Turning Effects of Forces (Cambridge (CIE) A Level Physics)

Which pair of forces act as a couple on the circular object?

Answer: A

• In diagram A, the forces are:

  • Equal in size

  • In opposite directions

  • Perpendicular to the distance between them

• B is incorrect as the forces are in the same direction

• C is incorrect as the forces are different in size

• D is incorrect as the distance between the forces is not perpendicular

The forces that make up a couple cannot share the same line of action. The line of action is a line through the point at which the force is applied. 

A steering wheel has a diameter of 40 cm, and the force of the couple needed to turn it is 10 N.

Calculate the torque on the steering wheel.

Answer:

Step 1: List the known quantities converting to SI units

• Force, F = 10 N

• Perpendicular distance between forces, d = 0.4 m

Step 2: Sketch a diagram of the scenario

Step 3: State the torque equation

Step 4: Substitute in the known values to calculate

τ = 4 N m

A ruler of length 0.3 m is pivoted at its centre. Equal and opposite forces of magnitude 4.0 N are applied to the ends of the ruler, creating a couple.

Determine the magnitude of the torque of the couple on the ruler at the position shown in the diagram.

Answer:

Step 1: List the known quantities

• Forces applied = 4 N at 30°

• Perpendicular distance between forces, d = 0.3 m

Step 2: State the equation for torque

Step 3: Determine the component of the force that is perpendicular to the distance between the forces

or

Step 4: Substitute in the known values to calculate

The forces given in a question might not always be perpendicular to the distance between them. In this case, remember to find the component of the force vector that is perpendicular.

You can recap how to do this in Scalars & Vectors.