# 5.1.2 Turning Effects of Forces

### What is a Moment?

• A moment is the turning effect of a force
• Moments occur when forces cause objects to rotate about some pivot
• The moment of a force is given by

Moment (N m) = Force (N) × perpendicular distance from the pivot (m)

• The SI unit for the moment is Newton metres (N m). This may also be Newton centimetres (N cm) depending on the units given for the distance
• An example of moments in everyday life is opening a door. The door handle is placed on the other side of the door to the hinge (the pivot) to maximise the distance for a given force and therefore a greater moment (turning force). This makes it easier to push or pull it

#### Exam Tip

If not already given, drawing all the forces on an object in the diagram will help you see which ones are perpendicular to the distance from the pivot. Not all the forces will provide a turning effect and it is not unusual for a question to provide more forces than required

### Couples

• A couple is a pair of forces that acts to produce rotation only
• Unlike moments of a single force, the moment of a couple doesn’t depend on a pivot, only on the perpendicular distance between the two forces
• A couple consists of a pair of forces that are:
• Equal in magnitude
• Opposite in direction
• Perpendicular to the distance between them Diagram of a couple

• Couples produce a resultant force of zero, so, due to Newton’s Second law (F = ma), the object does not accelerate
• The size of this turning effect is given by its torque

#### Exam Tip

The forces that make up a couple cannot share the same line of action which is the line through the point at which the force is applied. An example of this is shown in the diagram below ### Torque

• The moment of a couple is known as a torque
• You can calculate the torque of a couple with the following equation

Torque τ (N m) = one of the forces (N) × perpendicular distance between the forces (m)

#### Exam Tip

The forces given might not always be perpendicular to the distance between them. In this case, remember to find the component of the force vector that is perpendicular. You can learn more on how to do this in the ‘Resolving Vectors’ section of ‘Scalars & Vectors’ ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
Close Close