AQA A-Level Physics Paper 1, June 2024: Question 5

✅ Correct Answers

• T1: 480 N (Acceptable range: 470 N – 490 N)
• T2: 400 N (Acceptable range: 390 N – 410 N)

💡 Key Knowledge

• Equilibrium: Three coplanar forces acting at a point must form a closed vector triangle when the system is in static equilibrium.
• Scale Determination: The weight vector ( W = 350 N ) acts vertically downwards and sets the exact scale (e.g., 35 mm = 350 N or 10 mm = 10 N mm⁻¹ ).

🧠 Exam Technique

• Construct a closed vector triangle (or parallelogram) using the downward weight vector W and the lines of action of tensions T₁ and T₂ .
• Measure lengths of vector sides carefully with a ruler and convert using your chosen scale. Alternatively, apply sine/cosine rules using measured angles ( 34° and 11° ).

❌ Common Errors

• Failing to draw a complete, closed vector polygon/triangle.
• Inconsistent or unstated scale factors leading to wild conversion errors.
• Mixing up angle measurements relative to the horizontal vs. vertical axes.

✅ Correct Answer

• Average resistive force = 46 N (Accept answers rounding to 46 N)

📐 Step-by-Step Calculation

1. Find mass ( m ): m = W / g = 350 / 9.81 = 35.68 kg (expecting use of m = 350 / g yielding ~36 kg).
2. Calculate Initial Gravitational Potential Energy ( ΔE_p ): ΔE_p = m g Δh = 350 × 4.5 = 1575 J .
3. Calculate Final Kinetic Energy ( E_k ): E_k = 0.5 m v² = 0.5 × (350 / 9.81) × 6.5² = 753.6 J .
4. Determine Work Done against Friction ( W_friction ): ΔE_p - E_k = 1575 - 753.6 = 821.4 J .
5. Calculate Average Resistive Force ( F ): F = W_friction / distance = 821.4 / 18 = 45.63 N -> 46 N .

🧠 Exam Technique

• Always equate initial total energy to final total energy plus work done against non-conservative forces: Initial GPE = Final KE + Work Done (Friction) .
• Check significant figures: input data is given to 2 s.f. / 3 s.f., so round your final answer appropriately to 2 s.f. ( 46 N ).

❌ Common Errors

• Confusing force ( 350 N ) with mass ( kg ) when calculating kinetic energy ( 0.5 m v² ). Remember to divide weight by g !
• Dividing energy by height ( 4.5 m ) instead of total distance travelled along the cable ( 18 m ).

💡 Key Knowledge & Approaches

• Momentum Approach: Impulse F Δt = Δp . By increasing the impact/contact time ( Δt ) as the material compresses, the average force ( F ) exerted on the fragile item is reduced for a fixed change in momentum ( Δp ).
• Energy Approach: Work done F × d = ΔE_k . By increasing the stopping distance ( d ) over which the kinetic energy is dissipated, the average force ( F ) is reduced.

🧠 Exam Technique

• State the governing physical principle clearly upfront (Impulse-Momentum theorem or Work-Energy principle).
• Explicitly link the material property (compression / deformation) to either increased time or increased distance.
• Conclude explicitly how this decreases the resulting force on the fragile item, preventing damage.

❌ Common Errors

• Vague statements like "it absorbs the shock" without referencing underlying physics quantities (force, time, distance, momentum, energy).
• Stating that momentum or kinetic energy changes are reduced—remind yourself that the total change in momentum/energy to stop the object is fixed; the packing only alters the rate of change ( F = Δp / Δt ).