Art, That's what makes it a trick question that most of your 100 people will answer incorrectly. In the common definition of "wheel speed" being the surface speed and the mass center are the same number because of the zero slip at the contact point assumption and the assumption the ground is stationary. Those 2 simplifying assumption make understanding the actual definition unimportant generally. In this case however the definition is important because 1 of the normal assumptions cannot be applied so you must start the correct definition and then seen what simplifying assumption do apply do get the correct answer. Another very common example of the masses getting it completely wrong on common things is the difference between temperature and heat. Most people use them almost interchangeably becasue they are linked by simplifying assumptions but they are quite different. Temperature is a measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale. Heat is random energy that crosses a boundary due to a difference in temperature. There is no heat in an oven, there is only temperature and a vacuum oven has neither heat nor temperature….and I would guess all 100 of the people you ask would get that wrong as well. That's why when a technical question comes up in one of your the cases you don’t poll the jury or anyone else, you call an expert on the subject to the stand…at least I would hope that’s what you do. I would love to be home working on my engine but I’m stuck here at work teaching engineering 101
Mike: You'd be right if this were an engineering class. It isn't however. If we use standard english, not engineering english, then the non-movement is mandatory. If indeed, we have to delve into various definitions only apparent to those with an engineering background, you have effectively changed the question. That's my point. When I get a question that requires special expertise, we do hire experts, however, when it comes to definitions, etc., the Judge and the jury instructions prevail, and we do use standard english, and the definitions contained in Websters, not information from technical journals which may contain definitions usable soley by those in that field. I suggest that Jury Instruction 315 (California, Judicial Council) covers this point adequately: "You should assume that the parties intended the words in their contract to ahve their usual and ordinary meanings unless you decide that the parties intended the words to have a special meaning." Art
Art, Here's the problem and would cover it, there is nothing non-standard about the engineering definition. The word "speed" has many definitions in the dictionary and all require it to be in motion so it can't be non-moving by standard or engineering english. A wheel that is rotating about it's axis with no linear motion is spinning as in "spinning your wheels" and that would be both the standard and the engineering description of a wheel that is. It is a trick question and I would hope any good lawyer would never allow a jury to hear it to begin with, but even so Jury Instruction 315 would still lead to the same answer since the standard Webster published definitions work just fine...but it's still a trick question the expects the reader will make an incorrect assumption that "moving at x mph" and "spinning with a surface speed of x mph" are the same thing when clearly they are not. Mark
In the known universe any airplane placed on a treadmill the rotates in the opposite direction of the plane WILL take off. There are no recognized physical scientific or engineering principles that would keep the plane on the ground. CAN'T BE DONE. It appears that there are some that felt the plane wouldn't fly are now trying to justify their positions by arguing definitions and semantics. This isn't the first time that language is used to spin a foolish position. Wasn't there some famous that used the expression "It depends on what the definition of the word "is" is" to try to spin himself out of a bad position?
How can the plane move independantly of the wheels? Have you ever seen an aircraft on the runway, out accelerate the undercarrage, leave it behined, and still take off?? The plane is sitting on the runway. Engines off. What is it doing. Is it moving? As far as I can tell, its static. Now fire up the engines and apply throttle..the aircraft moves..why? Did it come off the undercarrage? No. The wheels are now turning. Now you have wheel speed..say 10mph. Now fire up the conveyor and set the conveyor speed to 10mph in the opposite direction. Hold the wheel speed and conveyor speed in the opposite direction at exactly 10mph, will the airplane move forward?
Yes. That is in fact how the Wright brothers did it. Now fire up the engines and apply throttle..the aircraft moves..why? Did it come off the undercarrage? No. The wheels are now turning. Now you have wheel speed..say 10mph. Now fire up the conveyor and set the conveyor speed to 10mph in the opposite direction. Hold the wheel speed and conveyor speed in the opposite direction at exactly 10mph, will the airplane move forward?[/QUOTE] Starting the conveyer will have no effect what so ever on the speed of the plane. No matter how fast you run the conveyer no force will be transmitted to the plane and the plane will continue on it's way. The wheels will continue to have a linear translation or "speed" equal to the plane but the rpm of the wheel will vary depending on the speed you have the conveyer set to.
