will it take off?

[Wayne County Hose]

This thinking stuff is too much like work for me. I'm still trying to figure out if the horse pulls the cart or pushes it. You guys are up to airplanes? And Egon claims to be behind the times.

 

[ray66v]

["I'm still trying to figure out if the horse pulls the cart or pushes it".]
LOL, How many posts will there be over this?

 

[David Cockey]

Welcome to the thread David. Nothing wrong with what youve deduced. However, there is another way of looking at this which makes more complete use of the physics. It is essentially an extrapolation of the conveyors effect on the situation. I believe it came up to counter the false claim that the conveyor could have no inhibiting effect on the thrust of the plane. The physics of this counter thrust via rotational acceleration of the wheels is described pretty well starting about 1/3 of the way in the thread in the 300's. I dont believe many understand it.
larry

The other way of looking at is wrong. The conveyor does not inhibit the thrust of the plane. The wheels and tires spinning at twice the normal speed due to the conveyor moving will slightly slow the acceleration of the plane, but the plane will still accelerate to take off speed. The inertia of the wheels and tires is effectively doubled compared to a takeoff on a normal runway, but the effect on acceleration of the rotational inertia is small compared to the inerita due to the mass of the airplane.

It seems that many don't understand a couple of things. The first is that the wheels on airplanes do not provide thrust. The thrust which moves the airplane comes from the propellers and engine exhaust. Also obviously misunderstood are basic physics, Newton's laws of motion in particular.

It is amazing how this thing hangs around.

 

[Wayne County Hose]

This whole argument is just inconceivable to me. This is the type of discussion that happens after a few beers at a bar, not on a forum full of intelligent (?) men. Mythbusters is a decent half hour show that drags on to an hour. I have some show suggestions for them.

1. I think they should unzip their flies and walk thru a chicken coop. Let's see if the early bird really does gets the worm.
2. See if the dog didn't stop to take a poop if he would have caught the rabbit anyway.
3. When we were kids and we said/did something dumb, my grandma would tell us to "go poop(edited) in our hat and pull it over our ears." Let them test if this would make them smarter.
4. They should set up microphones in the woods to listen for a tree falling. If nobody is around, will it make a sound?
5. Test if the amount of beer in a man's system is proportional to the vision impairment of beer goggles. On a side note, this will also determine whether a 2 at 10:00 is really a 10 at 2:00.
6. They should see if the following phrase is true: A skunk sat on a stump, the skunk thunk the stump stunk, the stump thunk the stump stunk.
7. Staying on their airplane theme, they could place a camera in the cockpit of their plane and see if he who fly upside down really do have crack-up.
8. Last but not least, they should check all the forms of capitol punishment, on themselves, to see if any of them are really "cruel and unusual."

 

[patrick_g]

Just to throw a stick in the ocean of theoretics:

Replace the wheels with skis (or ice skates) and ice the belt. There just went all the energy being applied to spinning up wheels, etc.

Harry K

Very good Harry, then the magic conveyor will really have to rev up so the friction of the skis will be sufficient to equal the prop thrust and hold the planes forward speed to zero and eliminate lift.

Pat

 

[SPYDERLK]

The other way of looking at is wrong. The conveyor does not inhibit the thrust of the plane. The wheels and tires spinning at twice the normal speed due to the conveyor moving will slightly slow the acceleration of the plane, but the plane will still accelerate to take off speed. The inertia of the wheels and tires is effectively doubled compared to a takeoff on a normal runway, but the effect on acceleration of the rotational inertia is small compared to the inerita due to the mass of the airplane.

It seems that many don't understand a couple of things. The first is that the wheels on airplanes do not provide thrust. The thrust which moves the airplane comes from the propellers and engine exhaust. Also obviously misunderstood are basic physics, Newton's laws of motion in particular.

It is amazing how this thing hangs around.

Your argument for your statement contradicts the statement. I agree with your argument. The wheels provide counter thrust.
larry

 

[CDsdad]

I've never been on a plane, but plenty of helicopters. The question of if it will take off is not important. The important question to ask if it takes off is: Will it safely land? If it won't take off, oh well, moot point.

 

[patrick_g]

patrick_g:

You said "Again, depending on the definitions of terms and ignoring the current state of the art in building conveyors and airgraft wheels/tires/axles, you picks your definitions and you gets your answer. Either contention can be supported, depending on your assumptions." I am very curious what the assumptions you see that would prevent takeoff.

