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On June 30 2012 06:24 klicken wrote:+ Show Spoiler +I don't know if you're trolls, stupid or ignorant. Here i drew you a pic, the only difference with the threadmill is that the wheels will spin twice as fast. The plane will still MOVE THE SAME SPEED RELATIVE TO THE AIR and thus take off as normal. ![[image loading]](http://i.imgur.com/OGPR8.png)
No shit if the wheels are spinning at twice the speed of the treadmill, the plane will take off.
The question was, if the wheels were spinning at the exact same perfect speed of the treadmill.
This can ONLY happen if the plane remains stationary relative to the air/observer.
That means the friction from the wheels rolling needs to cancel out the force applied by the propeller/engine of the airplane.
This is very very hard to do in the real world, but it's possible, and the question was structured such that this is the only possible scenario.
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On June 30 2012 06:19 Phael wrote:Then that invalidates the original premise of the question, which I quote here: Show nested quote +if a plane was on a conveyor belt, trying to take off, but the conveyor belt would match the speed of the planes wheels PERFECTLY in the opposite direction, would the plane ever take off? The only way that this condition is satisfied is if the plane is stationary on the treadmill, no other possible scenarios.
Sorry, I did not even notice but in this case the question is written incorrectly, which is why people have been saying it is an impossible scenario.
As soon as the engine generates force, the wheel speed will never match the treadmill speed. This can only happen if thee engine is not generating any force at all, and a plane cant take off without an engine. So, I guess you were correct in that the plane will not take off.
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.The question should be:
if a plane was on a conveyor belt, trying to take off, but the conveyor belt would match the speed of the plane PERFECTLY in the opposite direction, would the plane ever take off?
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United States7488 Posts
I've got a riddle
You have a thread which has posting guidelines to post riddles and answers to riddles only. There is a separate guideline that forbids arguing. How many posts does it take before posters begin breaking every rule simultaneously?
Answer:+ Show Spoiler +Seriously... get past the plane thing already
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Imagine this scenario:
You have a toy radio controlled plane, and it's sitting in a patch of ice. You have a leash attached to it, and you're standing outside the ice patch.
You turn the plane on, going forward, but pull the leash tight, so the plane is struggling against the leash. You are obviously significantly stronger than the plane so it doesn't move.
You pulling _________ plane's engine
<---------------------- Plane ------------------------>
Both forces cancel each other out, so plane doesn't move.
Obviously this is a possible scenario.
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Now lets extend it to the treadmill. If the wheels are rolling, there's going to be some frictional rolling resistance. The faster the wheels roll, the more resistance there is.
Imagine, if you will, you're holding a toy car (no engine) over a treadmill, holding it steady as the treadmill runs. At a low enough speed, it's easy. As the treadmill speeds up, there's more and more force trying to push the car back.
This is the same with the plane. Speed the conveyor belt up enough and you will get this scenario:
Rolling resistance ___ plane's engine
<---------------------- Plane ------------------------>
Where the forces cancel each other out perfectly, and bam, a plane with engine on but cannot fly.
The question can be worded in many ways. The one in which the riddle was posed gives the answer: no, it would not fly. The one that you posed would give the answer: yes, it would fly. Both are possible, neither question is "wrong" by itself ... it's definitely harder to meet the requirements of the original question in the real world, however.
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On June 30 2012 07:09 semioldguy wrote:I've got a riddle You have a thread which has posting guidelines to post riddles and answers to riddles only. There is a separate guideline that forbids arguing. How many posts does it take before posters begin breaking every rule simultaneously? Answer: + Show Spoiler +Seriously... get past the plane thing already
aahahaha. Great post
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On June 30 2012 05:01 Plexa wrote:+ Show Spoiler +On June 30 2012 02:43 mikell wrote:Show nested quote +On June 30 2012 00:13 klicken wrote:
Oh my god, not the airplane again.
Yes it will take off.
