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CAIE A-Level Physics 4.2.2 Conditions for Equilibrium

Practise testing translational and rotational equilibrium by combining force balance, torque balance, force diagrams, directions, units and verification.

Syllabus
2028–2030
Course
Physics 9702
Level
AS

Exam points

  • Recognise equilibrium when both resultant force and resultant torque are zero.
  • Use force balance and moment/torque balance together to solve a statics problem.
  • Explain the physical meaning of the equilibrium conditions and verify the result with directions and units.

4.2.2—That, when there is no resultant force and no resultant torque, a system is question 1

[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 .

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