The original post conditions are a plane on top of a conveyer like device that rolls its surface backward at exactly the speed the plane is moving forward. This is not a complicated device, certainly not magic.

By the way, my favorite teaching bowline is collapsing a slip not (simple noose) made back from the working end after putting the working end through the loop of the noose.

Steve

Steve, Attending to the high priority issue first and then getting to the magic conveyor belt:

That method of tying a bowline is a very good teaching technique as well as having practical importance. In the case of the former it lets you build on the simple familiar (overhand knot) and turn it into the desired new knot (bowline) and in the latter case addressing practicality it allows you to pre-tie part of the knot so yo can capture something and finish the knot before conditions interfere. For example you want to secure a flailing boom or tie to a bollard and conditions are just too wild to allow you to fully construct the bowline in place.

Now for the magic conveyor... IF you strictly define the conveyor's role to be ONLY to match the plane's forward speed but in opposite direction the plane (no magic involved), if it has just a little excess thrust above and beyond that required to take off with no conveyor it will indeed take off after just a little more take off roll. The wheels will be rotating at twice liftoff speed and we don't approach relativistic speeds moving us from Newtonian toward Einsteinian physics.

Much more fun was the version much discussed here where the conveyor's role was to speed up as required to prevent forward motion of the aircraft. This would work using either the rotational inertia of the wheels or friction of skis or whatever.

If you accelerate the conveyor sufficiently (non linear "jerk" where jerk is a defined physics term, the third derivative of distance with respect to time or the acceleration of acceleration if you will. If you do not limit the speed and the change of speed of the conveyor it can prevent the plane from taking off. If the conveyor is allowed to be "magic" and not self destruct then the plane's wheels will self destruct and long before relativistic speeds are achieved from plain old centrifugal force. If the plane's wheels are allowed to be magic and handle any RPM then the plane will run out of gas and not move forward.

Oh, bye the way, I think debate of this sort to be fun, and instructional and just plain good fun so long as we avoid the ad hominem approach. If we don't get too emotionally charged in favor of or opposing an idea, who knows someone might even learn something.

Unfortunately, much of what is posted seems to be in line with the story of the three blind men and the elephant, each touching a different part and disagreeing on what the elephant is like.

In the main we discussed a limited number of variants:

1. Conveyor moves to EQUAL the forward speed of the plane.

2. Conveyor moves to CANCEL the forward speed of the plane.

These are entirely different propositions!

In the first (1) the plane probably takes off with just a little extra roll and its wheels spin at twice its forward speed until after lift off.

In the second (2), The case of the "magic conveyor" the plane does not take off.

Pat

 

[SPYDERLK]

Very good Harry, then the magic conveyor will really have to rev up so the friction of the skis will be sufficient to equal the prop thrust and hold the planes forward speed to zero and eliminate lift.

Pat

Energy to energy: Conveyor to Ski #FxD/sec =~< Plane HPx33000/sec...Conveyor Distance[feet] /sec is very high.........Still, it seems to me that if #F is less than thrust the plane would forever move. Im getting a logic disconnect on this one. I believe its a mix of reference frames - the work done on the ski is mostly radiated; not mechanically transmitted from the conveyor to the airframe.
larry

 

[patrick_g]

Energy to energy: Conveyor to Ski #FxD/sec =~< Plane HPx33000/sec...Conveyor Distance[feet] /sec is very high.........Still, it seems to me that if #F is less than thrust the plane would forever move. Im getting a logic disconnect on this one.
larry

Yes, obviously, if the friction between the skis and conveyor produce less force at the conveyor's speed than the planes thrust then the plane will accelerate.

However IF the conveyor runs fast enough the counter force can equal thrust (or exceed it.)

Again, choose which propositioin you want to discuss:

1. Conveyor moves to equal the plane's forward speed or
2. Conveyor moves to stop the plane's forward speed.

These are vastly different propositions. Either can be achieved if in #2 we use a "magic conveyor" capable of unlimited acceleration and speed whether on skis or wheels or a channel filled with moving water and use a float plane.

I don't think we are in disagreement over physics, just maybe in precisely clarifying the initial conditions and givens.

Forget the tree. What if Helen Keller fell down in the woods. Would she make a sound?

