What Causes the Doppler Shift John Experiences?

[asl3589]

Homework Statement

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.

Correct Incorrect Both can be moving and have different speeds.
Correct Both can be moving, in opposite directions.
Correct Incorrect John is moving away from the horn at rest.
Correct Incorrect Both can be moving in the same direction at the same speed.
Incorrect The distance between John and the horn is decreasing with time.
Correct Both can be moving and have the same speed.

These are the answers i put, could anyone help me as where I am going wrong

 

[ideasrule]

Correct Incorrect Both can be moving and have different speeds.
Correct Both can be moving, in opposite directions.
Correct Incorrect John is moving away from the horn at rest.
Correct Incorrect Both can be moving in the same direction at the same speed.
Incorrect The distance between John and the horn is decreasing with time.
Correct Both can be moving and have the same speed.

Why did you put both correct and incorrect for some of them?

 

[asl3589]

sorry its supposed to be

Correct Both can be moving and have different speeds.
Correct Both can be moving, in opposite directions.
Correct John is moving away from the horn at rest.
Correct Both can be moving in the same direction at the same speed.
Incorrect The distance between John and the horn is decreasing with time.
Correct Both can be moving and have the same speed.

 

[ideasrule]

Correct Both can be moving and have different speeds.
Correct Both can be moving, in opposite directions.

Right.

Correct John is moving away from the horn at rest.

I'm confused by the wording. The horn certainly doesn't have to be at rest, but John does have to be moving away from the horn.

Correct Both can be moving in the same direction at the same speed.

If they're both moving in the same direction at the same speed, there would be no Doppler shift. (Do you understand why?)

Incorrect The distance between John and the horn is decreasing with time.
Correct Both can be moving and have the same speed.

Right.