A rocket is launched vertically from a launchpad. At the moment of launch, the rocket's engines produce a thrust force greater than the rocket's weight. As the rocket rises, its mass decreases because fuel is being burned. Explain why the rocket's acceleration increases as it continues to rise, even if the thrust force remains constant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A rocket is launched vertically from a launchpad. At the moment of launch, the rocket's engines produce a thrust force greater than the rocket's weight. As the rocket rises, its mass decreases because fuel is being burned.
Model answer (5 marks)
The resultant force on the rocket is the thrust minus its weight, so it is upwards because the thrust is greater than the weight.
As the rocket burns fuel its mass decreases.
Because weight is W = mg, a decrease in mass means a decrease in weight.
With thrust constant, the reduction in weight makes the resultant force larger.
According to Newton’s second law, a larger resultant force acting on a smaller mass gives a larger acceleration.
As the rocket burns fuel its mass decreases.
Because weight is W = mg, a decrease in mass means a decrease in weight.
With thrust constant, the reduction in weight makes the resultant force larger.
According to Newton’s second law, a larger resultant force acting on a smaller mass gives a larger acceleration.
Examiner tips
- State the resultant force first, then explain the mass/weight change, and finish with Newton’s second law.
- Use the exact terms: resultant force, thrust, weight, mass, acceleration, Newton’s second law.
Common mistakes
- Confusing thrust with resultant force; forgetting to subtract weight.
- Using the wrong sign for weight (downwards) or not linking the decrease in mass to the increase in acceleration.
Mark scheme (5 marks)
- The resultant force on the rocket equals thrust minus weight (accept: resultant force is upwards because thrust is greater than weight)
- As fuel is burned, the mass of the rocket decreases
- As mass decreases, the weight of the rocket also decreases (W = mg)
- As weight decreases (with constant thrust), the resultant force increases
- By Newton's second law (F = ma), a greater resultant force acting on a smaller mass produces a greater acceleration
Key terms in this question
weight · mass · acceleration · thrust
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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