Turning effect

A force may cause an object to turn about a pivot.

The turning effect of a force is called the moment of the force.

Moments act about a pivot in a clockwise or anticlockwise direction.

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.The anticlockwise moment acts downward on the left, and the clockwise moment acts downwards on the right

Calculating the moment of a force

The size of the moment of a force can be calculated using the equation:

moment of a force = force F x perpendicular distance from the pivot d

moment = F x d

  • force F is measured in newtons (N)
  • distance d is measured in metres (m)
  • moment is measured in newton metres (Nm)
Moment of a force = force F x perpendicular distance from the pivot d

Perpendicular distance from pivot to force d = 0.50 m.

Force F = 10 N

Moment = Fd

Moment = 10 N x 0.50 m

Moment = 5 Nm

This is a clockwise moment.

The force will rotate the object in a clockwise direction about the pivot.

curriculum-key-fact
It is important to remember that the distance d is the perpendicular distance from the pivot to the line of action of the force (see diagram).
Question

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

Calculate the moment of the force.

Perpendicular distance from pivot to force d = 12 cm = 0.12 m.

Turning force F = 15 N.

Moment = Fd.

Moment = 15 N x 0.12 m.

Moment = 1.8 Nm.

The moment of the force is 1.8 Nm.

Question

Question
A spanner is turning nut. The curved arrow at bottom of spanner indicates direction of turn.

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.

Perpendicular distance from pivot to force d = 30 cm = 0.30 m.

Turning force F = 40 N.

Moment = Fd.

Moment = 40 N x 0.30 m.

Moment = 12 Nm.

The moment is 12 Nm. This is a clockwise moment.