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Science · Topic

Forces, moments and motion

Moments balance: total clockwise equals total anticlockwise. Show the moment calculation.

2 original questions in the bank, worth 4 marks · above the two-per-topic floor

Drill this topic All forces, moments and motion questions

The questions, with the examiner reasoning

Calculate3 marksBGCSE

A uniform metre rule is pivoted at its 50 cm mark. A 2.0 N weight hangs at the 20 cm mark. A 3.0 N weight is hung on the other side to balance the rule. Calculate the position of the 3.0 N weight.

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Answer: At the 70 cm mark

  1. Moment of the 2.0 N weight: 2.0 N × 30 cm = 60 N cm
  2. Balance: 3.0 N × d = 60 N cm, so d = 20 cm from the pivot
  3. Position: 50 + 20 = 70 cm

Examiner technique: Never take moments about the end of the rule — take them about the pivot. The distance that matters is always the distance from the pivot.

Calculate1 markBoth boards

A car travels 150 m in 12 s. What is its average speed?

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Answer: 12.5 m/s

  1. Speed = distance ÷ time
  2. 150 ÷ 12 = 12.5
  3. Unit: m/s
  • A. 12.5 m/s — Correct: 150 ÷ 12 = 12.5, and the unit is metres per second.
  • B. 1800 m/s — The values were multiplied. Speed is distance divided by time.
  • C. 0.08 m/s — The division was inverted: 12 ÷ 150.
  • D. 25 m/s — A doubling slip in the division.

Examiner technique: Estimate before you divide: 150 m in 12 s must be a little over 10 m/s, which rules out three of the four options immediately.