Hey guys,
So question 3 on the AQA June 2013 Physics Unit 4 paper (Section A) is:A gas molecule of mass m moving at velocity u collides at right angles with the side of a container and rebounds elastically. Which one of the following statements concerning the motion of the molecule is incorrect?
A The magnitude of the change in momentum of the molecule is zero.
B The magnitude of the change in momentum of the molecule is 2mu.
C The force exerted by the molecule on the side of the container is equal to the force exerted by the container on the molecule.
D The change in kinetic energy of the molecule is zero.
Whilst the mark scheme gives the answer to be A, the examiner's report claims:
'In the perfectly elastic collision at right angles between a molecule and the side of a container, the velocity of the molecule is exactly reversed. The change in momentum is therefore mu − (−mu) = 2 mu. 70% ofthe candidates were aware of this, but almost a quarter of them thought the change in momentumwould be zero (and therefore chose distractor A).'
So there's clearly a discrepancy here. I'm inclined to agree with the examiner's report, but does anyone know the reason behind this discrepancy, and is B the correct answer?