Tricky question...

[Temperature101]

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A uniformly accelerating plane will take off in the shortest time, if it flies:

A. with the wind.
B. against the wind.
C. at 90 degrees to the wind.
D. during a calm spell.

Answer in white: B

Should I expect stupid question like that in the actual test?

 

[LoLCareerGoals]

Ok seriously:

It flies? It is not flying yet. Take off? Take off from where from dead stop? Take off from the ground?
How does wind affect anything? From movies I know planes don't take off the ground until a certain speed is reached, so it must be true :rollyeyes.
So if wind is against it...(what kind of wind? what are wind's kinematics?) it would lower the acceleration? Which would increase the amount of time for the plane to reach the necessary take off the ground speed. Basically see jpeg above.
Are you sure that was all the info provided?


Edit: Got this off Yahoo answers. How the f are we supposed to know that? Is there a physics chapter on this ish I haven't gone over yet? I seriously hope you got this question off a bad source or haven't posted all the relevant information.

"All a plane cares about is AIRSPEED, not ground speed
If a plane can rotate(pilot word for take off)at 150 mph, that means a 150 mph airspeed. So hey, if the winds were 150mph the plane would take off with NO ground speed, it'd lift straight up!(but 150mph winds would obviously mean like a hurricane, but you get my point I hope)"

 

[Temperature101]

Ok seriously:

It flies? It is not flying yet. Take off? Take off from where from dead stop? Take off from the ground?
How does wind affect anything? From movies I know planes don't take off the ground until a certain speed is reached, so it must be true :rollyeyes.
So if wind is against it...(what kind of wind? what are wind's kinematics?) it would lower the acceleration? Which would increase the amount of time for the plane to reach the necessary take off the ground speed. Basically see jpeg above.
Are you sure that was all the info provided?


Edit: Got this off Yahoo answers. How the f are we supposed to know that? Is there a physics chapter on this ish I haven't gone over yet? I seriously hope you got this question off a bad source or haven't posted all the relevant information.

"All a plane cares about is AIRSPEED, not ground speed
If a plane can rotate(pilot word for take off)at 150 mph, that means a 150 mph airspeed. So hey, if the winds were 150mph the plane would take off with NO ground speed, it'd lift straight up!(but 150mph winds would obviously mean like a hurricane, but you get my point I hope)"

This is from TBR physics...Chapter 7 (Fluid mechanics). Passage VI # 42 (2009 edition).

 

[BoxVersionAce]

This is just a terrible question. It is so odd and vague that isn't even specific enough for the AAMC style, more like random EK 1001 problems. TBR even took this problem out of the newer edition. Probably because its so vague. I wouldn't say this question is tricky, rather than just one of those things you read about.

Its a question of Bernoulli's principle and other complicated aspects of fliud dynamics that we just don't need to know. Just in general, wings of airplanes are designed in an asymmetric manner so that fluid flow (air) on top of the wing follows a different path than on the bottom (imagine streamlines). Theoretically, air would flow faster on the top side. Relating this to Bernoulli's principle, faster air flow dictates lower air pressure. This generates a pressure gradient that causes a "lift force." The way I look at is that fluid flows from areas of high pressure to low pressure. Air pushes up on the wing to move from the high pressure bottom to the top. This is most definitely a simplified explanation that is wrong in some fundamental ways.

In this problem, a lift force would probably be generated in both option A and B. Not sure about C or D (but they just look like terrible answers). Flying against the wind would generate the pressure gradient sooner by increasing the speed of the wind relative to a plane flying with the wind.

 

[cheesier]

I actually think it's a good intuitive problem. First off, it says uniformly accelerating, so you can fairly assume that the plane won't pick up speed if the wind is behind it. That means that when the nose of the plane lifts up off the ground, a headwind gives it more lift. You don't really need to think about pressure gradients or airplane design, it just intuitively makes sense that a tailwind would make lift more difficult because if wind is flowing over the tops of the wings in the same direction that the plane is moving, you need more force to lift the front of the plane.

