A force or a system of forces may cause an object to rotate.

The idea

A force makes an object turn when its line of action does not pass through the pivot. Push a door at the handle and it swings; push it exactly on the line running through the hinges and it will not turn at all, however hard you push. The further from the pivot the force acts, and the closer to a right angle it is applied, the bigger the turning effect. Two forces can also produce rotation with no overall push: the two hands on a steering wheel, or a force at each end of a spanner, act in opposite directions on opposite sides of the pivot, so they turn the object while the resultant force stays zero. That is why a system of forces, and not only a single force, is named here.

What earns the marks

• The force acting at a distance from the pivot, so its line of action misses the pivot.
• Stating that a force whose line of action passes through the pivot causes no rotation.
• A correct everyday example of a force causing rotation.
• Recognising that two opposing forces either side of a pivot can turn an object with no resultant force.

Common mistakes and how to put them right

• Assuming any force on an object makes it rotate, whatever its line of action.

• Measuring the distance from the pivot along the force itself rather than perpendicular to its line of action.

• Saying a bigger force always gives a bigger turning effect without mentioning the distance.

• Treating rotation as impossible unless there is a resultant force.

Exam tips

• Find the pivot before anything else; the geometry of the whole question follows from it.
• Push at right angles and as far from the pivot as possible is the standard way to maximise a turning effect.