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.

Answer is supposed to be A. However strict reading of the passage suggests that the ptich "sound the same as its true pitch as the car passes". In order for A to be right you have to "assume" the train blows it's horn the entire time both trains are passing each other. The only option that does not violate the conditions is E - although it is a pretty bland answer as it does not illustrate the Doppler effect. I guess my issue is that the passage and question appear loosely worded

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.

Interestingly, (C), (D), (E) presents three different scenarios, one of which must be true regardless of other westbound, southbound, northbound trains, if Doppler Effect has to be true. According the the passage, (E) is the only logical one, I find true. (A) shoud not be true as the trains are passing, not have passed.

There might be some wording problems in the passage.

you are right that choice (e) is true. in fact, it's quite true - no doubt about it. but here's the problem: it doesn't satisfy the criterion of the problem, which is to provide 'another illustration of the effect described above'. choice (e) makes no reference at all to the westbound train, and so can't possibly serve as an illustration of the doppler effect (which absolutely requires TWO bodies to be in motion relative to one another).

you do, however, have a valid point about the wording of the problem; it should be more clear that the horn's sound begins while the trains are approaching one another and doesn't end until the trains are moving away from one another. i will make sure that such changes are implemented to the problem. thank you!

Yes I agree with Ron. I made the same mistake of choosing E b/c the logic holds true but doesn't illustrate the Effect .

My reasoning was

A cannot be true b/c

A person sitting at the very end of the train will hear an increase --same -- decrease in the pitch as the E train blows it's horn while passing.

For A to be true, the person has to sit at the front end of the train and the train has to start blowing it's horn after it passes him.

wrong, on two counts.

first, and much more importantly, the passage gives absolutely no reason to expect an increase in pitch. the passage states that the initial sound of the approaching entity is higher than the 'true' pitch, but this statement in no way implies that the pitch is actually increasing; it just starts out high.

second, that isn't the way the actual doppler effect works (passages will never misrepresent factual information). for you to perceive an increase in pitch, the approaching entity would have to be not only approaching you, but accelerating toward you while doing so.

you are semantically correct in having minor quibbles with the interpretation of 'as they pass', but i hope you'll agree with me that those quibbles are trifling in comparison with the MAJOR problems that exist in the other answer choices.

I have been very confused even after reading all replies until I notice the statement:
"For example, the siren on a passing police car will sound higher than its true pitch as the car approaches...."
means 2 things (Police Car and a person) are in the same direction (Parallel). Is that what the trick in this question?

I agree with fellow students. These questions must be answered: how long are the trains, where does the passenger sit, and when and for how long does the eastbound train blow the horn. Without explicit information, a mere mention of passing cannot determine increase or decrease of the pitch. Also, logic requires C,D,E to have one correct answer regardless of how scientists define Doppler effect: you can only have three choices: same, higher, lower.

See Ron's note above as to the issues with the wording of the problem and how it has to be amended to clear up the ambiguity.

Just a test-taking point: yes, it's the case that either C, D, or E must be true in the real world. But that doesn't automatically mean that one of them must answer the specific question asked.

The trickiest type of wrong answer on CR or RC is one that is true... but just doesn't answer the given question. If I ask you, "How old are you?" and you tell me, "You have brown hair"... well, yes, that's completely true. So is the response, "You're either taller than me or you're shorter than me." But neither response answers my question!

_________________
Stacey Koprince
Instructor
Director of Online Community
ManhattanGMAT

Today, I got this question in manhattan test. This question is incorrect and should be modified.

Reason: A or B can be correct depending on where the passenger is sitting.

Suppose, trains have partially crossed each other. In this case, person sitting in front will hear pitch decreasing, whereas someone in the end, will hear it increasing.

You should fix this question.

I believe this question is being revised to be more clear. However, you are reading too much into the question. The argument does not provide any information regarding the train length, NOR does it indicate the positioning of the horn (you are assuming it's at the front of the train). The answer choices basically imply that the westbound passenger (the observer) hears the horn as eastbound train passes. In this case, the pitch of the horn decreases.

_________________
Ben Ku
Instructor
ManhattanGMAT

I took the MGMAT CAT 2 today, the question is still not properly worded..

This one has been removed from the pool till it is fixed.

_________________
Stacey Koprince
Instructor
Director of Online Community
ManhattanGMAT

Under which category does this Question fall?
Is there any CR Question in OG which falls under the same category as the above?

Not really sure if I have come across any CR question in OG which asks us to provide an example.

This is more common in RC though.

i see no way that any answer we can provide to this question could possibly help you prepare for the GMAT more effectively. if you disagree, please demonstrate how and we'll be happy to answer the question..

_________________
Tim Sanders
Manhattan GMAT Instructor

by letting us know the category of the question, we could mark it in our error log and when we come back to check the log, we could use the data to solve more questions of this category, thereby fixing our flaws.