Finding velocity of a vehicle using Doppler Shift for sound

[nawg04]

Homework Statement

You are standing on the side of the Trans-Canada Highway as the Physicsmobile approaches you. As it approaches, you hear an engine noise of 1100 Hz. After it passes, you hear an engine noise of 950 Hz. How fast was the Physicsmobile travelling?

Homework Equations

f2 = apparent frequency
f1 = actual frequency emitted by source
v = speed of sound in air
vs = speed of source

The Attempt at a Solution

I'm not sure how to re-arrange the formula. If I fill in all variables that I'm given, there are still two left blank (vs and f).

 

[mfb]

You have two unknown values (vs and f1) and two equations - one before and one after it passes you. You can solve this equation system to get vs.

 

[nawg04]

Does this look right?

 

[Borek]

Actually it looks terrible and is almost impossible to read. Please learn how to format your equations and formulas using LaTeX.

 

[Saitama]

Does this look right?

Your first equation is wrong. The source approaches the observer, so the frequency should increase. To increase the frequency, the denominator in the equation should decrease.