Doppler Effect : GMAT Verbal Section
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# Doppler Effect

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Manager
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05 Jun 2008, 13:12
The “Doppler effect” refers to the perceived change in pitch that occurs when the source of a sound is in motion relative to the observer. For example, the siren on a passing police car will sound higher than its true pitch as the car approaches, sound the same as its true pitch as the car passes, and sound lower than its true pitch as the car travels away from the observer.

If two trains pass each other going opposite directions on parallel east-west tracks, which of the following observations provides another illustration of the effect described above?

A. If the eastbound train blows its horn as they pass, passengers on the westbound train will hear a sound that decreases in pitch.

B. If the eastbound train blows its horn as they pass, passengers on the westbound train will hear a sound that increases in pitch.

C. If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that decreases in pitch.

D. If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that increases in pitch.

E. If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that is steady in pitch.

Source MGMAT. Will post OA after some discussion.
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Re: Doppler Effect [#permalink]

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05 Jun 2008, 13:27
Sunny143 wrote:
The “Doppler effect” refers to the perceived change in pitch that occurs when the source of a sound is in motion relative to the observer. For example, the siren on a passing police car will sound higher than its true pitch as the car approaches, sound the same as its true pitch as the car passes, and sound lower than its true pitch as the car travels away from the observer.

If two trains pass each other going opposite directions on parallel east-west tracks, which of the following observations provides another illustration of the effect described above?

A. If the eastbound train blows its horn as they pass, passengers on the westbound train will hear a sound that decreases in pitch.

B. If the eastbound train blows its horn as they pass, passengers on the westbound train will hear a sound that increases in pitch.

C. If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that decreases in pitch.

D. If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that increases in pitch.

E. If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that is steady in pitch.

Source MGMAT. Will post OA after some discussion.

Seems to be A.

While E is true, it doesnt really follow the force of the argument. Arguably I think E could be an answer though.
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Re: Doppler Effect [#permalink]

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05 Jun 2008, 15:05
A for me as well
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Re: Doppler Effect [#permalink]

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05 Jun 2008, 16:19
Thanks for posting responses, but dont you guys think that according to the answer choice A, if the horn is blown when the 2 trains PASS, then some part of the westbound train will already have passed( the passengers in that part are going away from the horn, so they hear a decreasing pitch), but the rest of the passengers who are in the part of the train that is yet to pass , they are coming towards the horn, hence they should hear a increasing pitch.

Thoughts?

OA a little later
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Re: Doppler Effect [#permalink]

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05 Jun 2008, 17:19
I think the best answer for the passenger in the part of the train that is yet to pass would be that they would hear an increasing pitch, and then an decreasing pitch, as the other traisn goes away. Once there's not that answer, the best answer is A for me too.
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Re: Doppler Effect [#permalink]

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06 Jun 2008, 06:40
OA is A.

But again, I dont fully agree with the MGMAT OA. As someone said, they are a bit rough on the edges with their CR. Only E makes some logical sense here among the choices...
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Re: Doppler Effect [#permalink]

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06 Jun 2008, 10:26
I think the key issue here is that the question is asking "which of the following observations provides another illustration of the effect described above".
Answer E does make more logical sense, but doesn´t illustrate the doppler efect
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Re: Doppler Effect [#permalink]

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19 Jun 2008, 08:23
Given example
------> Car A
-------> Car B

Car A blows siren... Car B hears increasing sound as A is approaching. ( Same direction )

Given Question

------> Train A
<------- Train B

Train A and B in oppopsite directions, therefore when A is approaching, B is in opposite directions hence due to doppler effect , it will hear sound decreasing in pitch.

Only A explains the same.
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19 Jun 2008, 09:12
Think of one person on the train...it is said that the horn is blown when the 2 trains pass...So think of a person who is at the end of the westbound. So when the eastbound train sounds the horn when they pass..the person in the last compartment of the westbound train will hear a increasing pitch first and then a decreasing pitch as the 2 trains completely pass...

The OA A comes closest in a very bad bunch. None of them are scientifically sound...but again one might argue that we are here efor CR questions...
Re: Doppler Effect   [#permalink] 19 Jun 2008, 09:12
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# Doppler Effect

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