A rocket is launched vertically from rest. As the rocket climbs higher, its acceleration increases even though the thrust force from its engines remains constant. Explain why the acceleration of the rocket increases as it climbs.

AQA GCSE Physics (8463) — 4.5.6 Forces and motion · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A rocket burns fuel at a steady rate, producing a constant upward thrust force throughout its climb.

Model answer (4 marks)

1. As the rocket burns fuel its mass decreases.
2. With a smaller mass, the same thrust gives a larger acceleration (F=ma).

Use the key phrase ‘mass decreases’ and link to Newton’s Second Law; mention the constant thrust; keep the answer concise and to the point.
Show the relationship between mass and acceleration explicitly – e.g. ‘a = F/m’ – to demonstrate understanding of the law.
Remember to mention the effect of air resistance decreasing with altitude, which also increases the net force.
Use correct UK spelling and scientific terminology.

Failing to state that the mass decreases as fuel is burnt; students often write only ‘thrust is constant’.
Confusing the role of air resistance – some students say it increases instead of decreases.
Not linking the decrease in mass to the increase in acceleration via Newton’s Second Law.

As fuel is burned, the mass of the rocket decreases
A smaller mass means a greater acceleration for the same force (Newton's Second Law)
The atmosphere becomes less dense at greater altitude, so air resistance decreases
A smaller opposing force means a larger resultant force, so acceleration is greater