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Hello:
I remembered yesterday an issue I have discussed intensely in the past but never solved to my complete satisfaction. Maybe someone here can help.
I would like to know how the following statement is to be read:
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“An airplane cannot take off from a runway which is moving backwards (like a treadmill)
at a speed equal to its normal ground speed during takeoff.â€
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1) An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to the airplane's normal ground speed during takeoff.
2) An airplane cannot take off at a speed equal to its normal ground speed during takeoff from a runway which is moving backwards (like a treadmill).
3) An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to the runway's normal ground speed during takeoff of any airplane.
I see the following differences:
1) The runway is moving backwards at a certain speed (equal to the airplane's normal ground speed) and the airplane cannot take off.
2) The runway is moving backwards (at any speed) and the airplane cannot take of at its normal ground speed.
3) The airplane cannot take off and the runway is moving backwards at a speed of zero since the runway's normal ground speed is zero.
Can the statement be read either way and be grammatically correct? Personally, I feel version 3 is grammatically the most correct version even though it is most contrary to what I assume is the writer's intention. Is there another possible version (even if it does not make sense physically) that is correct? Are multiple versions possible and acceptable grammatically? My spellchecker suggests that a comma is missing after the “which†or that the word “which†is replaced with “thatâ€. I feel cannot change the spelling of the sentence though. I would like to know how the current version is to be understood however clumsily it was written.
I have some opinions in regard to the technical issue involved. In my opinion, discussing or trying to solve this engineering challenge needs to happen after it is clear how the challenge is to be understood as it was written.
Welcome to the forum Karsten.
First, like many other test questions I've seen, even on the SAT, this is a poorly written question. I taught science for 30 years, wrote test questions for ACT, and have a background in physics, so I'm pretty sure I can help you out here. The main problem with that question is the ambiguity of the word "its." Depending on whether "it" is the plane or the runway, the interpretation would be different. Also, the question is what I'd call a run-on sentence. Finally, it appears to be intended as a "true or false" question, which are rarely pedagogically useful.
However, since the ground speed of a normal runway is zero, I think I see the question-writer's intent. I think that would be your version 1. If I rewrote that question, it would read like this:
An airplane's normal take-off speed is 100 mph. The airplane is placed on a runway that is moving backward like a treadmill at 100 mph. If the airplane is moving forward with a speed of 100 mph relative to the treadmill surface, will it be able to take off?
Now the situation is clear. Ambiguity is removed, and the answer should be apparent. That answer is clearly "NO." The airplane needs an airspeed of 100 mph to take off. A wheel-based speedometer would read 100 mph in the situation described, but an airspeed indicator (based on a pitot tube) would read zero.
Interesting. You accept that it is valid to have to guess in regard to the intent of the writer of the question. I struggle with this although I accept that it sometimes seems necessary. No language is perfect. I agree that my version 1 is probably the version most likely to be intended by the writers of the original statement. I think it is really not written to say that.
I think we both suspect that the treadmill is supposed to move backwards at the speed equal to the normal ground speed of the plane. But that would then not require that the airplane is required to take off at it normal ground speed. And if one accepts that the airplane is supposed to take off at its normal ground speed, the descriptor for any particular speed of the backwards moving runway falls away. The way it is written, "its" cannot describe the speed of the airplane AND the conveyor.
I agree this is a poorly written statement and that it hinges around the "its". I have also requested clarification regarding the definition of "ground speed" and received no answer. I can only assume that they mean it to be the speed of the airplane relative to the surface it sits on (as in measured by how fast the wheels turn while it is on the ground).
While I accept that the airplane will take off without great difficulties, it will not take off at its normal ground speed. Those little wheels will be spinning awfully fast until the plane gets airborne.
What is the best interpretation of the original statement if one DOES NOT try to interpret what the writers wanted to ask?
I agree with Heimhenge that the question is poorly worded and that the antecedent of "its" is vague, but I think logic can tell us the antecedent. Since, as has been pointed out, the runway has no ground speed, "its" must refer to the airplane.
However, the other vagary in this sentence is whether "the speed" is referring to the backward runway speed or the airplane speed. And even though I believe it refers to the runway speed, the more I read it, the more I can see it going either way, and this can make a difference in the answer. If the airplane is traveling at its normal ground speed, it can never take off because whatever speed the runway is going backwards, it is essentially slowing the plane by that much. If, however, the runway is traveling backward at the normal ground speed of the airplane, the plane would be able to take off by doubling its normal ground speed.
The statement, as Heimhenge as reworded it, is extremely clear because he assigns a speed to both the airplane and the backward runway. But if it is reworded like this:
An airplane's normal take-off speed is 100 mph. The airplane is placed on a runway that is moving backward like a treadmill at 100 mph. Will the airplane be able to take off?
This is also a clear question, although some may call it a trick question, with the opposite answer.
BTW I also object to the original statement being negative. It adds more confusion to an already confusing statement.
From my experience it is conventional, not exceptional, that 'it', 'its', 'he', 'his', etc. refers to one among multiple subjects and objects in the same statement — the way 'its' does in the test statement above.
