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CAIE A-Level Physics 4.2.1 Principle of Moments

Practise applying the principle of moments to beams, rods and supported loads, choosing pivots, solving unknowns and checking balance and units.

Syllabus
2028–2030
Course
Physics 9702
Level
AS

Exam points

  • State and apply the principle of moments for a body or system in equilibrium.
  • Choose a convenient pivot and set clockwise and anticlockwise moments equal with correct lever arms.
  • Solve for an unknown force, distance or angle and check the balance condition and units.

4.2.1—The principle of moments question 1

[Maximum number: 2]

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.

Friction between the disc and the pin is negligible.

Determine the angle θ\theta.
θ=∘\theta= { }^{\circ}

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