[Guest]

This is a very highly debated topic, even though scaled tests have been conducted. The popular Mythbusters show on discovery did it full scale, but some still dispute the way they did it. You can probably find thousands of forums that host this same question, and I wouldn't be suprised if it has even been posted here before. So here goes!

A large passenger jet sits on a tredmill the size of a normal runway that they would normally take off on. When the jet starts up, it climbs in speed slowly. The tredmill matches the jet's speed EXACTLY. For every mile an hour the jet climbs, the tredmill goes in REVERSE that exact amount as-well.

The grand question: Will the plane take-off?

Assume that:

The wind is at 0 MPH

The Jet is powered by turbines

Edited September 6, 2008 by RedRum

[Guest]

Isn't a plane's ability to fly a result of lift created by air moving over the wing? If so, with the plane standing still, no air is moving over the wing - therefore no lift is being created... On myth busters, the propeller was pushing air over the wings. in a jet - that is not the case.. I am no expert in flight physics, but this seems like a no brainer - the answer is NO - it will not take off...

It sounds like you're saying a jet plane could never take off, even if not on a treadmill. Jet planes take off because they build up enough speed on the ground and enough air passes over the wings giving it lift. It's no different in a prop plane. Prop planes must also get enough speed on land before they can leave the ground. To a very minor degree, the prop output flowing over the wings increases lift. Unless the props are pusher props located behind the wings, in which case they have negligible effect. Some jet aircraft also place the engines at least partly ahead of the wings to provide additional lift via the same mechanism. It's called "upper surface blowing".

In either case the effect is small, not nearly enough to lift the airplane off the ground.

If your contention were correct then the jets would not be needed at all to stand still. When the proposed scenario begins, leave the jets off- would the airplane not move backwards?

Jets aren't needed to stand still and no, the plane will not move backwards with the jets off, it will stand still. From the OP:

The tredmill matches the jet's speed EXACTLY. For every mile an hour the jet climbs, the tredmill goes in REVERSE that exact amount as-well.

The plane starts moving do to thrust by jets or a prop(s). The treadmill starts moving in the opposite direction. What's moving the plane is not it's wheels, so the wheels spinning is irrelevant.

[Guest]

It sounds like you're saying a jet plane could never take off, even if not on a treadmill. Jet planes take off because they build up enough speed on the ground and enough air passes over the wings giving it lift. It's no different in a prop plane. Prop planes must also get enough speed on land before they can leave the ground. To a very minor degree, the prop output flowing over the wings increases lift. Unless the props are pusher props located behind the wings, in which case they have negligible effect. Some jet aircraft also place the engines at least partly ahead of the wings to provide additional lift via the same mechanism. It's called "upper surface blowing".

In either case the effect is small, not nearly enough to lift the airplane off the ground.

Jets aren't needed to stand still and no, the plane will not move backwards with the jets off, it will stand still. From the OP:

The plane starts moving do to thrust by jets or a prop(s). The treadmill starts moving in the opposite direction. What's moving the plane is not it's wheels, so the wheels spinning is irrelevant.

I was with you on your first argument - you proved my point no movement - no lift. As to your contention that if the jets were off it would stand still - - how? -- thats like saying that if I got on a treadmill on my skateboard and turned the treadmill on, I would not move - only the wheels on skateboard would move...

the concept of the wheel speed being irrelevant is the flaw in your argument - it is irrelevant in that it is not what is causing the speed of the plane or lift

But the forward momentum of the aircraft, as defined by this riddle, is negated by the reverse direction of the treadmill - I don't care if the wheels are spinning 2000 mph, if the the treadmill is responding in kind - the forward velocity of the airplane is still net 0 mph - therefore no wind and no lift....

[Guest]

The other major flaw in the above argument of wheel speed being irrelevant assumes that the plane will move forward as soon as thrust is applied, regardless of the wheel speed. The moment the plane moves 1 foot in a forward direction - you have negated the premise of this riddle - for that to happen it would have to moving faster than the treadmill!

Edited September 6, 2008 by AAAsn888s

[Guest]

I was with you on your first argument - you proved my point no movement - no lift.

But there is movement. The plane does not move forward due to powered wheels; it moves forward due to thrust created by the jets.

As to your contention that if the jets were off it would stand still - - how? -- thats like saying that if I got on a treadmill on my skateboard and turned the treadmill on, I would not move - only the wheels on skateboard would move...

I told you how by quoting the OP. If the jets are off, the treadmill does not move. The treadmill only moves the exact opposite speed of the jet. If the jet is moving forward 0 mph, the treadmill is moving in the opposite direction 0 mph.

