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CAIE A-Level Physics 4.1.2 Moment of a Force

Practise defining and calculating moments as force times perpendicular distance, using signs, units and a suitable pivot in equilibrium problems.

Syllabus
2028–2030
Course
Physics 9702
Level
AS

Exam points

  • Define the moment of a force about a point as force multiplied by perpendicular distance.
  • Calculate moments with the correct lever arm, direction/sign and units.
  • Apply moments to equilibrium, turning or support problems and check the chosen pivot.

4.1.2—The moment of a force question 1

[Maximum number: 2]

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.

For the sphere at position Y , calculate the moment of its weight about point P .

moment =

Nm

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