This is because the plane accelerates with its engines through the AIR, not with its wheels through the ground.
You could compare it with a very aerodynamical car driving in heavy headwind, since the car accelerates through the ground, and due to its aerodynamical shape is isn't affected much by the wind and thus would drive just fine. ??? the question states that the conveyor belt is moving at the exact same speed as the aeroplanes tyres. the aeroplanes tyres are the only thing propelling the plane forward. the THRUST from the ENGINES are TRANSLATED to the TYRES. if the TYRES are doing ZERO work then the PLANE is not TAKING OFF.LIFT is not generated unless the plane is moving at an appropriate speed. lift is not generated by the engines. the engines serve as a means to propel the plane forward. lift is created from the pressure difference between the top and bottom of the wings. this lift is not generated if the air is not moving over the wings of the plane (which would not be the case if the plane was on a conveyor belt and NOT moving relative to the air). i am not sure why this is so puzzling to some. and that mythbusters episode as above didn't have the conveyor belt moving at the same speed as the plane, if that was the case, the plane would not be moving relative to the observer at any point. This is the bit that is incorrect. The tyres are not propelling the plane forward, it's the engines. If you removed the tyres and placed the plane on a frictionless surface it would be able to take off. Indeed, if tyres propelling the plane were correct then how does a plane fly through air without the tyres propelling it forward? The tyres are just a way for the plane to move while on the ground, and in a way which reduces as much extra drag as possible. Think of it this way, say you were extremely high up in the air (stationary) and dropped a plane from this height. Assuming the plane stays upright (i.e. doesn't begin rotating as it falls), if the plane turns on its engine will it be able to begin flying mid air?The difficulty in understanding this problem (or maybe why the problem is ill posed) is that plane do not use their tyres to generate forward motion. If they did, they would take off and be unable to sustain their speed with their engines and immediately crash. Thus its impossible for a tyre propelled plane to even fly, let alone take off from a treadmill.
The plane structure functions as a whole. The velocity of the plane is increased via force produced by the engines. The lift generated is a function of velocity - not force exerted by the engines. The plane will not take off unless the plane is moving relative to the air at a speed high enough to achieve lift. The tyres are a way for the plane to achieve this speed on ground efficiently (without dragging its chassis along the ground). The only losses between the thrust force induced by the engine is from heat and sound (in general) and friction losses due to slip in bearings/gears and in main point the wheels.
We are assuming, in this question, that the plane is moving on a treadmill which matches its ground speed exactly. The question was stated this way.
Planes require a headwind to lift. As above. And as stated by others.
The only condition in which this plane-conveyor belt leaves with the plane taking off are if:
- The conveyor belt moves so fast it begins dragging air backward into the plane (like in an air tunnel)
- The conveyor belt is only moving at the speed at which a plane would normally take off
In the case of your hypothetical question - the plane will begin to move again as the thrust generated by the plane is not encountering any resistance at all during its acceleration to an appropriate speed.
Think of it this way:
A plane is taking off - and half way along it's journey, the tarmac is dragged backward halting the motion of the plane completely. Would it take off? No, as the plane is no longer moving relative to the air and therefore no lift is generated. The conveyor belt just does this constantly (assuming the conveyor belt is spinning freely with the wheels).
I'm not sure why this is so confusing to some. A planes engines don't generate lift, they generate thrust. The wings generate lift, and this is only achieved when there is enough air passing across the wings to remove the ground from the surface of the earth. After that, the only friction loss is due to air resistance.
A frictionless surface would be great, as then the planes wouldn't need any wheels at all.
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On June 30 2012 07:09 semioldguy wrote:I've got a riddle You have a thread which has posting guidelines to post riddles and answers to riddles only. There is a separate guideline that forbids arguing. How many posts does it take before posters begin breaking every rule simultaneously? Answer: + Show Spoiler +Seriously... get past the plane thing already
How did you get a Carcasonne piece as your picture?
