Moments (CIE IGCSE Physics)

• The moment of a force is the turning effect produced when a force is exerted on an object

• Examples of the turning effect of a force are:
  • A child on a see-saw
  • Turning the handle of a spanner
  • A door opening and closing
  • Using a crane to move building supplies
  • Using a screwdriver to open a tin of paint
  • Turning a tap on and off
  • Picking up a wheelbarrow
  • Using scissors

• Forces can cause the rotation of an object about a fixed pivot
• This rotation can be clockwise or anticlockwise

Clockwise and anti-clockwise rotation

Consider the hands of a clock when deciding if an object will rotate in a clockwise or anti-clockwise direction

• A force applied on one side of the pivot will cause the object to rotate

Turning effect of a force about a pivot

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

The moment equation

• A moment is defined as:

The turning effect of a force about a pivot

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

• The forces should be perpendicular to the distance from the pivot
  • For example, on a horizontal beam, the forces which will cause a moment are those directed upwards or downwards

The turning effect of a force exerted on a spanner

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

• Increasing the distance at which a force is applied from a pivot decreases the force required
  
  • If you try to push open a door right next to the hinge, it is very difficult, as it requires a lot of force
  • If you push the door open at the side furthest from the hinge, then it is much easier, as less force is required

Forces required to open a door

A greater force is required to push open a door next to the hinges than at the door handle

• The principle of moments states that:

If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot

• The principle of moments means that for a balanced object, the moments on both sides of the pivot are equal 

clockwise moment = anticlockwise moment

Principle of moments

Imagine holding the beam about the pivot and applying just one of the forces. If the beam moves clockwise then the force applied is clockwise. This is the principle of moments

Extended tier only

• In the example below, the forces and distances of the objects on the beam are different, but they are arranged in a way that balances the whole system

Using the principle of moments

The clockwise and anticlockwise moments acting on a beam are balanced

• In the above diagram:
  • Force  causes an anticlockwise moment of about the pivot 
  • Force  causes a clockwise moment of about the pivot 
  • Force  causes an anticlockwise moment of  about the pivot

• Collecting the clockwise and anticlockwise moments:
  • Sum of the clockwise moments = 
  • Sum of the anticlockwise moments = 

• Using the principle of moments, the beam is balanced when:

sum of the clockwise moments = sum of the anticlockwise moments