In that case a question for the engineer. If the "speed of the wheels" is taken as the linear velocity (I use that word carefully ) of the wheels centre of mass then does that not mean that the plane has to stay stationary relative to the conveyor for this velocity to match that of the conveyor - i.e. the plane travels backwards relative to the air? If we use the word "speed" rather than velocity then the plane can move forward at belt speed (in the opposite direction) so then the planes ability to take off is governed by the conveyors linear speed.
I think the analogy of you standing on a treadmill with roller skates works best. If you are standing on a treadmill and the treadmill is going 5mph, you pull yourself with a rope forward, you WILL move forward. The speed of the treadmill has no effect on you moving or not. Its an outside force. The wheels on the skates will be going 5mph plus the speed you are pulling forward at but you are not going 5mph plus the speed you are pulling. So if you pull at 2mph you are not moving backwards at 3mph. Follow me? Antony
Hit the nail on the head there. Art, didn't you post this? Isn't it a little hypocritical for you to be criticising others for insulting those that disagreed with them, when you fell squarely into that category yourself? And for the record, you're wrong, I think you realise it now, and as vvvmd said, you're playing semantics. And if you *still* don't realise you're wrong, god help you, because I cannot.
Youre right that in physics 1 speed is not a vector and has no direction. The reality is that nothing can be in motion without a direction, so speed must actually be the instantaneous velocity and is in fact a vector. That aside, the motion of the conveyer is defined as dependent on the motion of the plane, so if a speed match were specified the conveyer would be required to move in the same direction as the plane. That would mean that the plane moves down the conveyer and takes off without the wheels ever turning. The harder question that Im surprised no one has asked is when it says The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation., what the heck is moving in the opposite direction of rotation. Technically you cant have a linear motion that is opposite to a rotation so the reader has to make the assumption that the author meant at the point of contact . but then for the conveyer to be going opposite to the direction the plane is trying to go, it would have to move in the same direction as the contact point of the wheel causing the wheel to spin twice as fast as normal. Moving opposite would make the wheel rpm zero. The wording is very confusing on many levels. The only way to answer is to go with intent of the question and the plane takes off just fine or select that flawed question choice.
Those comments are far cry from some of the insults that people were throwing around, which is what the reference refered to. Go back and read them. If you can't figure out the difference between the two, I know a decent remedial reading class I can recommend. Bottom line, is that the way the question is worded indicates that the plane probably isn't moving, and without wind across the lifting surface, it isn't going to fly. Mark had to adjust the meaning of the question to get to where he believes that it's possible the plane could fly. While I understand what he is refering to, I don't think that the wording of the question allows that intrepretation. A legitimate difference, leading to an appropriate discussion, not insults, as you seem want to deliver. Art
Dear me, really? Remedial reading? Looks like you're the more likely to benefit from that. Maybe a little remedial physics too while you're about it. And how about a little remedial humility? Tell me where the question states THAT. In fact, YOU made the following comment in post #1108 : You're sounding more than a little confused. What exactly do you believe : that the question DOES or DOES NOT allow the wheels to move forward? Does the center of mass of a single wheel move translationally, relative to the still air? Or do you think the question "forbids" it? And I *know* you're confused, because in that same post, you came up with this stellar analogy : All the arguments that people had already made to that point to establish the difference between a jet thrust scenario (like this one) and a friction driven wheel scenario (a car on a dyno), and you came up with *that* analogy? How can you not grasp a very simple concept like this? What's driving you forward relative to a fixed point on a treadmill belt is *friction*. What drives a jet forward relative to the air is *thrust*. And to be brutally honest, Art, your grasp of basic physics is not as strong as you seem to believe. Even a basic understanding of conservation of linear and angular momentum seems to have escaped you. Like in this post. Blah blah, show me the wording you're referring to, and how you think it doesn't allow your weird "interpretation". And I was hardly the one to deliver insults unprovoked. But I did get more than a little rankled when you, an obviously physics-challenged individual, chose to give others that appellation. And imply that we were not as smart as you were. BTW, I was Turb0flat4 before I registered under a new handle (an unfortunate consequence of a lost password to my account and a discontinued email account). Do go back and read post #970, I explained my position in excruciating detail.
