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AS Daily A Level Physics question

2026-01-14 OCR A Momentum & impulse (M3.5) 3.5.2(a) 3.5.2(b)

In a lab, a 1.0 kg trolley moving at 3.0 m/s on a straight, level track collides and couples with a 2.0 kg trolley initially at rest. External forces are negligible during the collision. Which outcome for their common speed immediately after is correct?

  1. A They move together at 1.5 m/s — the simple average of 3.0 and 0.0.
  2. B They move together at 3.0 m/s — the speed of the incoming trolley is unchanged.
  3. C They move together at 1.0 m/s, from conserving momentum: 1.0×3.0 = (1.0+2.0)×v. (correct)
  4. D They move together at 0.5 m/s by conserving kinetic energy instead of momentum.

Answer

The correct answer is C.

Correct: C — They move together at 1.0 m/s, from conserving momentum: 1.0×3.0 = (1.0+2.0)×v.. A averages speeds rather than conserving total momentum, ignoring that momentum depends on mass as well as speed. B assumes the speed is unchanged, but when total mass increases from 1.0 kg to 3.0 kg, the speed must decrease to keep momentum the same. C is the only option that applies conservation of momentum correctly for a 1D inelastic collision where the trolleys stick. D incorrectly applies conservation of kinetic energy; sticking is inelastic so KE decreases, and 0.5 m/s would not conserve the initial momentum.

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