A loudspeaker is producing a sound wave of constant frequency.Consider a tiny dust particle 1.0 m in front of the loudspeaker. Which one of the following diagrams best describes the motion of the dust particle?

Question

BlackTom AI · Accepted Answer

To approach this question, consider how a small dust particle behaves in a sound wave emitted by a loudspeaker. 
Option a) Oscillating randomly in all directions would imply the particle experiences forces in unpredictable directions, which does not occur in a coherent plane wave produced by a single speaker at a fixed distance. The air pressure variations in a sound wave are organized and propagate along a specific direction, so random motion is not expected.
Option b) Stationary would mean the particle does not move at all, contradicting the presence of a propagating sound wave that causes alternating pressure and velocity in the air, which tends to move particles back and forth.
Option c) Oscillating left and right corresponds to motion along the line of wave propagation. When a plane sound wave travels from the loudspeaker toward the particle, the local air pressure and velocity fluctuations induce the dust particle to oscillate parallel to the direction of travel, i.e., back and forth toward and away from the speaker, which is effectively left and right in the diagram.
Option d) Oscillating up and down would imply motion perpendicular to the direction of wave travel. In a simple one-dimensional sound wave from a single loudspeaker, there is no sustained vertical (transverse) oscillation of the particle; the primary motion is along the propagation axis, not perpendicular to it.
Overall, the physically correct description involves the particle following the longitudinal oscillations of the air, which align with the wave’s travel path. The reasoning hinges on the nature of a sound wave in air: compressions and rarefactions create particle motion along the direction of propagation, not perpendicular to it, and not in a completely random pattern.

A loudspeaker is producing a sound wave of constant frequency.Consider a tiny dust particle 1.0 m in front of the loudspeaker. Which one of the following diagrams best describes the motion of the dust particle?单项选择题

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