If you are on a skateboard on such a treadmill and standing still, the treadmill will also be still and you will not go backwards. If it's a motorized skateboard and you started it moving, the treadmill will move in the opposite direction at the same and you would be standing still. However, if I was standing on the ground and came along and pushed you forward, you would start moving. The treadmill will only move your wheels faster but I could push you along until you fell off the treadmill. Same with the airplane. Since the thrust forward momentum is not created by the wheels, all the treadmill will do is move the wheels faster.

the concept of the wheel speed being irrelevant is the flaw in your argument - it is irrelevant in that it is not what is causing the speed of the plane or lift

Exactly! But how is that a flaw in my argument? Since the speed of the airplane has zero to do with wheel speed, the plane takes off.

But the forward momentum of the aircraft, as defined by this riddle, is negated by the reverse direction of the treadmill

No, it's not. The forward momentum of the plane is caused by thrust given by jets. What it's free spinning wheels are doing below is irrelevant.

- I don't care if the wheels are spinning 2000 mph, if the the treadmill is responding in kind - the forward velocity of the airplane is still net 0 mph - therefore no wind and no lift....

Then tell me again why the plane on Mythbusters took off?

Consider this: If I attached a strong enough rope to the plane on the conveyor belt and I pulled it with a very powerful truck (not on the conveyor belt) traveling at 20 mph, and the conveyor belt turned in the opposite direction at the exact speed the truck was moving forward, would the truck stand still or would it move forward at 20 mph?

[Guest]

I am with you - and understand completely your arguments - let's disect your three arguments (I am conceding the turned off jets resulting in turned off tread mill, so I am refering to your other three..)

First - that on my motorized skateboard - I would stand still. - This is what is proving my argument - it is the closest correllation

your second and third arguments are adding an element not contained in the OP - that is - an object fixed, or held by friction, to the ground from which an additional force can be applied.

In either of your arguments, it would be impossible for the treadmill to match the speed - as soon as you push me or pull me from a fixed location - I am moving faster than the treadmill if I begin to move forward... hence a disconnect with the OP...

[Guest]

First - that on my motorized skateboard - I would stand still. - This is what is proving my argument - it is the closest correllation

It's not close; it's actually what makes the difference. The motorized skateboard moves forward due to its wheel speed in relation to the ground. If the ground moves in the opposite direction at the same speed, you stand still. Airplanes do not move forward due to its wheel speed in relation to the ground; they move forward due to thrust caused by jet engines. The plane's engines push air backwards. The resulting reaction pushes the plane forwards.

In either of your arguments, it would be impossible for the treadmill to match the speed - as soon as you push me or pull me from a fixed location - I am moving faster than the treadmill if I begin to move forward... hence a disconnect with the OP...

Not a disconnect. You move forward to to being pushed along by a means with has nothing to do with wheel speed. The motorized skateboard stays still because it relies on wheel speed in relation to the ground. If I pushed you, you would move forward due to a force other than spinning wheels. The same thing would happen if you were on the skateboard with a jet-pack on your back. You would be moved forward just as if I were pushing you. It wouldn't matter if the force were caused by my hand on your back or a jet-pack.

You didn't tell me why the plane on Mythbusters took off. Do you still think it's because the prop gave it enough lift? If so, you must think the plane went from 0 mph on the ground to being airborne. It didn't. It first gained enough speed on the belt which was moving the same speed as the plane in the opposite direction. It obviously moved forward regardless of the belt moving. All the belt did was cause the wheels to spin faster.

That's about all the explaining I can do for you on this topic. If you're still not convinced a plane on a treadmill will take off when given the parameters of the OP, maybe someone else would like to argue this with you. Later.

[Guest]

Even without the fixed object - the moment my plane/skateboard/hotwheel/whatever - starts moving forward (regardless of the source of the force causing the movement) - while on the ground still, I am exceeding the speed of the treadmill. I can therefore never gain any forward momentum and therefore cannot generate lift....

Not sure of all the parameters of the Mythbuster experiment - he did move forward ,however slightly, and he was in a much lighter airplane than a commercial jet - - with a front propeller sending air over the wings and a punch of acceleration as he was moving forward - my guess is he overshot the parameters of this riddle...

Edited September 6, 2008 by AAAsn888s

[Guest]

Even without the fixed object - the moment my plane/skateboard/hotwheel/whatever - starts moving forward (regardless of the source of the force causing the movement) - while on the ground still, I am exceeding the speed of the treadmill. I can therefore never gain any forward momentum without breaching the OP and therefore cannot generate lift....

