Question

John is listening to a horn. He knows the frequency of the horn is 300 Hz when both he and the horn are at rest. If he hears a pitch of 270 Hz, there are clearly several possibilities. For each of the following statements, indicate whether it is correct or incorrect.

The distance between John and the horn is decreasing with time.

Both can be moving, in opposite directions.

John and the horn are both moving in the same direction but John is ahead and moving faster than the horn.

Both can be moving and have different speeds.

John is moving away from the horn at rest.

Both can be moving and have the same speed.

Answer 1

Since John hears the horn at lower frequency (270 Hz) which means relative motion of the source and John will be away from each other.

John and the horn are both moving in the same direction but John is ahead and moving faster than the horn

in this case when John is moving faster and ahead of the horn which means relative speed of John is away from the source so pitch of a receding sound source will be lowered.

Both can be moving and have different speeds.

Yup, this is also possible that both the Source and John is moving in opposite direction. then also the relative motion between the is away fromm each other.

John is moving away from the horn at rest

A moving away source moves farther during period of the sound wave so the effective wavelength is increases, giving a low pitch pitch since the velocity of the wave is unchanged. Doppler Effect.