AQA GCSE Combined Science: Trilogy Physics Paper 2 (Higher), 2019: Question 6

✅ Correct answers

• Thrust decreases.
• This gives a resultant force opposite to the direction of motion so the plane slows down.
• Lift decreases because the plane is no longer needing to stay at constant height.
• This means there is a resultant downward force.

To get all 4 marks, students needed both horizontal and vertical changes.

💡 Key knowledge

• At constant speed and constant height, forces are balanced.
• For landing, the plane must slow down and descend.
• If thrust is reduced, air resistance can become greater than thrust, so the plane decelerates.
• If lift is reduced while weight stays the same, weight becomes greater than lift, so the plane moves downward.

🧠 Exam technique

• Use the word resultant to show you understand unbalanced forces.
• Link each force change to the effect: thrust down → slows down, lift down → descends.
• Think in pairs: thrust vs air resistance, lift vs weight.

❌ Common errors

• Saying only “the plane slows down” without explaining how the forces change.
• Writing that weight increases — the mark scheme says this is not correct here.
• Mixing up lift and weight.
• Forgetting to state that the resultant force must be downwards for landing.

💡 Key knowledge

• Use force = mass × acceleration.
• Rearrange to mass = force ÷ acceleration.
• Because the aeroplane is slowing down, acceleration is negative.
• In GCSE exam questions, correct handling of signs and units is essential.

🧠 Exam technique

• Write the formula first.
• Calculate acceleration before using force = mass × acceleration.
• Show every stage clearly: substitution, rearrangement, final answer.
• Include the unit kg in the final answer.

❌ Common errors

• Forgetting that the speed change is from 80 m/s to 10 m/s.
• Using 80 + 10 instead of finding the change in speed.
• Not dividing by time correctly.
• Leaving the answer in N instead of kg.
• Ignoring the negative sign and losing method marks.

✅ Correct answer

Mass = 300 000 kg

The mark scheme allows 300 000 kg even if a student uses the negative acceleration correctly, because mass is a positive quantity.

🧠 Why this earns 5 marks

• 1 mark for a correct acceleration equation.
• 1 mark for finding a = −2.5 m/s² .
• 1 mark for substituting into F = ma .
• 1 mark for rearranging correctly to find mass.
• 1 mark for the final correct value 300 000 kg .

💡 What strong answers did

• Named the force changes directly: thrust decreases, lift decreases.
• Explained the result: resultant force opposite direction of motion and resultant downward force.
• Used clear physics vocabulary rather than vague phrases.
• Showed a full calculation chain in 06.2.

❌ Where students lost marks

• Only describing the plane landing, not the forces.
• Saying weight increases, which the mark scheme rejects.
• Doing the maths but not showing the equation or rearrangement.
• Mixing up the direction of the resultant force.