Moments (Cambridge (CIE) O Level Physics): Revision Note

Moments

• As well as causing objects to speed up, slow down, change direction and deform, forces can also cause objects to rotate

• 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

Pivot Point on a Metal Bar

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

• A moment is defined as:

  The turning effect of a force about a pivot

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

M = F × d

• Where:

  • M = moment in newton metres (Nm)

  • F = force in newtons (N)

  • d = perpendicular distance of the force to the pivot in metres (m)

Moment of a Spanner

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

• This is why, for example, the door handle is placed on the opposite side to 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 would be much harder, and would require a lot more force

• Some other examples involving moments include:

  • Using a crowbar to prize open something

  • Turning a tap on or off

  • A wheelbarrow

  • Scissors