Not sure of all the parameters of the Mythbuster experiment - he did move forward ,however slightly, and he was in a much lighter airplane than a commercial jet - - with a front propeller sending air over the wings and a punch of acceleration as he was moving forward - my guess is he overshot the parameters of this riddle...

Edited September 6, 2008 by AAAsn888s

[Guest]

I thought of one other example I'd like to give you, so I'm back.

Even without the fixed object - the moment my plane/skateboard/hotwheel/whatever - starts moving forward (regardless of the source of the force causing the movement) - while on the ground still, I am exceeding the speed of the treadmill.

You can't exceed the speed of the treadmill; it matched the speed of the wheels exactly and instantaneously.

Not sure of all the parameters of the Mythbuster experiment - he did move forward ,however slightly,

No, the plane took off at the same speed the plane always takes off at, I don't know where you're getting "however slightly" from. The treadmill had zero detectable effect on the plane's normal manner of take-off and caused the wheels to spin twice as fast.

and he was in a much lighter airplane than a commercial jet - - with a front propeller sending air over the wings and a punch of acceleration as he was moving forward - my guess is he overshot the parameters of this riddle...

The weight of the craft is irrelevant.

Think about this. The most powerful jet airplane you can imagine is on this very accurate and instantaneous treadmill. It's jet engines are roaring with such force, that it would race down a runway at a very fast speed- but it's on a treadmill. What happens? do the wheels just start moving very quickly but the plane stands still? Or does the plane move very quickly down the treadmill due to the thrust caused by jet engines pushing the still air around it, with its wheels going twice as fast?

Don't know if the above example will help, but I can't imagine why anyone would think that all that thrust does is cause wheels to move and not push the craft forward. The thrust will push the plane forward due to the still air around it, not stay still because a treadmill is keeping up with the speed of its wheels.

[Guest]

your second and third arguments are adding an element not contained in the OP - that is - an object fixed, or held by friction, to the ground from which an additional force can be applied.

Yes, the reason I am able to push your back and apply force to you and move you forward if you are on a skateboard on a treadmill is because my feet have friction on the ground.

But why is this relevant to the fact that you will move forward if the treadmill keeps up with your wheel speed in the opposite direction? Your body does not know nor does it care how and why the force is applied. All that matters is that the force is there. You admit that me putting force on your back will cause you to move forward on the treadmill. If a jet-pack could apply the exact amount of force as my hand to your back, you would move exactly the same distance forward.

[Guest]

"You can't exceed the speed of the treadmill; it matched the speed of the wheels exactly and instantaneously."

I guess this is the ultimate area of contention - I addressed your argument about friction in my last post...

The moment the plane moves forward, regardless of the source of thrust, it is exceeding the speed of the treadmill. I don't understand how you can make any other argument...

[Guest]

I thought of one other example I'd like to give you, so I'm back.

Think about this. The most powerful jet airplane you can imagine is on this very accurate and instantaneous treadmill. It's jet engines are roaring with such force, that it would race down a runway at a very fast speed- but it's on a treadmill. What happens? do the wheels just start moving very quickly but the plane stands still? Or does the plane move very quickly down the treadmill due to the thrust caused by jet engines pushing the still air around it, with its wheels going twice as fast?

As soon as the plane begins to move quickly down the runway with its wheels moving twice as fast, you have breached the Original Premise - or am I missing something?

[Guest]

The moment the plane moves forward, regardless of the source of thrust, it is exceeding the speed of the treadmill. I don't understand how you can make any other argument...

I'm not following you. The OP states that the treadmill "matches the jet's speed EXACTLY".

The moment the plane moves forward, it DOES NOT exceed the speed of the treadmill. The treadmill moves at the exact speed in the opposite direction.

[Guest]

As soon as the plane begins to move quickly down the runway with its wheels moving twice as fast, you have breached the Original Premise - or am I missing something?

The wheels move twice as fast as they would be if the plane was taking off at the same speed on a still runway. They move at the same speed as the runway.

[Guest]

I'm not following you. The OP states that the treadmill "matches the jet's speed EXACTLY".

The moment the plane moves forward, it DOES NOT exceed the speed of the treadmill. The treadmill moves at the exact speed in the opposite direction.

