Doppler effect

Problem statement

(from a physics exam question, for pupils aged about 18)

An aircraft is north-east of you, flying north with a steady course and speed, half the speed of sound. It is emitting a loud sound with a pitch of 400 Hz. What pitch do you hear?

Obvious (?) answer: show

Alternative interpretation of question: show

Comments: show

Source: UK physics exam papers from the 1960s (wording modified)

This is one of several pages on puzzles and metapuzzles.