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CAIE A-Level Physics 4.1.4 Torque of a Couple

Practise defining and calculating couple torque from force and perpendicular separation, applying it to rotational equilibrium with signs, units and physical meaning.

Syllabus
2028–2030
Course
Physics 9702
Level
AS

Exam points

  • Define and calculate the torque of a couple from force and perpendicular separation.
  • Apply couple torque to rotational equilibrium or turning problems with direction and units.
  • Check the force spacing, sign convention and physical interpretation of the torque.

4.1.4—The torque of a couple question 1

[Maximum number: 1]

A thin disc of radius r is supported at its centre O by a pin. The disc is supported so that it is vertical. Three forces act in the plane of the disc, as shown in Fig.2.1.

Fig. 2.1

Fig. 2.1

Two horizontal and opposite forces, each of magnitude 1.2 N , act at points A and B on the edge of the disc. A force of 6.0 N , at an angle θ\theta below the horizontal, acts on the midpoint C of a radial line of the disc, as shown in Fig. 2.1. The disc has negligible weight and is in equilibrium.

State an expression, in terms of r, for the torque of the couple due to the forces at A and B acting on the disc.

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