Physics Question for you.....

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  • leehljp
    Just me
    • Dec 2002
    • 8429
    • Tunica, MS
    • BT3000/3100

    I do realize the tires generally have little to do with it beyond initial movement and that it will take off. Restating: if there is EVER any differential in inertia/speed between the two then it will take off. BUT, and that is a big but, if speeds are matched to the billionth power, exactly, totally, nothing different to the point that NO differences, then no.

    If the plane does start moving forward (or backwards due to wheel friction) in relation to the ground or tower, then it IS moving faster/different) than the rolling runway, it broke the inertia/kenetic energy (lost my long ago physics terminology) barrier of a standstill.

    Look at it this way, suppose the runway starts backwards first at 10 mph, after the plane has moved backwards 100 yards, the engine accelerates to bring it up to 10 mph making it effective ground speed as "0"; Keep doing this - accelerating the runway first until the plane is catching up with the runway speed on a 100 mile long runway. Eventually the runway will be doing 100mph, the plane is slowly catching up and finally reaches 100mph on the runway, the weight and minor wheel friction will still keep it on the runway because no wind is flowing over the wings yet.

    IF the plane were to speed up 1/10 of 1 mph faster than the runway, then it would begin forward movement and take off.

    HOWEVER, with the specific question that was asked, until a canceling inertial difference is overcome, it won't take off. But once an inertial difference is overcome, it will take off, - but then they are not equal.

    If a runway was 1000 miles long and was going 150 mph in one direction, could a 120 mph Cessna setting on it facing the other direction get up enough speed to ever take off?

    This is the OTHER side to the main question. Everything being exactly equal, no, but that kind of precision measurement doesn't exist, so for the question - no, for reality - yes - unless the runway does go backwards faster than the plane can accellerate.

    Lastly, if the plane is allowed to move forward as much as one inch, you should allow the possibility for the runway to move in the reverse at the beginning - faster than the plane can accelerate forward, taking it backwards. If this statement is considered not to be fair, then neither is a plane that accelerates forward beyond the speed of the runway. It boils down to timing - which comes first. If equal, no. If unequal, it is possible.
    Last edited by leehljp; 02-04-2008, 12:59 AM.
    Hank Lee

    Experience is what you get when you don't get what you wanted!

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    • Anna
      Senior Member
      • Feb 2006
      • 728
      • CA, USA.
      • BT3100

      Originally posted by leehljp
      If a runway was 1000 miles long and was going 150 mph in one direction, could a 120 mph Cessna setting on it facing the other direction get up enough speed to ever take off?
      Actually, yes. It pretty much doesn't matter what the speed of the runway is. If the plane's thrust exceeds something like 0.04 x weight of the plane, the plane will move forward. The "0.04" is the ballpark friction coefficient on the wheel (bearings and rolling).

      The statement of the problem is very misleading. It says that the conveyor belt will match the speed of the plane. That statement is misleading because even if it matches the ground speed of the plane, it will not be able to keep the plane from moving forward.

      Everyone who said that the wheels will only spin twice as fast was right.

      One other way to look at it is the case when the plane is landing. Say the runway is moving in the opposite direction with the same air speed as the plane. When the wheels touch the runway, is the plane going to stop moving with respect to the ground? I think the wheels will just spin more rapidly, but the forward motion of the plane is still the same.

      It's actually pretty amazing. I was really sure about my initial answer. Oh, well. At least this "problem" makes sense now.

      Comment

      • niki
        Senior Member
        • Nov 2006
        • 566
        • Poland
        • EB PK255

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        • leehljp
          Just me
          • Dec 2002
          • 8429
          • Tunica, MS
          • BT3000/3100

          Originally posted by Anna
          Actually, yes. It pretty much doesn't matter what the speed of the runway is. If the plane's thrust exceeds something like 0.04 x weight of the plane, the plane will move forward. The "0.04" is the ballpark friction coefficient on the wheel (bearings and rolling).

          The statement of the problem is very misleading. It says that the conveyor belt will match the speed of the plane. That statement is misleading because even if it matches the ground speed of the plane, it will not be able to keep the plane from moving forward.

          Everyone who said that the wheels will only spin twice as fast was right.

          One other way to look at it is the case when the plane is landing. Say the runway is moving in the opposite direction with the same air speed as the plane. When the wheels touch the runway, is the plane going to stop moving with respect to the ground? I think the wheels will just spin more rapidly, but the forward motion of the plane is still the same.

