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Mechanics and Materials

Moments

  • 4 things to know
  • Papers 1 and 3

Overview

The moment of a force, couples, the principle of moments, and the centre of mass.

What you need to know

Answer each one before you reveal it.

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Key term

What does a moment depend on, and why must the forces of a couple act on different lines?

Key term

What motion does a couple produce, and what is the condition for rotational equilibrium?

Key term

What is the centre of mass, and where does it lie in a uniform regular solid?

Key term

How is the moment of a couple defined?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    How is the moment of a force about a point defined?

    Model answer

    Multiply the force by the perpendicular distance from the point to the force's line of action.

  2. 2

    State the formula for calculating the moment of a force.

    Model answer

    M = F × d, where M is the moment, F is the force, and d is the perpendicular distance from the pivot to the line of action of the force.

  3. 3

    Calculate the moment when force = 73 N, perpendicular distance = 4.6 m.

    Model answer

    Moment = 73 × 4.6 = 336 N m.

  4. 4

    When calculating a moment, from where to where is the perpendicular distance measured?

    Model answer

    From the pivot point to the line of action of the force, not to the point of application of the force.

  5. 5

    Why must the distance used in moment calculations be measured to the line of action rather than to where the force is applied?

    Model answer

    Because the moment depends on how far the force's line of action is from the pivot; the actual point of application along that line does not affect the turning effect.

  6. 6

    What is a couple?

    Model answer

    A couple consists of a pair of equal and opposite coplanar forces that do not act along the same line.

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