Author Topic: Logic Puzzles  (Read 24128 times)

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Offline finlay

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Re: Logic Puzzles
« Reply #15 on: July 18, 2011, 02:02:15 pm »
I'm going to hazard a guess at yes, but only because my instinct says no and that's how these questions normally work. :P

Offline Simon

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Re: Logic Puzzles
« Reply #16 on: July 18, 2011, 02:30:06 pm »
No/Unsure. The airplane lifting force comes from interaction of wings and their surrounding air, and the wheels/conveyor belt are not part of this separate system. Conversely, the plane would lift with still wheels if there was a large fan blowing air towards it.

Unsure because it's not the wheels, but the the englines on the wings who provide acceleration. They shouldn't move enough air surrounding the wings though, they only pull in the air in front of them.

alfonz: I did not attempt the water level puzzle seriously before seeing its solution for the first time. I don't know whether I would have solved it correctly back then.

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Offline Johannes

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Re: Logic Puzzles
« Reply #17 on: July 18, 2011, 06:07:00 pm »
Someone's a Mythbusters fan :P

In which case I have to ask, how do you define "matching speed" of the conveyor belt?
Velocity of belt backwards = velocity of plane forwards, in regards to the ground?

Offline alfonz1986

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Re: Logic Puzzles
« Reply #18 on: July 18, 2011, 06:17:08 pm »
hehe, Ive seen mythbusters, but this puzzle far outdates that show. I guess if you saw that episode you know the answer, so dont tell the others yet :)

Offline ccexplore

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Re: Logic Puzzles
« Reply #19 on: July 19, 2011, 01:30:51 am »
Hey alfonz1986, based on people's question and response, I think part of your phrasing of the question caused some confusion.  Having seen the Mythbuster episode (well I've seen all of them from the beginning, and yeah, most of what they feature have to start off as a relatively well-known myth first, so the myth pretty much always outdates the corresponding TV episode), let me rephrase your puzzle to precisely describe the situation demonstrated on that episode (unless you purposely intended for your puzzle to differ from the myth as presented on TV):

[edit: completely rephrase again after Johannes corrected some fundamental misceptions I have around the setup of the scenario]

Imagine two absolutely identical airplanes.  Airplane "A" sits on a normal runway, while "B" sits on a giant conveyor belt.  We now start and turn up both airplane's propellers at completely matching rates at all times.  Obviously, A will start to move forward and eventually lift off.

Now imagine that with B, while we are ramping up its propellers at matching rate with A, we also have the conveyor belt turned on, and its speed ramp up at a rate always matching the speed of A, but going backwards.  What will happen with airplane B?  Can it takeoff or not, and why?

Offline Johannes

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Re: Logic Puzzles
« Reply #20 on: July 19, 2011, 08:28:48 am »
the conveyor belt ... speed is continuously adjusted to match the airplane's speed but in the opposite direction, so that relative to the runway, the airplane is neither moving forward nor backwards.
Not quite - the way you describe it it's quite obvious that the plane won't take off. The speed of the conveyor belt is adjusted to run backwards as fast as the plane moves forwards, yes. The question is, does this keep the plane stationary?

Offline ccexplore

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Re: Logic Puzzles
« Reply #21 on: July 19, 2011, 09:32:52 am »
the conveyor belt ... speed is continuously adjusted to match the airplane's speed but in the opposite direction, so that relative to the runway, the airplane is neither moving forward nor backwards.
Not quite - the way you describe it it's quite obvious that the plane won't take off. The speed of the conveyor belt is adjusted to run backwards as fast as the plane moves forwards, yes. The question is, does this keep the plane stationary?

Hmm, I sort of see your point now.  I think I sort of misunderstood this aspect of the setup but to be honest, I'm still not sure I quite get all the physical interactions around this scenario the more I think about it.

I've edited my scenario description hopefully to better capture the setup...maybe.  I need to think more about it. :-\

Offline Clam

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Re: Logic Puzzles
« Reply #22 on: July 19, 2011, 09:41:57 am »
(Not sure how "logic puzzles" came to mean physics problems, but anyway... :P)

This all depends whether you're talking about the plane's velocity relative to the ground, or to the conveyor belt. "Speed" isn't a good term to use, since movement has to be measured relative to something. That said, given what's been said in the above posts, I'm guessing we mean velocity relative to the ground.

Relative to the conveyor belt, the plane is travelling at twice its apparent (relative to the ground) speed: enough to balance the action of the conveyor belt, and the same amount again to produce actual movement (which, again, is what I'm guessing the conveyor belt is supposed to match). In other words, the conveyor belt effectively halves the speed of the plane. So, assuming the plane needs to attain a given velocity relative to the ground in order to take off, it needs to be able to attain twice that speed on a normal runway without the conveyor belt. Whether it can do this probably depends on the plane in question. (For example, if it's a Harrier Jump Jet, the entire question is moot since it can take off vertically :P)

Offline ccexplore

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Re: Logic Puzzles
« Reply #23 on: July 19, 2011, 09:50:41 am »
(Not sure how "logic puzzles" came to mean physics problems, but anyway... :P)

Well, so far only alfonz1986 has actually been posing physics problems.  Not that I mind; while not what I think of as "logic" puzzles, they can nevertheless be pretty thought-provoking, simply because our human intuition about physics is not very good, and at the same time, accurate applications of the various principles, laws and equations of physics can be subtle sometimes.