How can that be possible? - If the treadmill matches the speed exactly, the plane will remain motionless - the only way for the plane to advance is if it exceeds the speed of the treadmill. (regardless of the source of the force moving the jet.) I guess I am not understanding how it is physically possible for the jet to advance unless it is going faster than the treadmill. I conceed that the jet could roar down the runway by increasing its thrust, but for the plane to move forward it HAS to move faster than the treadmill...

Edited September 6, 2008 by AAAsn888s

[Guest]

The wheels move twice as fast as they would be if the plane was taking off at the same speed on a still runway. They move at the same speed as the runway.

that is a totally contradictory argument - they either move the same speed as the runway therefore the plane remains motionless - OR they move twice as fast, in which case the plane moves forward at its normal takeoff rate, but as stated earlier, to do so you have to go twice as fast as normal, i.e. twice the speed of the runway thereby breaching the Premise!!.

Edited September 6, 2008 by AAAsn888s

[Guest]

If the OP stated that the Jet was on a treadmill where both plane and treadmill have frictionless bearings, could it take off? The answer is yes. I believe that is the premise you are equating your arguments to...

But that is not the premise, the premise assumes the plane weighs however many tons and is sitting on the runway and as the plane begins to move forward the treadmill matches the speed in the opposite direction - net result - 0 forward momentum. As soon as the plane begins to move forward it is exceeding the speed of the treadmill - I cannot conceive how physics can contradict this assertion....

with no forward momentum - no lift.

Edited September 6, 2008 by AAAsn888s

[Guest]

How can that be possible? - If the treadmill matches the speed exactly, the plane will remain motionless - the only way for the plane to advance is if it exceeds the speed of the treadmill. (regardless of the source of the force moving the jet.) I guess I am not understanding how it is physically possible for the jet to advance unless it is going faster than the treadmill. I conceed that the jet could roar down the runway by increasing its thrust, but for the plane to move forward it HAS to move faster than the treadmill...

I think you are confusing the speed of the plane with the speed of the wheels. The plane's speed is equivalent to the speed of the treadmill. The wheels of the plane are moving twice as fast as the plane and the treadmill. The wheels' speed is basically the combination of the speed of the plane and the treadmill. Since the wheels aren't what drive the plane, they DO NOT affect the speed of the plane. Therefore, the impact that that the treadmill has on the wheels has no effect on the plane, just like the impact the plane has on the speed of the wheels has no effect on the speed of the treadmill. That's why the wheels are moving twice as fast, they are the combination of the speeds of the two moving objects. The plane would move forward and take off. The plane wouldn't be moving faster than the treadmill, only the wheels would.

Here's one way to think about it:

Say that the plane is already flying in the air, and for some reason has its wheels out. If the wheels were spinning at the same speed as the plane due to some force(not wind), that would not affect the speed of the plane because the wheels are not what give the plane speed. The turbines are. That is essentially what is happening here.

Edit: While friction may have some minor impact, it would be incredibly small, and of little importance to this question(if any).

Edited September 6, 2008 by Frost

[Guest]

Phrased another way - according to Scraff's assertions, if the plane just sat on the runway and the runway started moving in the opposite direction the plane would stand still because the wheels are "free spinning" - well, it wouldn't. It would require a force equal to the total force moving it backward to keep it in a stationary position relative to a fixed object sitting next to it on the tarmac. The only way the plane advances is if the force advancing it increased, thereby causing it to move FASTER than the runway - again breaking the premise...

PS - hope there is no offense Scraff - I am enjoying the conversation - just not agreeing...

[Guest]

I think you are confusing the speed of the plane with the speed of the wheels. The plane's speed is equivalent to the speed of the treadmill. The wheels of the plane are moving twice as fast as the plane and the treadmill. The wheels' speed is basically the combination of the speed of the plane and the treadmill. Since the wheels aren't what drive the plane, they DO NOT affect the speed of the plane. Therefore, the impact that that the treadmill has on the wheels has no effect on the plane, just like the impact the plane has on the speed of the wheels has no effect on the speed of the treadmill. That's why the wheels are moving twice as fast, they are the combination of the speeds of the two moving objects. The plane would move forward and take off. The plane wouldn't be moving faster than the treadmill, only the wheels would.

Here's one way to think about it:

Say that the plane is already flying in the air, and for some reason has its wheels out. If the wheels were spinning at the same speed as the plane due to some force(not wind), that would not affect the speed of the plane because the wheels are not what give the plane speed. The turbines are. That is essentially what is happening here.

Edit: While friction may have some minor impact, it would be incredibly small, and of little importance to this question(if any).

