F1 and F3 are supplying anticlockwise moments
• Hence:

F2 x d2 = F1 x d1 + F3 x d3

Extended Only

Example of The Principle of Moments

• The principle of moments doesn’t just apply to seesaws – it is important in many other situations as well such as, for example, a shelf:

To prevent the shelf from collapsing, the support must provide an upward moment equal to the downward moment of the vase

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.