Anyway, I completely re-edited how I framed the problem, because after what Johannes said, I feel my original way of trying to frame the problem may actually be totally off from what's intended, despite reading superficially similar.  I'm also going to post a follow-up question (in spoilers to avoid hinting one way or another at original problem) to see if I really understand this better now. :-\

Offline Simon

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Re: Logic Puzzles
« Reply #24 on: July 19, 2011, 10:30:10 am »
(Not sure how "logic puzzles" came to mean physics problems, but anyway... :P)
they can nevertheless be pretty thought-provoking, simply because our human intuition about physics is not very good, and at the same time, accurate applications of the various principles, laws and equations of physics can be subtle sometimes.

Hrm. The main thoughts they provoke are clarification requests. :> And without grinding through enough theory, one can never verify answers despite being interested in it, only believe the eventually presented solution. Both issues (clarification and relative theory-heaviness) rarely happen with logic puzzles; maybe sometimes with math (note that I require being interested in the question at hand), but still much less than with physics.

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Offline alfonz1986

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Re: Logic Puzzles
« Reply #25 on: July 19, 2011, 11:01:39 am »
@CCE The newly editted way you describe the problem is basically identical to the way I described it. I guess these are physics related problems but I don't believe you need to have studied much physics to solve it if you have strong logic.

@ClamSpam The plane's velocity relative to the ground is what we're talking about in this case. Assume the plane is a small aircraft with two propellers, not a harrier jump jet  :)

I'll give you guys a hint (invisible) Imagine yourself in a swimming pool up to your armpits in water. You're standing on a conveyor belt on the swimming pool floor, wearing a pair of rollerblades. If the conveyor turns on, what will happen? If you breakstroke forward and the conveyor matches your speed, what will happen?

Offline ccexplore

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Re: Logic Puzzles
« Reply #26 on: July 19, 2011, 11:29:18 am »
@CCE The newly editted way you describe the problem is basically identical to the way I described it.

I agree. I didn't think your original phrasing was ambiguous, it's just that some of the first answers and questions to your original phrasing makes me wonder if people even understand the basic description correctly.  My 1st rephrasing is mainly an attempt to describe the frame of reference more precisely, to eliminate confusion about "speed" (and to make it clear that the airplane propellers are indeed in operation).  And then Johannes raised a point about my 1st rephrasing which makes me think (I'm still thinking :-\) maybe it is a bad formulation, so then in my 2nd rephrasing, I decided to use 2 airplanes as a way around his objections.

I guess these are physics rated problems but I don't believe you need to have studied much physics to solve it if you have strong logic.

I'm not so sure, I think you may be overestimating people's ability to properly analyze physical situations.  Our intuition in most cases are just plain wrong, as history have readily shown through the "natural philosophies" of Aristotle etc.

But I never majored in physics, so maybe you can enlighten me.  For example, how would you explain your 1st problem about water level to someone w/o asserting Archimedes' principle?  (or alternatively, how to explain that principle purely in terms of logic?)

Offline Johannes

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Re: Logic Puzzles
« Reply #27 on: July 19, 2011, 11:36:28 am »
I've edited my scenario description hopefully to better capture the setup...maybe.  I need to think more about it. :-\
I really like your new scenario description, as it still poses the same core problem but avoids all the definition issues and physical impossibilities usually accompanying this "riddle".  :thumbsup:

Offline Clam

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Re: Logic Puzzles
« Reply #28 on: July 19, 2011, 11:52:16 am »
I'll give you guys a hint (invisible) Imagine yourself in a swimming pool up to your armpits in water. You're standing on a conveyor belt on the swimming pool floor, wearing a pair of rollerblades. If the conveyor turns on, what will happen? If you breakstroke forward and the conveyor matches your speed, what will happen?

Well...
Quote from: in response to hint
When the conveyor belt turns on, won't the wheels of the rollerblades just spin, while you go nowhere? Doesn't this analogy break down because of (a) the massive difference in density between air and water (and thus force required to move you through it), and (b) your buoyancy in water, which takes much of the weight off your feet?

Anyway, is the assessment in my above post (twice normal take-off velocity, otherwise you stay on the ground) correct?


Also, I agree with Simon's sentiment that physics puzzles will result in a lot of back-and-forth and clarifications. But, on the other hand, they are fascinating in their own right :)

Offline Johannes

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Re: Logic Puzzles
« Reply #29 on: July 19, 2011, 12:10:26 pm »
Anyway, is the assessment in my above post (twice normal take-off velocity, otherwise you stay on the ground) correct?

No, not correct.

Quote
Hint: Is the aircraft exerting a force (aka pushing) on the conveyor belt to move forward?