The only question is does the context provide enough clues for the reference to be unambiguous? In the current example — I have to disagree with Karsten and Heimhenge and Dick here — the give-away is quite plain: 'its normal ground speed during takeoff.'
RobertB said:
From my experience it is conventional, not exceptional, that 'it', 'its', 'he', 'his', etc. refers to one among multiple subjects and objects in the same statement — the way 'its' does in the test statement above.
The only question is does the context provide enough clues for the reference to be unambiguous? In the current example — I have to disagree with Karsten and Heimhenge and Dick here — the give-away is quite plain: 'its normal ground speed during takeoff.'
Are you saying that the "its" refers to the airplane and the the ground speed mentioned does not describe the backwards speed of the runway?
Is it possible (or permissible) that the "its" refers to two items in one sentence at once?
Karsten said:
Does it seem proper to restructure the original statement (without adding or deleting words) as written below?
“An airplane cannot take off at a speed equal to its normal ground speed during takeoff from a runway which is moving backwards (like a treadmill).â€
There is still an ambiguity. Now we wonder if we're talking about ground speed while on that moving runway.
Face it … many sentences will depend on context to remove ambiguity, or become so complex the reader has to diagram them to figure out the meaning. Good writing demands that the reader be considered, and superfluous verbiage eliminated. I stand by my rewrite (2nd post in this thread) as an unambiguous restatement of the original question.
I have found that breaking a long sentence into two or three shorter sentences often helps in that regard. The antecedent of a pronoun should be clear and not leave the reader puzzling over its meaning.
Heimhenge said:
There is still an ambiguity. Now we wonder if we're talking about ground speed while onthat moving runway.
Face it … many sentences will depend on context to remove ambiguity, or become so complex the reader has to diagram them to figure out the meaning. Good writing demands that the reader be considered, and superfluous verbiage eliminated. I stand by my rewrite (2nd post in this thread) as an unambiguous restatement of the original question.I have found that breaking a long sentence into two or three shorter sentences often helps in that regard. The antecedent of a pronoun should be clear and not leave the reader puzzling over its meaning.
I agree that breaking the sentence into smaller sentences is better and that the statement is written badly but I feel I cannot just rewrite the whole thing or break it down. I am in the position to challenge (or accept) the statement, not to change the statement.
By the way, I agree that your rewritten statement is what the writers intended to say. I just feel they don't say it and I challenge them on those grounds.
I will be at peace soon. 
Karsten said:
Does it seem proper to restructure the original statement (without adding or deleting words) as written below?
“An airplane cannot take off at a speed equal to its normal ground speed during takeoff from a runway which is moving backwards (like a treadmill).â€
Heimhenge said:
There is still an ambiguity. Now we wonder if we're talking about ground speed while on that moving runway.
I do not agree that there is ambiguity. The reworded statement is true. Heimhenge wonders if we are talking about ground speed while on the runway, but the reworded statement says, "ground speed during takeoff from a runway which is moving backwards." You can not even try to take off from a runway unless you are on it. I believe this reworded statement is acceptable, but it is only one of many interpretations of the original statement.
BTW you can see Mythbusters interpretation and solution to this question here:
Robert said:
Maybe the word 'ground' is where some of the confusion is. Dropping it, you are left with a description of an airplane:
'its normal speed during take off'
There are at least two speeds with which an airplane's speed can be described. Air speed and ground speed. The ground speed is relevant here.
If you eliminate the reverse treadmill and talk about normal operation of an airplane, the ground speed and air speed are equal on a day with no wind blowing. If you have a headwind, the airspeed increases by the speed of the wind and you can take off at a slower ground speed. If you have a tailwind, the airspeed decreases by the speed of the wind and you must travel at a faster ground speed to take off. Obviously, headwinds are preferred.
This points out a fallacy in this statement. There is no such thing as "normal ground speed." I define normal as the speed at which takeoff occurs. Ground speed at takeoff will change depending on the speed and direction of the wind. Air speed at takeoff will remain the same and can have a "normal." This may be Robert's train of thought. But this contrived situation has no wind speed mentioned making us assume there is no wind, therefore ground speed and air speed are the same. Also, a moving runway must be factored in so ground speed is the only relevant speed that is needed here.
None of this has anything to do with the way the statement is worded, only to show that a contrived situation cannot account for every factor in real life. Also, I hope this shows Robert why dropping "ground" does not help to clarify the statement.
Don't you think all these talks about airplanes validate more and more the main point-- that the original statement with the 'its' unambiguously refers to the airplane?
If you purposely analyze every angle, every nuanced sense, probably 99.99% of the language has some amount of ambiguities. So at some point you got to employ the legal concept of 'beyond a reasonable doubt,' which I think is apt for this discussion.
Ps. Somehow I have different names depending on on mobile or laptop, so Robert is also me.
I think "its normal ground speed" definitely refers to the airplane's normal ground speed. But what can not be decided from the statement as originally written is, "What is traveling at the airplane's normal ground speed?" Is it the airplane or the backwards runway? I think this is not knowable until the statement is reworded. It goes back to your original post and the first two of three options. The answers to these two options will be different.
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