AQA A-Level Physics Paper 1, June 2022: Question 9

Estimating Kinetic Energy of a Motorway Car

What this question tests

This question assesses your ability to recall physical equations, convert units efficiently, and make reasonable order-of-magnitude estimations for everyday quantities (Physics AO1). You need to combine standard values for an ordinary car's mass and speed into the kinetic energy formula.

Question 09

Exam Breakdown & Solution

✅ Correct Answer

B ( 10⁶ J )

Marks available: 1 mark

💡 Key Knowledge

Kinetic energy equation: E_k = 0.5 × m × v²
Typical mass of a standard car m ≈ 1000 kg (or 10³ kg).
Speed conversion from km h⁻¹ to m s⁻¹ .

🧠 Exam Technique

For estimation questions, do not waste time seeking exact figures. Use sensible approximations (Fermi estimation skills) and order-of-magnitude working to quickly narrow down choices to the nearest power of ten.

❌ Common Errors

Failing to convert km h⁻¹ into m s⁻¹ before squaring the velocity.
Misjudging the mass of a car (e.g., guessing 10 kg or 100,000 kg).
Forgetting to square the velocity term in 0.5mv² .

📐 Step-by-Step Calculation

Estimate Mass (m): A typical car has a mass around 1000 kg ( 10³ kg ).
Convert Speed (v):
Speed = 100 km h⁻¹
= (100 × 1000 m) / 3600 s
≈ 28 m s⁻¹ (Roughly 30 m s⁻¹ or 10¹ m s⁻¹ for order-of-magnitude).
Substitute into Kinetic Energy Formula:
E_k = 0.5 × m × v²
E_k ≈ 0.5 × 1000 × (30)²
E_k ≈ 500 × 900
E_k ≈ 450,000 J = 4.5 × 10⁵ J .
Match Order of Magnitude: 4.5 × 10⁵ J rounds up to an order of magnitude of 10⁶ J (Option B).

AQA A-Level Physics Paper 1, June 2022: Question 9

Question

Mark scheme

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