The Turning Effect

When forces are used to open doors, steer and pedal bicycles or turn a tap they are causing turning. The effect that a force has in turning an object round depends on:

• the size of the force
• the perpendicular (shortest) distance between the force line and the pivot (the axis of rotation).

Both of these factors are taken into account when measuring the turning effect, or moment, of a force.

Moment of a force = force × perpendicular distance from line of action of force to pivot.
The moment of a force, also known as torque, is measured in N m.

The ‘line of action’ of a force is the line drawn along the direction in which the force acts.

The diagram shows how the same force used to open a door can produce very different effects, according to the direction in which it is applied.

Image

The moment of a force is a vector that can only have one of two directions; either clockwise or
anticlockwise.

When turning on a tap or steering a bicycle, two forces are normally used. The forces act in opposite directions, but they each produce a moment in the same direction. A pair of forces acting like this is called a couple. The combined moment is equal to the sum of the moments of the individual forces.

If the forces that make the couple are equal in size, the moment of the couple = size of one force × shortest (perpendicular) distance between the force lines.