PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Waves Sound and Physical Optics · Waves Sound and Physical Optics
Problem
At an outdoor physics demonstration, two identical loudspeakers mounted on small carts emit the same continuous tone of rest frequency f0. Three students—A, B, and C—stand on a straight sidewalk as the carts move.
• Speaker 1 starts 40m to the left of student A and is pushed directly toward A at 3m⋅s−1.
• Speaker 2 starts 40m to the right of student C and is pushed directly toward C at 5m⋅s−1.
• Students A, B, and C stand still, in that order, equally spaced 20m apart along the sidewalk.
Take the sidewalk as a straight line; ignore reflections and wind.
(a) At the instant both speakers are 20m away from student B, rank the frequencies heard by students A, B, and C from highest to lowest. Justify your ranking qualitatively.
(b) At a later instant both speakers are 10m away from student B, moving in the same original directions. Without recalculating any numeric values, state whether each student now hears a frequency higher than, lower than, or equal to the frequency they heard in part (a). Provide reasoning for each student.
(c) Identify any pair of students who hear the same frequency at either instant and explain the physical conditions that lead to equality.