Interesting physics/logic riddle

[AdnanVCS]

It depends, how long is the conveyer belt? :tongue: If its not longer than effective minimum takeoff roll it won't matter.

*like someone else said, assuming it can overcome the 'treadmill' and gain sufficient airspeed. Ground speed is irrelevant.*

Hmmm forgot this:
This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

Again this would be a good one for Mythbusters: http://dsc.discovery.com/fansites/mythbusters/mythbusters.html
This is the page for show ideas:
http://community.discovery.com/1/OpenTopic?a=frm&cdra=Y&s=6941912904&f=9701967776

 

[DocL]

Again this would be a good one for Mythbusters: [/QUOTE] I don't think that it's a...ould be a good one for Mythbusters. :biggrin:

 

[AdnanVCS]

I don't think that it's a big enough myth for them to test. On the other hand, the myth that an Indy car has so much down force that it can drive upside-down would be a good one for Mythbusters. :biggrin:

It's allways worth a try!

 

[Aero]

well in that case its a simple problem if the plane is kept stationary by the treadmill it won't go anywhere. Just like if you keep the brakes on.

but it would waste a lot of fuel.

 

[Vega$ NSX]

Guys, not to sound cocky but the answer is in my post above. You are arguing the solution to a question that doesn't have enough given information to make an answer. All answers can be justified depending on what set of assumptions you want to take. It's like if I said there is a red house, a blue house and a third house. What color is the third house? You can't rationalize a definitive answer.

 

[Vega$ NSX]

well in that case its a simple problem if the plane is kept stationary by the treadmill it won't go anywhere. Just like if you keep the brakes on.

but it would waste a lot of fuel.

The initial question doesn't say the plane stands still (you are assuming that), it says that the conveyer moves backwards at the same speed as the wheels are trying to move forward. If the wheels were completely frictionless, the conveyer would exert no forces to hold the plane back. Study an engineering statics and dynamics class if you need proof.

 

[Ski_Banker]

The initial question doesn't say the plane stands still (you are assuming that), it says that the conveyer moves backwards at the same speed as the wheels are trying to move forward. If the wheels were completely frictionless, the conveyer would exert no forces to hold the plane back. Study an engineering statics and dynamics class if you need proof.

Yeah, but what if the wheels on the plane are on backwards!!? Oh yeah, who's yo daddy now...

:biggrin:

 

[clr1024]

I don't think that it's a big enough myth for them to test. On the other hand, the myth that an Indy car has so much down force that it can drive upside-down would be a good one for Mythbusters. :biggrin:

Thats not a myth, at least not for formula one...assuming we are talking about the same thing though. Its part of the reason they can corner so well, increased normal force equates to increase in the friction between the tires and the pavements.

 

[Malibu Rapper]

Guys, not to sound cocky but the answer is in my post above. You are arguing the solution to a question that doesn't have enough given information to make an answer. All answers can be justified depending on what set of assumptions you want to take. It's like if I said there is a red house, a blue house and a third house. What color is the third house? You can't rationalize a definitive answer.

I agree, we don't know the model of the plane, the weight, the thrust, the friction, etc. If you have an extreme condition on any of those factors it can completely change the outcome. Plus who is to say that the riddle presented at the beginning hasn't gone through the game of telephone with its presentation.

 

[Aero]

The initial question doesn't say the plane stands still (you are assuming that), it says that the conveyer moves backwards at the same speed as the wheels are trying to move forward. If the wheels were completely frictionless, the conveyer would exert no forces to hold the plane back. Study an engineering statics and dynamics class if you need proof.

Needless to say I've done my share of engineering classes. :tongue: but like you said its in what assumptions are taken from the way the problem is stated.

 

[jond]

No. The plane will be stationary and thus no wind over the wings to create lift.

 

[docjohn]

Did'nt Capt. Kirk solve the unsolveable with the Kobiashi-maru test,,kinda like this one

 

[TomCat]

"The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"

Aircraft motion, relative to the air mass, creates lift...not wheel speed, etc...
Therefore, if the airplane moves ("The plane moves in one direction") it flies! (assuming no excessive friction, etc...). If however, a big FAN (not the conveyer belt) pushed air to matched the forward velocity...the aircraft would "hover" in the moving airmass over the ground.

 

[12AMNSX]

Engines, either prop or jet, do not make an airplane fly (ie gliders, space shuttle coming back from space). The shape of the aircraft, wings or lifting body, moving forward through the air is what creates lift. The speed of the ground moving under the plane is irrelevant. Therefore, if the aircraft is moving forward through the air fast enough to create enough lift to overcome the weight of the plane, it will fly.

 

[12AMNSX]

Here is another one for you to ponder.