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I have a problem with the solution of The Warden question on Pg. 12.
+ Show Spoiler +Nobody knows whether Switch A starts out as UP or DOWN.
We also do not know who of the twenty three inmates will go in first.
What if the first person was not the counter and saw that Switch A was down? He would then turn it back on, thinking the counter had turned it down already. Then, the counter would come in and see that the switch is on. He would never know whether or not he was the first one to enter the room or not, nor would he know that the switch had started down and someone had turned it up before him. This would screw up his count. The person who had gone first would never touch switch A again, leaving the count at 21 forever....
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On July 11 2012 13:59 vizi wrote:I have a problem with the solution of The Warden question on Pg. 12. + Show Spoiler +Nobody knows whether Switch A starts out as UP or DOWN.
We also do not know who of the twenty three inmates will go in first.
What if the first person was not the counter and saw that Switch A was down? He would then turn it back on, thinking the counter had turned it down already. Then, the counter would come in and see that the switch is on. He would never know whether or not he was the first one to enter the room or not, nor would he know that the switch had started down and someone had turned it up before him. This would screw up his count. The person who had gone first would never touch switch A again, leaving the count at 21 forever....
+ Show Spoiler +The solution is that you "count" through everybody twice, or if the switch started in the "count" position then you count everybody twice except for one person who you count once. You will always reach this number of counts, and you will never reach it before everyone enters at least once.
I am too lazy to get my words right to go into more detail.
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On July 11 2012 13:59 vizi wrote:I have a problem with the solution of The Warden question on Pg. 12. + Show Spoiler +Nobody knows whether Switch A starts out as UP or DOWN.
We also do not know who of the twenty three inmates will go in first.
What if the first person was not the counter and saw that Switch A was down? He would then turn it back on, thinking the counter had turned it down already. Then, the counter would come in and see that the switch is on. He would never know whether or not he was the first one to enter the room or not, nor would he know that the switch had started down and someone had turned it up before him. This would screw up his count. The person who had gone first would never touch switch A again, leaving the count at 21 forever....
+ Show Spoiler +Every non-counter flips the switch twice. There can be at most 1 flip before counter enters.
The counter counts 43 resets after his first visit, regardless of what he does on his first visit.
After 43 counts and no one has flipped before he enters 21 people would have flipped twice and 1 flipped once.
After 43 counts and someone flipped before he enters 22 would have flipped twice.
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On July 11 2012 16:13 GaiaCaT wrote:+ Show Spoiler +Every non-counter flips the switch twice. There can be at most 1 flip before counter enters.
The counter counts 43 resets after his first visit, regardless of what he does on his first visit.
After 43 counts and no one has flipped before he enters 21 people would have flipped twice and 1 flipped once.
After 43 counts and someone flipped before he enters 22 would have flipped twice.
+ Show Spoiler +Ah, I see. So rather than pulling once, each person pulls the lever twice to negate the problem caused by the specific scenario........very smart. Thanks
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On June 30 2012 06:28 Phael wrote:The question was, if the wheels were spinning at the exact same perfect speed of the treadmill.
This can ONLY happen if the plane remains stationary relative to the air/observer.
Only true for linear speed if you consider the speed of the wheel matches the speed of the plane, which is not a given. Let's say you block the wheels, so that they cannot move at all (park brakes). Your treadmill does not move either, so you fulfill your requirements. Is the power of the engines enough to reach takeoff speed on blocked wheels ?
(not taking into account vertical-takeoff planes :p)
To get a "real" answer, you have to look at the max propulsion force vs max braking force of the plane. If brakes can resist maximum thrust, your plane will stay stationary. If not, it will move and eventually take off.
