Sound propagates because unbalanced forces accelerate neighboring air cells in sequence

Evidence

Each lozenge has mass m and is pushing out on its two neighbors; they push back just as hard in the quiescent state. If a disturbance arrives, a lozenge momentarily pushes on its neighbor a little harder, which now feels an unbalanced net force F as it begins to accelerate according to F = ma. The acceleration in turn induces a harder push on the next lozenge, and so on down the line, leading to propagation of the pulse. — Eric J. Heller, Why You Hear What You Hear- An…, 109

Categorization

• DEFINE: Concretizes how Newton’s F = ma operates locally in each air cell to propagate a pressure pulse down a pipe.