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CAIE A-Level Physics 4.2 Equilibrium of Forces

Practise resolving forces, applying equilibrium and moments to determine unknown forces, angles or tensions in supported rigid bodies.

Syllabus
2028–2030
Course
Physics 9702
Level
AS

Exam points

  • resolve forces and use vector addition or a vector triangle to represent the resultant force
  • apply equilibrium conditions to determine unknown forces, angles or tensions in a system at rest
  • combine moments and force balance to analyse supported bodies, including non-uniform loads and contact forces

Covered syllabus points

4.2.1—The principle of moments4.2.2—That, when there is no resultant force and no resultant torque, a system is4.2.3—A vector triangle to represent coplanar forces in equilibrium

4.2 Equilibrium of forces question 1

[Maximum number: 3]

Question (a)

(a)

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.

[ 3 ]

Question (i)

(i)

Friction between the disc and the pin is negligible.

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

[ 2 ]

Question (ii)

(ii)

State the magnitude of the force of the pin on the disc.
force = N

[ 1 ]

4.2 Equilibrium of forces question 2

[Maximum number: 1]

A pendulum consists of a solid sphere suspended by a string from a fixed point P , as shown in Fig. 3.1.

Fig. 3.1 (not to scale)

Fig. 3.1 (not to scale)

The sphere swings from side to side. At one instant the sphere is at its lowest position X , where it has kinetic energy 0.86 J and momentum 0.72 Ns in a horizontal direction. A short time later the sphere is at position Y , where it is momentarily stationary at a maximum vertical height h above position X.

The string has a fixed length and negligible weight. Air resistance is also negligible.

State and explain whether the sphere is in equilibrium when it is stationary at position Y .

4.2 Equilibrium of forces question 3

[Maximum number: 1]

An object is held in equilibrium by three forces. The forces all act in the same plane. The diagram

shows two of the forces that act on the object.

The third force is missing from the diagram.

What is the third force?

A

Option A shown in diagram

B

Option B shown in diagram

C

Option C shown in diagram

D

Option D shown in diagram

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