A moving trolley collides with a stationary trolley on a frictionless track. After the collision, the two trolleys stick together and move as one object. State the principle of conservation of momentum and use it to explain why the combined trolleys move more slowly than the original trolley was moving before the collision.

Cambridge International IGCSE Physics (0625) — 1.6 Momentum · State and explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The total momentum of a system remains constant if no external forces act.
The stationary trolley had zero momentum before the collision, so the total momentum before equals the momentum of the moving trolley alone.
After the collision the same total momentum is carried by a greater mass (the two trolleys together).
Because momentum = mass × velocity, a greater mass with the same momentum must have a smaller velocity, so the combined trolleys move more slowly.

Examiner tips

  • State the conservation law first, then link each point to the marks; use the exact wording "total momentum of a system remains constant".
  • Show the reasoning step‑by‑step: zero momentum of stationary trolley, same total momentum, greater mass, hence lower velocity.
  • Use the formula p = mv to justify the final conclusion.

Common mistakes

  • Confusing momentum with kinetic energy; writing that the speed increases instead of decreases.
  • Omitting that the stationary trolley’s momentum is zero before the collision.
  • Using vague terms like "the system’s momentum stays the same" without specifying the condition of no external forces.

Mark scheme (4 marks)

  1. The total momentum of a system remains constant / is conserved provided no external forces act
  2. The stationary trolley had zero momentum before the collision, so total momentum before equals momentum of moving trolley alone
  3. After the collision, the same total momentum is shared by / carried by a greater (combined) mass
  4. Because momentum = mass × velocity, a greater mass with the same momentum must have a smaller velocity / moves more slowly

Key terms in this question

momentum · conservation of momentum

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