Quantities and units in mechanics (A2 Unit 4: Applied Mathematics B) — WJEC A-Level Mathematics
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Quantities and units in mechanics (A2 Unit 4: Applied Mathematics B) explained
The moment of a force measures its turning effect about a chosen point or axis of rotation.
Read the full explanation
The moment is defined quantitatively as the product of the force magnitude F and the perpendicular distance d from the pivot to the line of action of the force: Moment = F × d. The derived SI unit for a moment is the newton metre (N m). Moments possess rotational sense: clockwise or anticlockwise. Candidates must distinguish moments (measured in N m, vector torque) from energy or work done (measured in joules, J = N m, scalar), and ensure that distances are consistently converted to metres before computing moments.
Your focus
- Define the moment of a force as the product of force and perpendicular distance to the pivot.
- Apply the derived SI unit of newton metres (N m) and assign clockwise or anticlockwise sense.
- Determine perpendicular distances from pivot points to inclined force lines of action.
Quantities and units in mechanics (A2 Unit 4: Applied Mathematics B) exam tips
Marking Points
- stating the definition of a moment as force multiplied by perpendicular distance: Moment = F × d
- stating the correct derived SI unit for a moment as newton metres (N m)
- assigning correct rotational sense (clockwise or anticlockwise) to calculated moments
Examiner Tips
- 💡Always check that distance is perpendicular to the line of action of the force: use d sin θ if inclined.
- 💡State units clearly as N m (with a space) and specify rotational direction: clockwise or anticlockwise.
Common Mistakes
- using the direct distance between the pivot and the point of force application instead of the perpendicular distance
- writing the unit of moment as joules (J) rather than newton metres (N m)