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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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17 Jun 2008, 11:40
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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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17 Jun 2008, 11:48
It's A.

Quote:
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. (true, and the effect is illustrated))

B If the eastbound train blows its horn as they pass, passengers on the westbound train will hear a sound that increases in pitch. (not true: if the train travels away the sound should be lower)

C If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that decreases in pitch. (not true: passengers on the eastbound train are not in motion relative to the train they are in)

D If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that increases in pitch. (not true: passengers on the eastbound train are not in motion relative to the train they are in)

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. (that is true, but not illustrate the Doppler effect)

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17 Jun 2008, 11:50
Confusing....

Could come down to A and E, but from there had a difficult time to eliminate, but then convienced my self to pick A b'coz E talk about steadt pitch, but the arg says true pitch. True pitch is different from steady pitch.

(let me know if i'm wrong)

Last edited by sumanamba on 17 Jun 2008, 12:17, edited 1 time in total.

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17 Jun 2008, 11:53
http://www.gmatclub.com/forum/11-p476288

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17 Jun 2008, 12:14
A it is. I have to admit that my first pick was E though.

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17 Jun 2008, 14:07
fresinha12 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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17 Jun 2008, 14:11
C, D & E are out. In the example given it tells us how an "observer" hears the pitch of an approaching police car.
It never talks about how the person sitting in the police car hears it. C, D and E the observer is sitting in the East Bound Train analogous to the police car.

B is out because the pitch would start off higher as they approach, and drops as they go past each other. So its decreasing not increasing.

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19 Jun 2008, 08:55
Not a very clean OA here...

refer to the old discussion that we had on this one...I am just raising the same concern I had before

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.

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Re: CR-mgmat trains   [#permalink] 19 Jun 2008, 08:55
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# The Doppler effect refers to the perceived change in pitch

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