# Finding velocity of a vehicle using Doppler Shift for sound

• nawg04
So the correct equation is f2 = f1/(1 - vs/v).In summary, the Physicsmobile was travelling at a speed of 55 m/s, based on the change in engine frequency heard by the observer. This was calculated using the formula f2 = f1/(1 - vs/v), where f1 is the actual frequency emitted by the source, vs is the speed of the source, and v is the speed of sound in air.
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).

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.

Does this look right?

nawg04 said:
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.

## 1. What is the Doppler Shift for sound?

The Doppler Shift for sound is a phenomenon in which the frequency of a sound wave changes depending on the relative motion between the source of the sound and the observer.

## 2. How does the Doppler Shift affect the sound of a moving vehicle?

The Doppler Shift causes the frequency of a sound wave produced by a moving vehicle to increase as it approaches an observer and decrease as it moves away from the observer. This results in a change in the perceived pitch of the sound.

## 3. How can the Doppler Shift be used to find the velocity of a vehicle?

By measuring the change in frequency of the sound wave produced by a moving vehicle, the velocity of the vehicle can be calculated using the Doppler Shift equation. This is known as the Doppler Effect.

## 4. What factors can affect the accuracy of finding the velocity of a vehicle using Doppler Shift for sound?

The accuracy of finding the velocity of a vehicle using Doppler Shift can be affected by factors such as the frequency of the sound wave, the speed of the vehicle, and any obstructions or reflections that may alter the sound wave's path.

## 5. In what real-life situations is the Doppler Shift for sound used to find the velocity of a vehicle?

The Doppler Shift for sound is commonly used in traffic monitoring systems, speed guns used by law enforcement, and in navigation systems for vehicles and aircraft. It can also be used in meteorology to measure wind speeds and in astronomy to study the motion of celestial bodies.

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