A rocket is launched vertically upwards from the ground. At the moment of launch, the thrust force from the rocket engine is greater than the weight of the rocket. As the rocket rises, the thrust force remains constant but the weight of the rocket decreases. Explain how the motion of the rocket changes from launch until the rocket engine is switched off.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
At launch the thrust exceeds the weight, so the resultant force is upward and the rocket accelerates upward.
As the rocket ascends it burns fuel and its mass – and therefore its weight – decreases while the thrust remains constant. The downward weight force is therefore smaller, so the upward resultant force becomes larger and the upward acceleration increases.
When the engine is switched off the thrust disappears; only the weight acts downward, giving a downward resultant force that decelerates the rocket.
As the rocket ascends it burns fuel and its mass – and therefore its weight – decreases while the thrust remains constant. The downward weight force is therefore smaller, so the upward resultant force becomes larger and the upward acceleration increases.
When the engine is switched off the thrust disappears; only the weight acts downward, giving a downward resultant force that decelerates the rocket.
Examiner tips
- State the initial upward resultant force and acceleration. Explain how decreasing weight increases the resultant force and acceleration. Describe the change when thrust stops.
- common_mistakes
- :
- Forgetting that the weight decreases as fuel is burnt. Confusing acceleration with velocity. Assuming the rocket continues to accelerate after engine cut‑off.
Mark scheme (5 marks)
- At launch, the resultant force is upward because thrust is greater than weight
- An upward resultant force causes the rocket to accelerate upwards
- As the rocket rises, weight decreases (fuel is used up / rocket loses mass)
- The resultant force increases as weight decreases (thrust stays constant), so acceleration increases
- When the engine is switched off, thrust becomes zero so the resultant force acts downward (weight only), causing the rocket to decelerate
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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