Answer will vary from one plane to the next. The regulation on brakes for civil planes (JAR 25.735d) states:
The aeroplane must have a parking control that, when set by the pilot, will without further attention, prevent the aeroplane from rolling on a paved, level runway with take-off power on the critical engine
As far as I know, the main issue is with the "critical engine" part: This has been interpreted for most multi-reactor planes as a braking force sufficient for 1 full thrust engine. Meaning there are multi-reactor planes that could move on parking control with all reactors at full thrust (although there are systems to prevent that).
I would say:
- all 1 reactor civil planes with park brakes will not move on a static treadmill even with full thrust.
- some multi-reactors will start to move even with stopped wheels. Will they reach take-off speed ? Probably : as soon as speed creates a lift, maximum braking power diminishes
- most military jets will take off
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probably been posted before but here goes anyway
A prisoner in prison is forced to carry a heavy sack of sand across the courtyard and back every day. After a while he decides he cant manage it anymore, so he puts something in the sack to make it lighter. What does he put in?
+ Show Spoiler +not necessarily an object
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On July 11 2012 23:01 ThatGuy89 wrote:probably been posted before but here goes anyway A prisoner in prison is forced to carry a heavy sack of sand across the courtyard and back every day. After a while he decides he cant manage it anymore, so he puts something in the sack to make it lighter. What does he put in? + Show Spoiler +not necessarily an object
+ Show Spoiler +
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On July 11 2012 23:13 garbanzo wrote:Show nested quote +On July 11 2012 23:01 ThatGuy89 wrote:probably been posted before but here goes anyway A prisoner in prison is forced to carry a heavy sack of sand across the courtyard and back every day. After a while he decides he cant manage it anymore, so he puts something in the sack to make it lighter. What does he put in? + Show Spoiler +not necessarily an object + Show Spoiler +
yea, pretty easy one tbh ^_^
Ok heres another stupid one. What one word fills both gaps in this sentence
My _________ is _________than my car
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+ Show Spoiler +yea, pretty easy one tbh ^_^ Ok heres another stupid one. What one word fills both gaps in this sentence My _________ is _________than my car
I'm assuming it has to do with a brand of car? I'm not sure since there is almost nothing to go off of.
On June 30 2012 07:09 semioldguy wrote:I've got a riddle You have a thread which has posting guidelines to post riddles and answers to riddles only. There is a separate guideline that forbids arguing. How many posts does it take before posters begin breaking every rule simultaneously? Answer: + Show Spoiler +Seriously... get past the plane thing already
You aren't supposed to put in the answer in the question ;_;
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I'm looking for riddles with a theme of "confusing" wording, where just hearing the riddle can give you a headache. I'll give an example below. If anyone knows similar riddles, please post them as well.
What day would yesterday be if Thusday was four days before the day after tomorrow?
Answer: + Show Spoiler +
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On July 11 2012 23:22 ThatGuy89 wrote:Show nested quote +On July 11 2012 23:13 garbanzo wrote:On July 11 2012 23:01 ThatGuy89 wrote:probably been posted before but here goes anyway A prisoner in prison is forced to carry a heavy sack of sand across the courtyard and back every day. After a while he decides he cant manage it anymore, so he puts something in the sack to make it lighter. What does he put in? + Show Spoiler +not necessarily an object + Show Spoiler + yea, pretty easy one tbh ^_^ Ok heres another stupid one. What one word fills both gaps in this sentence My _________ is _________than my car
+ Show Spoiler +
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I have a clock with the hour hand pointing to 12.
If I move the hour hand forward by 5 hours it is at 5 O'clock, if I do this again it is at 10 O'clock, then 3 O'clock, then 8, 1, 6, 11, 4, 9, 2, 7 and then back again to 12 O'clock. Thus landing on every number on the clock once and only once.
Q1:
Which other numbers will this work for and what is the pattern?
Q2:
what about for a clock with M different positions where I move the hand by N positions each turn? prove that the pattern for Q1 is necessary and sufficient for the general case
Answers:
I forget... But I know that I have worked it out once. So if need be I could probably figure it out again.
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EPT
