9.6P Describe situations where forces can cause rotation

The idea

This statement is about turning effects. A force can cause an object to rotate about a pivot or turning point when the force acts at a distance from that pivot and is not directed through it. The turning effect, called the moment, increases when the force is larger or when it acts further from the pivot. Typical situations include pushing a door near its handle rather than near the hinge, using a spanner on a nut, turning a steering wheel, or a see-saw pivoting about its centre. A force applied directly through the pivot produces no rotation. When choosing an answer, identify the pivot, check that the force line does not pass through it, and consider how force size and distance from the pivot change the turning effect.

What earns the marks

• Identifies the pivot or turning point about which the object rotates.
• States that a force causes rotation when its line of action does not pass through the pivot.
• Describes a named situation, such as opening a door or turning a spanner, in which a force produces rotation.
• Explains that a larger force or a greater distance from the pivot increases the turning effect.

Common mistakes and how to put them right

• Assumes any applied force causes rotation.

  Put it right: Recognise that a force acting through the pivot produces no turning effect.

• Confuses the distance from the pivot with the length of the object.

  Put it right: Measure the perpendicular distance from the pivot to the line of action of the force.

• Describes a situation without naming the pivot.

  Put it right: State the pivot and the force that turns the object.

• Claims a larger force always rotates an object faster regardless of position.

  Put it right: Link the turning effect to both force size and distance from the pivot.

Exam tips

• Locate the pivot first, then check whether the force line passes through it.
• Use everyday examples such as a door, spanner or steering wheel to test each option.
• Remember that increasing either the force or its distance from the pivot increases the turning effect.