Pat

 

[Iplayfarmer]

Yes, obviously, if the friction between the skis and conveyor produce less force at the conveyor's speed than the planes thrust then the plane will accelerate.

However IF the conveyor runs fast enough the counter force can equal thrust (or exceed it.)

Again, choose which propositioin you want to discuss:

1. Conveyor moves to equal the plane's forward speed or
2. Conveyor moves to stop the plane's forward speed.

These are vastly different propositions. Either can be achieved if in #2 we use a "magic conveyor" capable of unlimited acceleration and speed whether on skis or wheels or a channel filled with moving water and use a float plane.

I don't think we are in disagreement over physics, just maybe in precisely clarifying the initial conditions and givens.

Forget the tree. What if Helen Keller fell down in the woods. Would she make a sound?

Pat

Thanks for summarizing and consolidating the views. Here's the original question again for dissection...

a plane is standing on a movable runway (something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.

the question is

will the plane take off or not?

The scenario states the direction and speed of the plane and conveyor but makes no statement about the effect of the conveyor. The effect of the conveyor is the question.

In order to say that the the plane won't fly one has to argue that the question states the effect of the conveyor is "to stop the forward speed".

The "Fly" camp says "I think it will fly based on the conditions stated in the question"

The "No Fly" camp says "I think the question must have more to it since I think it won't fly"

Whether a conveyor could stop a plane or not is an interesting and educational discussion, but it's not the question at hand.

I reconfirm that this question should be filed in the same category with "If a Rooster lays an egg in the center of a rooftop, what side will it roll off?" and "If a plane crashes on the border between Canada and the USA, where do you bury the survivors?"

 

[SFish]

Pat:

I agree about fun being most important. I didn't realize a second definition of the conveyer had been added to the original post.

I teach the slip knot method because you can contrast the quality of a noose with the bowline for beginners, and they will then remember it. I have emphasized that this method allows wrapping the working end around oneself and getting the loop the correct size in an emergency situation. Also useful is using this method to tie two ropes together when only a very short length of one is available to tie to (I forget what this knot is actually called).

Steve

 

[patrick_g]

I reconfirm that this question should be filed in the same category with "If a plane crashes on the border between Canada and the USA, where do you bury the survivors?"

A better question is where do we bury the survivors of this thread!!!

Even with any negitatives it shure wuz fun most of the time, huh?

Pat

 

[Egon]

[QUOTE "If a Rooster lays an egg in the center of a rooftop, what side will it roll off?" [/quote]

Who knows but there will surely be one surprised Rooster!! Will he crow is another matter.

The survivors may eventually be buried somewhere as per the wishes of next of kin or depending on the will instructions. That is if they are buried. They may be cremated! Or the body may be lost to some sort of mishap and never be recovered!

 

[patrick_g]

Pat:

I agree about fun being most important. I didn't realize a second definition of the conveyer had been added to the original post.

I teach the slip knot method because you can contrast the quality of a noose with the bowline for beginners, and they will then remember it. I have emphasized that this method allows wrapping the working end around oneself and getting the loop the correct size in an emergency situation. Also useful is using this method to tie two ropes together when only a very short length of one is available to tie to (I forget what this knot is actually called).

Steve

Could your mystery knot be the sheet bend which is used for joining two lines together and resembles the bowline in appearance? If you use it with man made fiber rope (slipperier than natural fiber) you will want to use a double sheet bend, especially if the knot is subject to getting wet.

I was taught how to tie a bowline around my waist with one hand (by a grand mother in her 70's who had to learn it to go mountaineering with an Outward Bound group) and subsequently taught it in all marlinespike seamanship classes I taught. It can be a life saver in the outback as well as at sea.

Regarding the conveyor.. all sorts of dynamic modifications were made, assumed, employed, etc. Me, I don't have an axe to grind either way. You pick your initial conditions and assumptions (givens) and the answer is immediately apparent. The reason controversy raged so long on this topic is that not everyone (a) stuck with the given info and sometimes inserted their opinion and prejudice in place of the stated problem and (b) not everyone is as well grounded in Newtonian physics as would be a credit to their school system.

Still, on ballance a fun outing. What I don't understand is why everyone wants to bury the survivors. Haven't they suffered trauma enough?

