AQA AS Level Physics Paper 2, June 2022: Question 25

✅ Correct Answer

Row A

Acceleration before: increasing upwards

Acceleration immediately after: constant downwards

💡 Key Knowledge

• Newton's Second Law: rearranged as a = (T - mg) / m = (T / m) - g .
• As fuel burns, mass ( m ) decreases while thrust ( T ) remains constant.
• This causes the upward acceleration to continually grow.
• Once fuel is used up, thrust becomes zero, leaving only gravity acting on the rocket.

🧠 Exam Technique

Break the question into two distinct physical phases:

1. Before fuel runs out: Analyse the equation a = (T/m) - g . As m decreases, the term T/m increases, so a increases upwards. Eliminate rows C and D.
2. Immediately after fuel runs out: T = 0 . The only force is weight downwards. Acceleration is equal to g downwards (constant). This rules out row B.

❌ Common Errors

• Assuming constant acceleration: Students often mistakenly think a constant thrust gives a constant acceleration, forgetting that Newton's second law depends on the net force divided by the varying mass.
• Confusing velocity with acceleration: Students selecting row B often mix up a decreasing rate of mass loss or changing speed with a "decreasing acceleration", failing to recognise that acceleration becomes completely uniform ( 9.81 m s⁻² ) once airborne without thrust.