Suppose there is a scale that is incredibly accurate and can weigh the weight of a plane while in flight. (Or, another way to look at it, can measure the amount of lift being created). Now suppose, there is a fly flying around in the airplane while it is flying. You take a weight measurement and record it. Now, the fly lands on the floor in the plane while the plane is still flying. You take another weight measurement. Does the plane weigh more now that the fly has landed than it did when the fly was flying around?

 

[robr]

I've followed this whole thread and the off-site explanation but I'm still confused. I see people arguing about the plane being stationary vs. it's not, but wheels turn faster.

Do I misunderstand this piece of the puzzle? This sounds to me like we're being told the plane is being kept stationary:

"This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)"

If plane speed = X, the conveyor moves at speed of -X. I interpret this to mean that the plane is always stationary. It doesn't mention wheel speed, only plane speed. What am I missing?

 

[Aero]

Here is another one for you to ponder.

Suppose there is a scale that is incredibly accurate and can weigh the weight of a plane while in flight. (Or, another way to look at it, can measure the amount of lift being created). Now suppose, there is a fly flying around in the airplane while it is flying. You take a weight measurement and record it. Now, the fly lands on the floor in the plane while the plane is still flying. You take another weight measurement. Does the plane weigh more now that the fly has landed than it did when the fly was flying around?

Yes the fly increases the weight when it lands. And if the Aero engineers didn't account for new lift required it the plane will crash immediately.

 

[12AMNSX]

I've followed this whole thread and the off-site explanation but I'm still confused. I see people arguing about the plane being stationary vs. it's not, but wheels turn faster.

Do I misunderstand this piece of the puzzle? This sounds to me like we're being told the plane is being kept stationary:

"This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)"

If plane speed = X, the conveyor moves at speed of -X. I interpret this to mean that the plane is always stationary. It doesn't mention wheel speed, only plane speed. What am I missing?

The riddle does not state if the plane is kept stationary by some means and leads you to ASSUME that the plane remains stationary due to the movement of the conveyor belt. But, if indeed, it is only a conveyor belt under the wheels and you ASSUME that there is nothing attached to the plane to hold it still, it will move forward, create lift, and fly.

 

[12AMNSX]

Yes the fly increases the weight when it lands. And if the Aero engineers didn't account for new lift required it the plane will crash immediately.

You may have been snoozing in your engineering class when they covered this topic. :biggrin:

 

[AdnanVCS]

Yes,

A VSTOL style air craft does not need a run way! so it would work great with this one. But those kinda plains don't count in this one i find!

 

[Aero]

You may have been snoozing in your engineering class when they covered this topic. :biggrin:

I've taught the class. Get on a scale. Jump up in the air. What does it read when you're in the air. Same exact thing. Different point of reference. And the the lift/generated required (pushing down on the floor) to keep the fly aloft does not directly equal the same mass required as the fly at rest.

 

[AdnanVCS]

Here is another one for you to ponder.

Suppose there is a scale that is incredibly accurate and can weigh the weight of a plane while in flight. (Or, another way to look at it, can measure the amount of lift being created). Now suppose, there is a fly flying around in the airplane while it is flying. You take a weight measurement and record it. Now, the fly lands on the floor in the plane while the plane is still flying. You take another weight measurement. Does the plane weigh more now that the fly has landed than it did when the fly was flying around?

Nice one!

Yes,

The plain would weigh more because the fly is no resting on it so the plain needs to resist it's gravitational pull!

That's why when a plain is in parabolic flight you feel no gravity as you are actually dropping to earth at the same speed as the air plain.

 

[FeetFxr]

If planes could take off, and potentially land, this way then why do we use or need long runways at airports or aircraft carriers? I'm not an engineer, but would have to assume that most jet engines cannot easily create enough air flow over their wings, basically sitting still, to lift off. Otherwise, Harrier jets and other vertical take-off jets wouldn't need nozzles that can turn vertically to the ground for take off. I would have to assume that a plane taking off this way would be very unstable, dangerous and difficult to control.

 

[12AMNSX]

I've taught the class. Get on a scale. Jump up in the air. What does it read when you're in the air. Same exact thing. Different point of reference.

Well, you are more of an expert than me.

However, the difference is that by jumping in the air you are not creating lift, you are just a mass in motion. A fly flying around, in an airplane or otherwise, IS creating lift So, the answer is that the plane will weigh exactly the same whether the fly is flying or sitting on the floor. While flying, the wings are creating a downward force sufficient to lift the weight of the fly. That downward force would be recorded by the scale just as if the fly were sitting on the floor.

 

[clr1024]

I've taught the class. Get on a scale. Jump up in the air. What does it read when you're in the air. Same exact thing. Different point of reference.

Assuming its a spring scale, when you are on it it will show your weight when you jump in the air it will zero again, how does this demonstrate your point