SPEEDING TRAIN w/2 frequencies given while person is standing at rest. Please help.

In summary, the conversation discusses using the Doppler formula to calculate the speed of a train based on the change in frequency of its whistle as it passes by a person standing close to a railroad crossing. The frequency of the whistle before and after the crossing is used to determine the ratio of the frequency, which does not include the frequency of the train at rest. The basic Doppler formula is needed, along with the speed of sound in air and the speed of the train, in order to solve for the speed of the train.
  • #1
Tensionfreek
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1. A person standing close to a railroad crossing hears the whistle of an approaching train. He notes that the pitch of the whistle drops as the train passes by and moves away from the crossing. The frequency of the distant approaching whistle is 555 Hz; it drops to 472 Hz after the train is well past the crossing. What is the speed of the train? Use 340 m/s for the speed of sound in air.

HINT: Calculate the ratio of frequency of the whistle before and after the crossing. That ratio does not include the frequency of the train at rest.



2. i end up with this 472(340/340-Vs)=555(340/340+Vs) but do not know how to solve for Vs. i do not even know if this is set up correct I've tried numerous ways already and this seems most logical.


3.
 
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  • #2


Can you show the equation(s) you are starting from in order to get that expression?
You need the basic Doppler formula that includes the frequency of the whistle at rest, together with the speed of sound and the speed of the train. How did you then rearrange and manipulate?
 

1. How do you calculate the speed of a train with two frequencies given while a person is standing at rest?

The speed of the train can be calculated by using the formula: speed = frequency x wavelength. In this case, you will need to use the two frequencies given and the known wavelength of the train.

2. What is the Doppler effect and how does it relate to train speeds?

The Doppler effect is the change in frequency or wavelength of a wave when the source of the wave is moving relative to the observer. This effect is commonly observed with sound waves, such as the change in pitch of a siren as it passes by. In the case of a train, the Doppler effect can be used to calculate the train's speed based on the change in frequency of the train's sound as it approaches and passes by a stationary observer.

3. How does the distance between the train and the observer affect the calculation of speed?

The distance between the train and the observer does not directly affect the calculation of speed. However, the closer the train is to the observer, the greater the change in frequency will be, making it easier to accurately calculate the speed of the train.

4. Can the speed of the train be accurately calculated if the person is not standing at rest?

Yes, the speed of the train can still be accurately calculated even if the person is not standing at rest. However, the person's movement may affect the accuracy of the calculation, as their changing position may alter the perceived frequency of the train's sound.

5. Is there a way to calculate the speed of a train without using frequencies?

Yes, there are other ways to calculate the speed of a train without using frequencies. One method is to use the time it takes for the train to travel a known distance, along with the distance itself, to calculate the speed. Another method is to use radar or other tracking technology to directly measure the train's speed.

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