Now I am understanding the argument he is making. The breakdown for me is how are we defining the speed of the plane. As long at its wheels are on the ground, we are talking groundspeed. The fact is that the wheels ARE on the ground. I conceed that the plane can move forward at 200 mph while the treadmill is moving in the opposite direction at 200 mph and the plane can take off. The problem with this scenario relative to the OP is that for this to happen, the groundspeed relative to the treadmill is 400MPH (twice the speed of the treadmill.) If we measure the speed of the plane relative to a fixed object on the ground yes - it could take off - If we measure the speed of the plane relative to the treadmill (which I believe is inherent in the phrasing of the OP) - then I stand on my assertion that the answer is no...

[Guest]

Here's one way to think about it:

Say that the plane is already flying in the air, and for some reason has its wheels out. If the wheels were spinning at the same speed as the plane due to some force(not wind), that would not affect the speed of the plane because the wheels are not what give the plane speed. The turbines are. That is essentially what is happening here.

Edit: While friction may have some minor impact, it would be incredibly small, and of little importance to this question(if any).

That is a better way to explain it Frost- ppl get caught by the standing start. - you may have killed the debate

[Guest]

Fair enough?

[Guest]

That is a better way to explain it Frost- ppl get caught by the standing start. - you may have killed the debate

I don't get caught by the standing start - that is part of what is defining the speed.....

[Guest]

I don't get caught by the standing start - that is part of what is defining the speed.....

The thrust of the jets are not the same as the drive of the wheels on the treadmill... taliesan explained it well i think

[Guest]

How can that be possible? - If the treadmill matches the speed exactly, the plane will remain motionless - the only way for the plane to advance is if it exceeds the speed of the treadmill.

How can it be possible? What do you think I've been explaining all this time? Exactly that! Planes are driven by thrust, not by mechanically driven wheels. The plane moves forward because thrust pushes still air and drives it forward. A conveyor matching the speed of the plane has no noticeable effect on take-off.

You ask how can it be possible. How can it be possible for me to move you forward by pushing on your back if you are on a skateboard that's on a treadmill that matches your speed? You understand how that works; you should understand that engines pushing the plane forward is functionally the same.

(regardless of the source of the force moving the jet.) I guess I am not understanding how it is physically possible for the jet to advance unless it is going faster than the treadmill.

I explained it many times. All the treadmill will do is move its wheels.

I conceed that the jet could roar down the runway by increasing its thrust, but for the plane to move forward it HAS to move faster than the treadmill...

So this huge jet plane with all this thrust pushing still air will just remain still because it's free spinning wheels are resting on a treadmill that will match its speed in the opposite direction? Sheesh!

that is a totally contradictory argument - they either move the same speed as the runway therefore the plane remains motionless - OR they move twice as fast, in which case the plane moves forward at its normal takeoff rate, but as stated earlier, to do so you have to go twice as fast as normal, i.e. twice the speed of the runway thereby breaching the Premise!!.

What? Picture a plane going down a runway at 100 mph. Picture an identical plane going down a treadmill as described in the OP at the same 100 mph (even though you think it's impossible). The wheels of the plane on the treadmill will be going twice as fast as the plane on the runway.

[you have breached the Original Premise - or am I missing something?

Actually, you breached the original premise in your second post in this thread:

it is on a tread mill that is matching its speed in the opposite direction - the only thing moving on the plane is its wheels...

If its wheels are moving, I assume you acknowledge that the treadmill is moving. How can the plane be still while the treadmill is moving when the OP states "The tredmill matches the jet's speed EXACTLY".

The treadmill matches the speed of the jet, not its wheels.

Here's this huge jet with its thrusters fully engaged and it's staying still on a treadmill? Why on Earth are its wheels spinning? The wheels of car spin because they are mechanically driven. The wheels of a plane spin because the plane is moving due to thrust pushing air while the wheels are in contact with the ground, forcing them to spin. If the plane doesn't move forward, the wheels don't spin.

The jets are pushing air! The jet will move forward.

I conceed that the plane can move forward at 200 mph while the treadmill is moving in the opposite direction at 200 mph and the plane can take off. The problem with this scenario relative to the OP is that for this to happen, the groundspeed relative to the treadmill is 400MPH (twice the speed of the treadmill.)

You're contradicting yourself. If you concede that the plane moves forward at 200 mph while the treadmill moves backwards at 200 mph and still takes off, then it doesn't make sense to say that the plane and treadmill moving away from each other at a combined speed of 400 mph is a problem. It either works and is not a problem or doesn't work and is a problem.