Question

A red car moving west on some road is creating a loud sound with a frequency of $700Hz$. A blue car is also travelling west along the same road. A person in the blue car hears the loud sound from the red car.
From the following list of frequencies, what possible frequency or frequencies might the person in the blue car hear?
1) $710Hz$
2) $700Hz$
3) $690Hz$

• A. $700Hz$ only
• B. $710Hz$ only
• C. $690Hz$ only
• D. $710Hz$ and $690Hz$ only
• E. Any of the three frequencies might possibly be heard by the person in the blue car

Answer 1

The correct option is E Any of the three frequencies might possibly be heard by the person in the blue car

Case 1 : $V_{Blue}>V_{Red}$

Then the person in blue car would hear frequency greater than original frequency.

Case 2 : $V_{Blue}=V_{Red}$

Then the person in blue car would hear frequency equal to the original frequency.

Case 3 : $V_{Blue}<V_{Red}$

Then the person in blue car would hear frequency less than original frequency.

Thus option E is correct.