How to calculate moments

This video can not be played

To play this video you need to enable JavaScript in your browser.

A force or system of forces may cause an object to turn. A is the turning effect of a force. Moments act about a point in a clockwise or anticlockwise direction. The point chosen could be any point on the object, but the – also known as the fulcrum – is usually chosen.

A plank balances on a pivot. There are identical boxes, or objects at each end of the plankm, with two arrows point downwards from the boxes. Curved arrows at the ends indicate possible rotation. — Figure caption,
The anticlockwise moment acts downward on the left, and the clockwise moment acts downwards on the right

The magnitude of a moment can be calculated using the equation:

moment of a force = force × distance

\(M = F \ d\)

This is when:

moment (M) is measured in newton-metres (Nm)
force (F) is measured in newtons (N)
distance (d) is measured in metres (m)

A plank balances on a central pivot, with boxes at each end. Lines cross through the middle of the boxes and pivot, with arrows showing the distance between them. — Figure caption,
The normal (perpendicular) distance is the shortest distance between the pivot and the line of action of the force

Example

A force of 15 N is applied to a door handle, 12 cm from the pivot. Calculate the moment of the force.

A simple image of a door with handle shows a curved arrow indicating that handle is pushed down on, or turned.

First convert centimetres into metres:

12 cm = 12 ÷ 100 = 0.12 m

Then calculate using the values given in the question:

\(M = F \ d\)

\(M = 15 × 0.12\)

\(M = 1.8 \ Nm\)

Question

A force of 40 N is applied to a spanner to turn a nut. The perpendicular distance is 30 cm. Calculate the moment of the force.

��ý

Effects of forces - EdexcelHow to calculate moments