AQA AS Level Physics Paper 2, June 2025: Question 17

1 mark · Medium difficulty · Multiple Choice

Identify which combination of units is not equivalent to the newton.

Practise this question

Question

Question 17 asks: 'Which is not equivalent to the newton?' followed by four options with check boxes: A: J m⁻¹, B: kg m s⁻², C: W s⁻², D: W s m⁻¹. The question is worth 1 mark.

Mark scheme

Show the mark scheme Mark scheme for question 17 showing the correct answer as option C (W s⁻²), referencing assessment objective AO1.

How to answer it

SI Units: Equivalence to the Newton

📌 What this question tests

This question tests your ability to express derived physical units (the newton, N) in terms of other derived units (joules, watts) and SI base units (kg, m, s) by applying fundamental mechanics relationships:

  • Newton’s 2nd Law: F = ma (Base SI units of force)
  • Work Done: W = Fs (Relating force to energy)
  • Mechanical Power: P = Fv and P = W / t (Relating force to power)

Question 17 Analysis

Identifying the expression NOT equivalent to the newton [1 Mark]

✅ Correct Answer: C (W s⁻²)

Option C is not equivalent to the newton. The unit W s⁻² equates to kg m² s⁻⁵ in base units, which does not match force ( kg m s⁻² ).

Award 1 mark for selecting option C. Assessment Objective: AO1 (recall and application of fundamental scientific units).

📐 Option-by-Option Breakdown

  1. Option A: J m⁻¹
    Use work done: Work = Force × distance (W = F × s)
    Rearrange for force: F = W / s
    Units: J / m = J m⁻¹ .
    👉 Equivalent to N.
  2. Option B: kg m s⁻²
    Use Newton's 2nd Law: F = m × a
    Mass is in kg , acceleration is in m s⁻² .
    Units: kg × m s⁻² = kg m s⁻² .
    👉 Equivalent to N (this is the base unit definition).
  3. Option C: W s⁻²
    Since Power = Work / time (P = W / t), 1 W = 1 J s⁻¹ .
    Substitute watts: W s⁻² = (J s⁻¹) s⁻² = J s⁻³ .
    Substitute base units of joules ( kg m² s⁻² ): (kg m² s⁻²) s⁻³ = kg m² s⁻⁵ ≠ N .
    👉 NOT equivalent to N.
  4. Option D: W s m⁻¹
    Use power and velocity: P = F × v
    Rearrange for force: F = P / v
    Velocity is in m s⁻¹ , so: W / (m s⁻¹) = W s m⁻¹ .
    Alternatively: W s = (J s⁻¹) s = J , hence (W s) m⁻¹ = J m⁻¹ = N .
    👉 Equivalent to N.

💡 Key Knowledge

  • Base Units of Force: 1 N = 1 kg m s⁻²
  • Energy: 1 J = 1 N m = 1 kg m² s⁻²
  • Power: 1 W = 1 J s⁻¹ = 1 kg m² s⁻³
  • Always link back to an equation you know that contains Force (F).

🧠 Exam Technique

  • Read the bold text carefully: Questions that ask which is not equivalent are designed to catch out students moving too quickly.
  • Use simple physics equations: Rather than converting everything down to base units directly, test options against known formulas like F = P/v or F = W/s to save precious time in multiple choice sections.

❌ Common Traps

  • Selecting Option A or B: Rushing students recognise J m⁻¹ or kg m s⁻² as valid newtons and immediately mark it without noticing the word not.
  • Index errors: Confusing W / v = W / (m s⁻¹) = W s m⁻¹ with W s⁻² due to mismanaging negative powers in denominators.

Topics

Physics · 3.1 Measurements and their errors · 3.4 Mechanics and materials

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.