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# The Doppler effect refers to the perceived change in pitch

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The Doppler effect refers to the perceived change in pitch [#permalink]

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08 Dec 2009, 02:41
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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.
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08 Dec 2009, 03:37
While E seems ok, it doesn't demonstrate Doppler effect.

Therefore, (A) wins.

east Hzzz..
======>...
.......<======
west

West train is passing east, and passangers in west WILL hear decreasing in pitch horn. Since just after passing, east travels away from passangers in west

So, A

skim 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.
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08 Dec 2009, 08:57
I think it should be A. E is true but is not an example of the doppler effect.
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08 Dec 2009, 11:30
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21 Dec 2009, 09:39
We can eliminate C, D, E because there is no mention of the second train, and to illustrate the Doppler effect, we need two sources to ensure the relativity between the source of the sound and the receiver of this sound.
About A, it's not so sure, because it's depend where you sit. For example if you sit at the end of the westbound train, you will hear a increase in pitch because the train is approaching, then a decrease in pitch because the train is going away. Therefore the answer can be also B, depending where you sit.
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21 Dec 2009, 21:55
I would assume based on the stem that the trains are approaching each other and therefore by Doppler effect passengers in the westbound train would find an increase in sound.

Thanks,
Tuhin
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21 Dec 2009, 23:44
blows its horn as they pass

This means that the trains are passing each other and at that point of time the horn is honked.

The engine part of the train situated at the front will have the horn. This would mean the passengers on the westbound train will hear a honk from the engine of the east bound and as the engine passes the each compartment of the westbound train hence the passengers westbound train will experience the increase in the pitch as it passes.

As option B states the same, IMO it is B.
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22 Dec 2009, 06:10
The OA is A, it is a question from Manhattan GMAT questions bank, however the MGMAT staff admits this question is ambiguous, and should be revised to make it clearer. so my previous post makes sense.

Source :
As a new member, I can't post an U R L, but you can refer to Manhattangmat forum website, then serch the doppler effect
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22 Dec 2009, 06:56
shud be A .....
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22 Dec 2009, 11:01
I totally disagree with A, actually none of the answer choices seem correct.

Question states
1) sound higher than its true pitch as the car approaches,
2) sound the same as its true pitch as the car passes,
3) sound lower than its true pitch as the car travels away

All the answer choices say as they pass, The only valid answer is E, however it doesn't illustrates the effect of Doppler effect, as the listeners and the train are not in relative motion.
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28 Dec 2009, 14:33
IMO its A if i consider my prev knowledge of doppler effect but going LITERALLY by the question as pointed by atish it seems none of the choices...
Ambiguous it is then!!!
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29 Jan 2011, 02:37
One can find the ambiquity of this test question when one brings together opinions of ludwigfraboulet and atish above!

ludwigfraboulet wrote:
We can eliminate C, D, E because there is no mention of the second train, and to illustrate the Doppler effect, we need two sources to ensure the relativity between the source of the sound and the receiver of this sound.
About A, it's not so sure, because it's depend where you sit. For example if you sit at the end of the westbound train, you will hear a increase in pitch because the train is approaching, then a decrease in pitch because the train is going away. Therefore the answer can be also B, depending where you sit.

atish wrote:
All the answer choices say as they pass, The only valid answer is E, however it doesn't illustrates the effect of Doppler effect, as the listeners and the train are not in relative motion.
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31 Jan 2011, 07:43
I still remember Doppler effect until now. So A easily.
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31 Jan 2011, 18:15
Given the choices...A is best
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Re: The Doppler effect refers to the perceived change in pitch [#permalink]

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17 Nov 2012, 08:30
what kind of CR question is this?
under which category does this fall?
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Re: The Doppler effect refers to the perceived change in pitch [#permalink]

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24 Apr 2013, 00:01
Its A, which can be clearly inferred from the argument.
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Re: The Doppler effect refers to the perceived change in pitch [#permalink]

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24 Apr 2013, 12:38
I beg to differ from all of you. In case of E, since the passengers in the east bound train are sitting in the train, their relative velocity with respect to the train is zero. Hence, they should hear the horn at a steady pitch.

Hence, in my opinion answer should be E. Eagerly awaiting for the OA.
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Re: The Doppler effect refers to the perceived change in pitch [#permalink]

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24 Apr 2013, 20:35
' If the eastbound train blows its horn as they pass' Does this say, whether two trains are approaching or going away from each other?

If they are going away, Answer is (A) ===A==> <====B===
If trains are approaching, Answer is (B) <==B== ===A==>
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Re: The Doppler effect refers to the perceived change in pitch [#permalink]

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24 Apr 2013, 21:08
itsmeabhi99 wrote:
I beg to differ from all of you. In case of E, since the passengers in the east bound train are sitting in the train, their relative velocity with respect to the train is zero. Hence, they should hear the horn at a steady pitch.

Hence, in my opinion answer should be E. Eagerly awaiting for the OA.

Dear itsmeabhi99, the argument does not mention the effect that has been stated in option E. If it were in practical world, I would have said yes in a general discussion, but here we have to strictly stick to what has been mentioned in the stimulus.

I hope you get it right..
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Re: The Doppler effect refers to the perceived change in pitch [#permalink]

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03 Jun 2013, 13:14
hi,

eastward tr==========>
west word train <=1======2===.

1,2 represent passengers with the position of the trains.

for 1 pitch will decrease whereas for 2 it will increase(at least till train pass 2)
I feel OA would be E and question is ambiguous.
Re: The Doppler effect refers to the perceived change in pitch   [#permalink] 03 Jun 2013, 13:14

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