Molecular inertia makes sound particle velocity trace a cosine, smoothly not abruptly

Evidence

The velocity does not switch instantly from one direction to another, due to the inertia of the molecules involved; instead, it accelerates smoothly from a stationary to a moving state and back again. The velocity component reaches its peak between the compressions and rarefactions, and for a sine wave displacement component, the associated velocity component is a cosine. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 160

Categorization

• DEFINE: Explains the displacement-to-velocity (sine-to-cosine) phase relation in a sound wave via a concrete golf-ball mental model.
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 160