          It's actually pretty amazing. I was really sure about my initial answer. Oh, well. At least this "problem" makes sense now.

          Your are right on this illustration (I think). After thinking it over, I used a "not so good" analogy.

          What we need here are not theorists or physicists, but lawyers and judges with a grammarian thrown in. They would figure out the wording.

          As it is stated, if the
          Hank Lee

          Experience is what you get when you don't get what you wanted!

          Comment

          • williamr
            Forum Newbie
            • Dec 2006
            • 56
            • Mazatlan or Toronto
            • BT3000SX

            Originally posted by Anna
            I don't really know much about planes, so the suggestion that the plane's wheels are freely spinning threw me off a bit. Having thought about this some more, doing the requisite free body diagrams, and learning a little more about planes and their vital statistics in the process... I have to concede.

            The plane will take off.


            Women are always right -- so you are right again. QED

            ---
            will

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            • williamr
              Forum Newbie
              • Dec 2006
              • 56
              • Mazatlan or Toronto
              • BT3000SX

              Originally posted by leehljp
              Your are right on this illustration (I think). After thinking it over, I used a "not so good" analogy.

              What we need here are not theorists or physicists, but lawyers and judges with a grammarian thrown in. They would figure out the wording.

              As it is stated, if the
              The heart of a good riddle is a dash of misdirection.

              ---
              will

              Comment

              • Russianwolf
                Veteran Member
                • Jan 2004
                • 3152
                • Martinsburg, WV, USA.
                • One of them there Toy saws

                19 pages........ Do I hear 20??


                Yep Anna, You got it. In fact if you watch a video of a plane landing, before ground contact is made the wheels aren't spinning at all. That's why you get that screach on the landing, the rubber skids instantaneously until the wheel speed can catch up to air speed. It only takes a split second , but it's noisey.

                I think the Youtube Video shows it pretty good. He had the conveyor running faster than the plane is capable of accelerating, yet the plane can still move forward almost as if the conveyor was off. I bet the wheel on the hot wheels car were spinning pretty good though.
                Mike
                Lakota's Dad

                If at first you don't succeed, deny you were trying in the first place.

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                • Alex Franke
                  Veteran Member
                  • Feb 2007
                  • 2641
                  • Chapel Hill, NC
                  • Ryobi BT3100

                  So long as the engines provide enough thrust to overcome the friction in the wheels, then "a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)" is essentially impossible to achieve. (Because if the plane appears to be moving forward, then the control system is not working.)

                  Is this right?
                  online at http://www.theFrankes.com
                  while ( !( succeed = try() ) ) ;
                  "Life is short, Art long, Occasion sudden and dangerous, Experience deceitful, and Judgment difficult." -Hippocrates

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                  • JeffG78
                    Established Member
                    • Jan 2007
                    • 385
                    • Northville, Michigan - a Detroit suburb
                    • BT3100

                    Originally posted by Alex Franke
                    So long as the engines provide enough thrust to overcome the friction in the wheels, then "a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)" is essentially impossible to achieve. (Because if the plane appears to be moving forward, then the control system is not working.)

                    Is this right?
                    Sort of. The conveyor cannot ever achieve the speed of the plane's TIRES since they will always be going faster than the conveyor while the plane is moving relative to the control tower. The original question simply states that the conveyor matches the PLANE'S speed. Airplane's speed is measured as air speed, not wheel speed like in a car. Since the wheels of a plane only come into play during take-off and landing, it would be rather useless to track their speed. The speed of the plane can indeed be matched by the conveyor if that speed is the air speed.

                    Comment

                    • Anna
                      Senior Member
                      • Feb 2006
                      • 728
                      • CA, USA.
                      • BT3100

                      Originally posted by Russianwolf
                      19 pages........ Do I hear 20??
                      Hey, this is educational.

                      I learned quite a bit. Took a while, but I'm convinced. Besides, I didn't know that the wheels on a Hot Wheel are free spinning, too.

                      Alex, I think the "trick" to the question is that the plane's wheels are practically frictionless. I mentioned in one of my posts that if the system is frictionless, the plane will takeoff. I didn't realize that in the real world case, the system does approximate a frictionless system.

