Calculating Truck Speed from Frequency Shift

In summary, the conversation discusses how to calculate the speed of a truck based on the change in dominant frequency of its noise as it approaches and passes by a listener. The suggested solution involves using the doppler shift formula and setting up two equations to solve for the unknowns of the source frequency and the truck's speed.
  • #1
confusedinphys
3
0

Homework Statement



You are standing by the roadside as a truck approaches, and you measure the dominant frequency of the truck noise at 1100 Hz. As the truck passes, the frequency drops to 950 Hz. What is the truck's speed?

Homework Equations




f'= f (vsound)
-------------------
(vsound-velocity of source)

1100 = 950(340)
------
(340-vs)

The Attempt at a Solution



I rearrange for velocity of source and get 47 m/s, but my book says 25 m/s
 
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  • #2
f'= f (vsound)
-------------------
(vsound-velocity of source)
Is one relationship - in this case for frequency change of a sound moving toward the listener/receiver for which there is an increase in frequency

So f' = f (vs/(vs-v)) = 1100 Hz,

for the case where the source passes and is traveling away from the listener/receiver

f' = f (vs/(vs+v)) = 950

Take on equation and rearrange for f

one should obtain

1100 = 950 [tex]\big(\frac{v_s\,+\,v}{v_s\,-\,v}\big)[/tex]
 
  • #3
Here, you have two unknowns. 1100 Hz, and 950 Hz are both frequencies you observe, neither is the actual frequency of the source. That is, they are both values of f', one for when the truck is approaching you, the other for when the truck is moving away from you.

So, as I said, their are two variables you do not know, the source frequency, and the speed of the source.

HINT: Since you have two unknowns here, you are going to need two equations to solve this problem? Using the doppler shift formula you have above, can you set up two equations corresponding to this situation?

EDIT: (Astronuc you beat me to it!)
 
  • #4
I really have no idea what you mean by two equations here, can someone fill me in?
 
  • #5
Look back at Astronuc's post.
Can you summarize his suggestion?
 

1. How does frequency shift relate to truck speed?

Frequency shift is a measure of the change in frequency of a sound wave as it reflects off a moving object. In the case of calculating truck speed, this shift in frequency is caused by the Doppler effect and can be used to determine the speed of the truck.

2. What factors affect the accuracy of calculating truck speed from frequency shift?

Several factors can affect the accuracy of this calculation, such as the angle at which the sound waves hit the truck and the distance between the source of the sound and the truck. Additionally, environmental factors like wind speed and temperature can also impact the accuracy of the measurement.

3. Is it possible to accurately calculate truck speed from frequency shift in real-time?

Yes, it is possible to calculate truck speed from frequency shift in real-time using specialized equipment, such as radar or sonar devices. These devices can accurately measure the frequency shift and calculate the truck's speed based on that information.

4. Can this method be used to calculate the speed of any type of vehicle?

Yes, this method can be used to calculate the speed of any moving object, including vehicles. However, the accuracy may vary depending on the size and shape of the object, as well as the distance and angle at which the sound waves hit it.

5. How can the calculated truck speed from frequency shift be used in practical applications?

The calculated truck speed can be used for various purposes, such as traffic monitoring, speed control, and accident prevention. It can also be used in scientific research and engineering to study the movement and behavior of vehicles.

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