The diagram shows a gas particle about to collide elastically with a wall - AQA - A-Level Physics - Question 12 - 2018 - Paper 2

To determine the correct change in momentum during the elastic collision, we need to consider how momentum is defined. Momentum ( extbf{p}) is given by the formula:

$p = mv$

When the particle collides elastically with the wall, its velocity changes direction, but the magnitude remains the same. The change in momentum ($\Delta mv$) can be represented as a vector difference, where the initial momentum is directed towards the wall, and the final momentum is directed away from it.

Given the direction of the particle's velocity before and after the collision, the correct representation of the change in momentum is shown in Diagram A, as it accurately depicts the vector change from an incoming momentum $mv$ to an outgoing momentum $-mv$, resulting in the correct vector triangle for $riangle mv$.