 

[BoxVersionAce]

I actually think it's a good intuitive problem. First off, it says uniformly accelerating, so you can fairly assume that the plane won't pick up speed if the wind is behind it. That means that when the nose of the plane lifts up off the ground, a headwind gives it more lift. You don't really need to think about pressure gradients or airplane design, it just intuitively makes sense that a tailwind would make lift more difficult because if wind is flowing over the tops of the wings in the same direction that the plane is moving, you need more force to lift the front of the plane.

Why does the nose of the plane lift up off the ground again?

 

[LoLCareerGoals]

I actually think it's a good intuitive problem. First off, it says uniformly accelerating, so you can fairly assume that the plane won't pick up speed if the wind is behind it. That means that when the nose of the plane lifts up off the ground, a headwind gives it more lift. You don't really need to think about pressure gradients or airplane design, it just intuitively makes sense that a tailwind would make lift more difficult because if wind is flowing over the tops of the wings in the same direction that the plane is moving, you need more force to lift the front of the plane.

I would agree if the problem stated: "As the nose of the plane lifting up, which wind direction helps the nose lift up faster." How are we supposed to know what "take off fastest" mean? From dead stop to losing touch with the ground? From first point of breaking contact with the ground to being fully airborne? Terrible question.

 

[cheesier]

Why does the nose of the plane lift up off the ground again?

During takeoff? All I could really tell you is that the force on the lower side of the wing is greater than on the upper when the nose lifts, but you don't need to know that much physics to answer this question. If you've been in an airplane before you should be able to reason out the answer.

 

[BoxVersionAce]

During takeoff? All I could really tell you is that the force on the lower side of the wing is greater than on the upper when the nose lifts, but you don't need to know that much physics to answer this question. If you've been in an airplane before you should be able to reason out the answer.

Point being... Relating to the question at hand, what is the difference between takeoff and the nose lifting off the ground? Just trying to understand your logic.

Also, what presuppositions are you allowed to make when answering this question?

 

[cheesier]

Point being... Relating to the question at hand, what is the difference between takeoff and the nose lifting off the ground? Just trying to understand your logic.

Also, what presuppositions are you allowed to make when answering this question?

My interpretation was, from the time the nose lifts up to the time the plane is airborne is the total time for takeoff. That's really the only thing that takeoff could mean in the context of this question.

It's the mcat. You obviously don't need to know about complicated fluid dynamics, airplane design, or complex applications of bernoulli's principle, so you'll just run down the clock if that's your train of thought with a question like this.

My assumptions were:

1) Acceleration, and therefore velocity, is the same at the time that the plane starts to lift up in all of the scenarios.

2) The more lift the plane has, the easier it is to get it off the ground, and the faster it takes off.

From there, if you've ever been in a plane before, you can just reason out the answer. When the plane starts to lift up, a headwind will push on the underside of the wings (versus on the upper side of the wings in a tailwind) and give it lift (with a tailwind it would push the plane down). Giving it lift means it gets off the ground faster.

 

[nowanmd]

just think about flying a kite - you run INTO the wind with your kite to get it to fly if you run with the wind-it ain't gonna fly 🙂

 

[dontbeanegaton]

Simplest explanation: Takeoff occurs when the air moving over the wings hits a critical speed. Speed of air over the wings is velocity of plane - velocity of air (if air is in same direction).
It's that simple.

 

[BoxVersionAce]

My interpretation was, from the time the nose lifts up to the time the plane is airborne is the total time for takeoff. That's really the only thing that takeoff could mean in the context of this question.

It's the mcat. You obviously don't need to know about complicated fluid dynamics, airplane design, or complex applications of bernoulli's principle, so you'll just run down the clock if that's your train of thought with a question like this.

My assumptions were:

1) Acceleration, and therefore velocity, is the same at the time that the plane starts to lift up in all of the scenarios.

2) The more lift the plane has, the easier it is to get it off the ground, and the faster it takes off.

