A gas atom strikes a wall with speed \(v\) at an angle \(\theta \) to the normal to the wall. The atom rebounds at the same speed \(v\) and angle \(\theta \).

Which of the following gives the magnitude of the momentum change of the gas atom?

A.     zero

B.     \(2mv\sin \theta \)

C.     \(2mv\)

D.     \(2mv\cos \theta \)

D

A large majority of the candidates opted for A, indicating perhaps that they misunderstood the question as asking for the change in the magnitude of momentums. It is worth noting that such an interpretation has no physical significance, whereas the question being asked relates to the force of the ball on the wall.