Pat

 

[SPYDERLK]

Yes, obviously, if the friction between the skis and conveyor produce less force at the conveyor's speed than the planes thrust then the plane will accelerate.

However IF the conveyor runs fast enough the counter force can equal thrust (or exceed it.)Pat

I just dont see how this can ever happen with skis. I would expect the frictional coefficient would remain near constant; unlike with tires, where the hysteresis in increasingly rapid flexure continues to add resistance. With skis I can see at high speed that tremendous energy is dissipated, but I cant see how that can increasingly factor against thrust as conveyor speed rises. How does that work??

Youre right. Its the mind exercise of considering the possibilities that makes this fun. Were doing alot better on this than MythBusters.
larry

 

[laurencen]

The prop will pull the aeroplane forward, no matter how fast the conveyer goes the aircraft will advance, the only way the aircraft would remin stationary is if wheels are driven such as a vehicle, my Cherokee will lift off at 55mph, the airspeed would be 55mph however the conveyer could be going negative 55mph, thus wheels will be going 110mph, the aircraft is going to do 55mph forward and will lift, now if you reverse the conveyer and propel the aircraft at 55mph it would still lift off and if the motor is at idle I expect it to float and land, if one revved the motor it would fly, this is the way aircraft carriers launch aircrafts, steam cylinders propel the aircraft to takeoff speed in a short distance, the engines then take over, I suspect many modern jets could not launch to remain flying in such a short distance if it was the jet engines alone.

 

[daTeacha]

"...as the plane moves..." pretty well tells us that the plane, regardless of whatever may be motivating it, changes position with respect to some unnamed point of reference. "...the conveyor moves in the opposite direction..." tells us any given point on the surface of the conveyor, regardless of whatever may be motivating it, also moves with respect to said unnamed point of reference.

In the original premise, the motion of the conveyor seems to be based on the motion of the plane, although they may be simultaneously and instantaneously controlled by some third entity about which we know nothing.

If the motion of the conveyor is indeed based on the motion of the plane, and if the motion of the conveyor adversely affects the motion of the plane, that adverse effect would have to be be conveyed (no pun intended) to the conveyor, causing it to slow. Carrying this line of thought to it's extreme, the conveyor and plane would simply come to a halt.

Anything that slows the plane with respect to the unnamed point of reference would have to slow the conveyor. That anything would include the conveyor.

As far as sounds in the woods, "sound" can be defined as either a series of waves in a medium OR the interpretation of those waves by a brain of some kind. The tape recorder would prove nothing. A CD, tape, or whatever makes no sound until the information is transformed into compressions and rarefactions in the air and those in turn are converted to electrochemical impulses transmitted through nerves to a brain which interprets them as sound.

Many of us, thanks to our love for internal combustion devices, loud music, firearms etc. and just plain age can "hear" a high pitched whistle even in complete silence due to damage to our hearing apparatus. Is that a sound or a physiological phenomenon? Similarly, we can "see" various colors by pressing on our eyes. Is that light or a physiological phenomon?

The waves in the air resulting from the falling of an object are just waves until a brain interprets them as sound.

If you connected your optic nerve to the auditory center of the brain, and the auditory nerve to the optic center, would that truly seem to be a "loud" tie you got for Christmas? And when fiddling with the equalization of your music system, could you really make the music "brighter"??

 

[Erik_in_Hoyt]

Egon - it was a trick question.
you don't bury the SURVIVORS.

 

[RobS]

The prop will pull the aeroplane forward, no matter how fast the conveyer goes the aircraft will advance, the only way the aircraft would remin stationary is if wheels are driven such as a vehicle, my Cherokee will lift off at 55mph, the airspeed would be 55mph however the conveyer could be going negative 55mph, thus wheels will be going 110mph, the aircraft is going to do 55mph forward and will lift, now if you reverse the conveyer and propel the aircraft at 55mph it would still lift off and if the motor is at idle I expect it to float and land, if one revved the motor it would fly, this is the way aircraft carriers launch aircrafts, steam cylinders propel the aircraft to takeoff speed in a short distance, the engines then take over, I suspect many modern jets could not launch to remain flying in such a short distance if it was the jet engines alone.

Ah, but this is the debatable point in the excercize. One could argue that there is some resistance in the wheels that will counteract the prop thrust. While unlikely, the possiblility is certainly there.