A rocket is launched vertically upward from the ground. Initially, the thrust force from the rocket engines is greater than the weight of the rocket. As the rocket rises, its engines continue to provide the same thrust force, but the rocket accelerates more and more quickly. Explain why the rocket accelerates, and why the acceleration increases as the rocket rises.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A rocket burns fuel to produce a large downward thrust. As it rises through the atmosphere, air resistance is small enough to be ignored.
Model answer (5 marks)
1. The thrust force exceeds the weight, giving a net upward force.
2. By Newton’s Second Law, a net force produces acceleration.
3. As fuel is burnt the rocket’s mass decreases.
4. With the same thrust, a smaller mass means a larger acceleration (a=F/m).
5. The weight also falls, so the net force grows, further raising the acceleration.
2. By Newton’s Second Law, a net force produces acceleration.
3. As fuel is burnt the rocket’s mass decreases.
4. With the same thrust, a smaller mass means a larger acceleration (a=F/m).
5. The weight also falls, so the net force grows, further raising the acceleration.
Examiner tips
- Show the chain: thrust>weight → net force → acceleration; then link mass loss to increased a.
- Use the formula a=F/m to justify the increase; mention weight decreases too.
Common mistakes
- Confusing thrust with acceleration; forgetting to mention weight decreases.
- Using the wrong sign for forces or ignoring the weight term in the net force calculation.
Mark scheme (5 marks)
- The thrust force is greater than the weight / there is a resultant (net) upward force
- A resultant force causes acceleration (Newton's Second Law / F = ma)
- As fuel is burned, the mass of the rocket decreases
- For the same (constant) thrust force, a smaller mass produces a greater acceleration
- The resultant force also increases (as weight decreases with decreasing mass), further increasing acceleration
Key terms in this question
thrust · weight · acceleration
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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