AQA A-Level Physics Paper 1, June 2024: Question 24

Jet Engine Momentum & Force Calculation

Question 2.4 • Multiple Choice • 1 Mark

What this question tests

This question assesses your understanding of Newton's second law in terms of momentum (Force equals rate of change of momentum), unit conversions (kilonewtons to newtons, minutes to seconds), and rearranging equations to solve for mass over a given time interval.

Question Part 2.4

Calculate mass of air passing through a jet engine in one minute

✅ Correct Answer

Option D: 4700 kg

Awarded 1 mark for selecting option D.

💡 Key Knowledge

Newton's Second Law: F = d(mv)/dt
For a constant mass flow rate, force can be written as F = (Δm/Δt) × Δv or F = (m/t) × v .
Prefix conversions: 1 kN = 10³ N
Time conversion: 1 minute = 60 s

🧠 Exam Technique

In multiple-choice calculation questions, always write down your rearranged formula before plugging in numbers. Pay careful attention to time units—forgetting to convert minutes into seconds is the #1 trap in rate-of-momentum questions.

📐 Step-by-Step Calculation

State the formula relating force to rate of change of momentum:
F = (Δm / Δt) × v
Rearrange to make total mass ( Δm ) the subject:
Δm = (F × Δt) / v
Substitute the values with correct units:
Force F = 42 kN = 42 × 10³ N
Time Δt = 1 minute = 60 s
Velocity change v = 540 m s&sup{-1}
Calculate:
Δm = (42 × 10³ × 60) / 540
Δm = 2,520,000 / 540 = 4666.67 kg
Round to appropriate significant figures:
Given data ( 42 kN , 540 m s&sup{-1} ) are given to 2 sig figs, giving 4700 kg .

❌ Common Errors & Distractor Traps

Forgetting to multiply by time (60 s): Calculating just 42000 / 540 yields 77.8 kg (leading students to incorrectly choose Option C, or get 7.7 × 10&sup{-2} kg if mixed up).
Power of 10 errors: Forgetting to convert kN to N (omitting the × 10³ factor).