# 5.4.1 Moments

### Forces & Rotation

• As well as causing objects to speed up, slow down, change direction and deform, forces can also cause objects to rotate
• A system of forces can also do this
• An example of a rotation caused by a force is on one side of a pivot (a fixed point that the object can rotate around)
• This rotation can be clockwise or anticlockwise

The force will cause the object to rotate clockwise about the pivot

• Further examples are:
• A child on a see-saw
• Turning the handle of a spanner
• A door opening and closing
• If two forces act on an object without passing through the same point, then the object can still rotate
• If the forces are equal and opposite, this is known as a couple

The above forces are balanced, but will still cause the object to rotate clockwise as they don’t act through a common point

### The Moment of a Force

• moment is defined as:

The turning effect of a force about a pivot

• The size of a moment is defined by the equation:

M = Fd

• Where:
• M = moment in newton metres (Nm)
• F = force in newtons (N)
• d = perpendicular distance of the force to the pivot in metres (m)

The moment depends on the force and perpendicular distance to the pivot

• This is why, for example, the door handle is far away from the hinge
• This means for a given force, the perpendicular distance from the pivot (the hinge) is larger
• This creates a larger moment (turning effect) to make it easier to open the door
• Opening a door with a handle close to the pivot will be much harder, and would require a lot more force

#### Exam Tip

The units of moment is Nm, but can also be Ncm (where the distance is measured in cm instead). If the exam question doesn’t ask for a specific unit, always convert the distance in metres

### 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.
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