Unlike some, I waffled on this question for a very long time and eventually wandered firmly into the "yes, it'll fly" category. I think that past comparison of a car on a dyno is laughable at best- once I read the roller skate/rope/treadmill scenario it made complete sense (because I've done it before).
Issue: Wheel speed matched in reverse by the treadmill precludes the movement of the plane if you take the exact wording of the question. While it doesn't preclude movement of the plane, a simple reading of the question means that if the speeds are equal, then probably movement isn't going to happen. And when that occurs, no lift, no flight. It has little to do with physics, rather it has a lot to do with the intrepration of the question, which is english. Sorry you got your panties in such a twist. Art
The answer is not yes. The answer is not no. The answer is the conditions imposed by the question create an impossible situation. The ONLY way the planes wheels can have any speed is if the plane moves forward, as the wheels are not powered. If the treadmill EXACTLY matches the speed of the wheels then they can't ever move forward, otherwise the speeds wouldn't match. IMPOSSIBLE. Why can't anyone else see this? It's like asking this question: If you move the wheel to the right one foot while simultaneously moving it to the left one foot, then where is it located ? Can't answer, impossible situation.
I've been reading numerous posts referring to wheel speed in terms of lateral motion. In my world wheel speeds are a measurement of rotational velocity or surface velocity and have nothing to do with the lateral movement of the wheel through space or what it is attached to. The fact that a moving surface (treadmill/conveyor) is provided for the unpowered wheel to rest further disconnects any meaningful relationship between wheel rotation and aircraft movement. The conveyor can go forward, backward or stand still but it exerts no meaningful forces on the aircraft. The wheels are unpowered and are in contact with the conveyor. The surface speeds will match anytime movement happens and no skidding occurrs. Merely stating that the conveyor prevents motion will not make it so.
You move the wheel on foot to the right by its axle the wheel will rotate to the right. Simultaneously the GROUND underneath the wheel moves one foot to the left at the same speed the wheel is moving to the right. The axle of the wheel will still move to the right one foot. the wheel will have ROTATED about its axel twice as much as if it would have if the ground underneath it would not have moved. Even accepting the arguement that there would be no forward motion is logically inconsistant. If there is no forward motion then by definition the treadmill will not go backwards. So we have a plane sittin on a treadmill at full throtle just sitting there on a treamill. there is no forward motion so the treadmill does not move everything just sits there lots of thrust being generated nothing except a magic treadmill and free wheeling wheels just sitting there. No movement how is that possible? Interesting that one of the biggest nay sayers is a plaintiffs lawyer. Never let science or physical facts get in the way of an arguement. If the facts or science don't jive with your case argue semantics to convince people you are right. The original question was about the difference with thrust and traction in providing forward motion. With thrust the ground underneath is irrelevant, with traction the interface with the ground becomes critical Those that still can't see that they gotthe science wrong are no trying to confuse everyone with semantics.
Let me make it simpler for you: I have a plane that must follow the rules. The rules say the plane can't move. Will the plane fly?
I have re-read the rules several times. NO WHERE does it say that the plane can't move. That is an assumption on your part. The rules state that the treadmill will go backward at the same forward speed as the wheel, exactly matching the speed.. That means if the wheel moves forward at 10mph the treadmill will go backward at 10mph. If there is no motion of the wheels there will be no motion of the treadmill. your interpretation of the rules mean that the plane would be standing still on motionless tires on a motionless treadmill going full throttle. Once again, if this is a problem about science and mechanics the plane will take off one hundred percent of the time. If you want to argue semantics then once again what is the mass of all the angels that can fit on the head of a pin. You can argue that forever too.
