Paper 1 Q4 — Elastic collisions: approach = separation

Collisions — relative speed of approach equals relative speed of separation; speed vs velocity

What the examiners said

“The idea of ‘speed of approach = speed of separation’ for elastic collisions is one that candidates sometimes find challenging. The most common incorrect answer was option A, which implies confusion between speed and velocity.”

Try the question (reconstruction)

Ball X (moving right at 5.0 m s⁻¹) collides elastically head-on with ball Y moving left at 3.0 m s⁻¹. What is the relative speed of separation after the collision?

Examiner report: “The common error subtracted the two speeds — treating opposite velocities as if same-signed.”

Explore — approach speed = separation speed

Set each ball’s velocity (sign = direction). Run the elastic collision and confirm the relative speed is unchanged in magnitude — only reversed.

u_X (m s⁻¹, + = right): 5 u_Y (m s⁻¹): -3

Because velocities carry a sign, two balls closing at 5 and −3 approach at 8 m s⁻¹, and separate at 8 m s⁻¹ — the elastic-collision signature.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H2 Physics 9749 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org