A rocket is launched vertically upwards from the ground. After the engines cut out, the rocket continues to move upwards before eventually falling back to the ground. State and explain how the speed of the rocket changes from the moment the engines cut out until the rocket reaches its highest point, and explain why the displacement of the rocket at the moment it lands is zero.

Cambridge International IGCSE Physics (0625) — 1.2 Motion · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

After the engines cut out the only force on the rocket is its weight, so the resultant force is downward. The rocket therefore decelerates and its speed decreases until it reaches zero at the highest point.

The displacement of the rocket when it lands is zero because displacement is a vector that depends on the change in position. The rocket returns to its starting point, so the change in position is zero.

Examiner tips

  • Use the word ‘decelerates’ to show you understand the change in speed. Mention that the only force after cut‑out is gravity. State that displacement is a vector and that the final position equals the initial position.
  • common_mistakes
  • :
  • Writing that the speed increases after cut‑out. Forgetting to mention that the only force is gravity. Saying the rocket’s displacement is zero because it ‘stops’ rather than because it returns to its starting point.

Mark scheme (4 marks)

  1. The speed of the rocket decreases (after engines cut out) / the rocket decelerates
  2. Because the resultant force acts downwards (due to gravity/weight) opposing the upward motion
  3. Displacement is a vector quantity / displacement has both magnitude and direction
  4. The rocket returns to its starting position, so its overall change in position is zero / final position equals initial position

Key terms in this question

speed · displacement

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