Problem with basic doppler effect question

[PsychonautQQ]

Homework Statement

The source of a 1 kilohertz sound is getting closer to the listener at a speed of .9 times the speed of sound. What frequency does the listener hear?

Homework Equations

fl = ((v-vs) / (v + vl))fs

The Attempt at a Solution

((343+343(.9)) / 343)*1000=1900 Hz
this is the wrong answer, what did I do wrong? it's supposed to be 10,000 Hz

 

[besulzbach]

Correct formula:
[itex]f = \frac{c\ +\ v_{r}}{c\ +\ v_{s}} \cdot f_{0}[/itex]

In your problem the source is getting closer, so [itex]v_{s}[/itex] is negative.

As you didn't miss that, your error is in the formula. Use the one above.

You also SUMMED values that you should have SUBTRACTED.

Remember:

[itex]v_{r}[/itex] is the velocity of the receiver relative to the medium; positive if the receiver is moving towards the source;

[itex]v_{s}[/itex] is the velocity of the source relative to the medium; positive if the source is moving away from the receiver;

 

[STEMucator]

Homework Statement

The source of a 1 kilohertz sound is getting closer to the listener at a speed of .9 times the speed of sound. What frequency does the listener hear?

Homework Equations

fl = ((v-vs) / (v + vl))fs

The Attempt at a Solution

((343+343(.9)) / 343)*1000=1900 Hz
this is the wrong answer, what did I do wrong? it's supposed to be 10,000 Hz

Use this equation : ##f = f_0( \frac{v}{v ± v_s} )##

Where ##f## is the stationary frequency. ##f_2## is the apparent frequency detected. ##v## is the speed of sound in air and ##v_s## is the speed of the source.

Since the source is moving closer to your listener, you want to use ##v - v_s## ( Doppler effect ). Don't forget to convert kHz to Hz otherwise this won't work!

I got ##f = 10000 Hz##.