Turning Effects of Forces (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

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

$\tau = 4 \mathrm{N} \mathrm{m}$

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