From there, if you've ever been in a plane before, you can just reason out the answer. When the plane starts to lift up, a headwind will push on the underside of the wings (versus on the upper side of the wings in a tailwind) and give it lift (with a tailwind it would push the plane down). Giving it lift means it gets off the ground faster.

Yes, you don't need to know complicated applications of bernoulli's principle for the mcat. But I guess its just good nature to try to go further than intuition/logic when studying, even if that is all you have in the end.

From what you say, isn't it feasible to say that the earlier the nose of an airplane would lift, then faster the time it takes to have takeoff? Is it possible that the airplanes have their noses lifting off at different times? If so, wouldn't this be an important part to consider with intuition?

 

[cheesier]

From what you say, isn't it feasible to say that the earlier the nose of an airplane would lift, then faster the time it takes to have takeoff?

I think so.

Is it possible that the airplanes have their noses lifting off at different times? If so, wouldn't this be an important part to consider with intuition?

Imagine they all pull the control yoke back at the same time and the noses all start to lift. They have to reach some angle before they actually are airborne. The headwind will push the plane back and it will become airborne faster.

 

[BoxVersionAce]

Woah, woah, woah. I for one know nothing about piloting a plane or a control yoke. Is that necessary for this question? For the mcat? The control essentially allows the nose of the plane to be lifted by natural forces?

 

[cheesier]

Woah, woah, woah. I for one know nothing about piloting a plane or a control yoke. Is that necessary for this question? For the mcat? The control essentially allows the nose of the plane to be lifted by natural forces?

No it's not necessary lol. I had to google 'plane controls' to find out what the joystick thing is called. I again have no idea how a plane works but at the risk of sounding like a broken record if you've seen one take off before or ridden in one you know what you need to know to answer this question. I stand by it being a fine question, and I have laid out the very simple reasoning behind my answer choice.

 

[BoxVersionAce]

No it's not necessary lol. I had to google 'plane controls' to find out what the joystick thing is called. I again have no idea how a plane works. I stand by it being a fine question, and I have laid out the very simple reasoning behind my answer choice.

Ahh I see. I feel like they could have done a better job in asking the question. There may be more than one way in explaining scientific phenomena but mcat questions are better when it can be interpreted by all (withstanding those without strong science backgrounds). Which, for the most part AAMC does a good job of doing. Perhaps it'd be better to ask a question that specifically inquires about Bernoulli's principle/streamlines. I stand by the jpeg a poster used above.

 

[gtbROX]

The question isn't as "stupid" as people say it is. I'll admit that it's unnecessarily hard/tricky, but it's definitely not incorrect....
A plane flies because the speed of air above the wings is moving faster than the speed of air below; this leads to the whole bernoulli effect of the pressure difference above and below the wings. There's a certain threshold pressure gradient that you need to take off which is roughly (if I'm not remembering it incorrectly):
(Pbelow-Pabove)*Area of Wing > Weight of the Plane
If the plane is on the ground, it can take off faster if it is moving faster (wings going through air molecules to cause bernoulli effect). This is effect is aided by a wind velocity against plane because that means that the air is flowing past the wings so less velocity is needed to actually take off meaning it takes less time for the plane to accelerate to the point at which it can take off. I hope this helps.

 

[BoxVersionAce]

The question isn't as "stupid" as people say it is. I'll admit that it's unnecessarily hard/tricky, but it's definitely not incorrect

Its not hard or tricky. Its not a good question, in trying to ask about the very principles you've tried to describe in your post. The posts above explain why this question isn't good.

 

[gtbROX]

Its not hard or tricky. Its not a good question, in trying to ask about the very principles you've tried to describe in your post. The posts above explain why this question isn't good.

My definition of hard/tricky questions are questions that expect the test-taker to bring in many concepts just to get an answer. In terms of the concepts behind it, its not beyond the scope of what they expect us to know. I'll clarify my original post to this:
-Bernoulli's Equation
-P/A=F
Understanding both concepts, versus just memorizing, could have led anyone to the correct answer.
Their Qstem was straightforward and not badly worded, but I will agree that they should have put this Q in a passage with formulas.