Physics (AS & A Level) · May / June 2020 · Paper 11 · Question 25

A source emitting sound of a single frequency $f_s$ travels at constant speed directly towards an observer. The source then passes the observer and continues to move directly away from the observer. The velocity of the source remains constant. Which graph represents the variation with time of the frequency $f_o$ of the sound heard by the observer?

1. AGraph A: $f_o$ constant below $f_s$, then steps up to constant above $f_s$
2. BGraph B: $f_o$ constant above $f_s$, then steps down to constant below $f_s$
3. CGraph C: $f_o$ increases linearly from below $f_s$ to above $f_s$
4. DGraph D: $f_o$ decreases linearly from above $f_s$ to below $f_s$

As printed in the paper