                      The conveyor belt will not be able to "pull" on the plane to keep it from taking off. We can have the conveyor going 10x the speed of the plane, and the plane will still take off because it takes very little thrust to overcome the small amount of friction between the wheels and the conveyor belt.

                      The ballpark numbers:

                      Friction force ~ 0.04 x weight of plane
                      Plane thrust ~ 0.3 x weight of plane

                      For a 10,000 lb small jet, you'll need about 400 lbs of force to keep the plane from moving with a conveyor belt, no matter how fast the belt goes. I don't think the wheels will slip because of the plane's wheel design. If you apply 401 lbs, the plane should start to move forward. You'll need a little more to counteract the drag force eventually, but the idea remains the same.


                      P.S. I have to add: All the arguments about the plane's thrust acting on the air were also misleading. It doesn't matter where the plane is getting the force from. You can tie a rope on the plane's nose with the plane's engine off, apply enough force, whether by several guys pulling or with a truck or whatever, and the plane will still move. Move fast enough, and the plane will take off. Of course, with the engines off, bets are off on how long it stays afloat. But it will take off.
                      Last edited by Anna; 02-04-2008, 01:33 PM.

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                      • JeffG78
                        Established Member
                        • Jan 2007
                        • 385
                        • Northville, Michigan - a Detroit suburb
                        • BT3100

                        Originally posted by Russianwolf
                        19 pages........ Do I hear 20??

                        This is nothing Mike. Over at the Discovery Channel's site, they were nearing 100 pages just a few days after Mythbusters aired. They have probably doubled that by now.

                        Comment

                        • Alex Franke
                          Veteran Member
                          • Feb 2007
                          • 2641
                          • Chapel Hill, NC
                          • Ryobi BT3100

                          Originally posted by JeffG78
                          The original question simply states that the conveyor matches the PLANE'S speed. Airplane's speed is measured as air speed, not wheel speed like in a car. . . .The speed of the plane can indeed be matched by the conveyor if that speed is the air speed.
                          Hmm. This is why I'm not a physicist. To me, the original question suggests that the plane would somehow appear stationary to the tower.

                          Originally posted by Anna
                          Alex, I think the "trick" to the question is that the plane's wheels are practically frictionless. I mentioned in one of my posts that if the system is frictionless, the plane will takeoff. I didn't realize that in the real world case, the system does approximate a frictionless system
                          Yeah, I think I get the whole friction part. Regardless of how that plane is connected to the ground, there's some about of friction, no matter how trifling. Once the engines overcome this friction, it'll take off -- unless the conveyor can somehow add more friction to compensate (e.g. plane on skids and conveyor getting more and more rubbery).

                          But once the initial friction is overcome by thrust, you can't add more drag just by moving the conveyor faster.

                          Is that right?

                          So it's physically impossible for any conveyor (unless it can dynamically increase the friction) to make a plane appear stationary to the tower. Right?

                          So it's how you read the question. All technicalities aside, if you read it as "The plane will somehow be kept stationary as viewed from the tower", then it would be correct to conclude that the plane won't take off. After all, you can make leaps like this in thought experiments, can't you?

                          P.S. I know this is making me a better woodworker at some level. I just know it!
                          online at http://www.theFrankes.com
                          while ( !( succeed = try() ) ) ;
                          "Life is short, Art long, Occasion sudden and dangerous, Experience deceitful, and Judgment difficult." -Hippocrates

                          Comment

                          • Russianwolf
                            Veteran Member
                            • Jan 2004
                            • 3152
                            • Martinsburg, WV, USA.
                            • One of them there Toy saws

                            Originally posted by Alex Franke
                            So it's how you read the question. All technicalities aside, if you read it as "The plane will somehow be kept stationary as viewed from the tower", then it would be correct to conclude that the plane won't take off. After all, you can make leaps like this in thought experiments, can't you?

                            P.S. I know this is making me a better woodworker at some level. I just know it!
                            Nope, it's not how you read the question, it's your knowledge of all the parts that are factors and your assumptions from them.

                            Most people don't realize that the wheels are freespinning and assume that the tire rotation is what moves the plane. The reality is the plane's movement and the wheels; contact with the ground is what rotates the wheels.

                            The first assumption will lead you down the wrong path and the second assumption send you along the correct one.

                            The question is worded in a way that MIGHT encourage the wrong assumptions, but all the information you need is there.
                            Mike
                            Lakota's Dad

                            If at first you don't succeed, deny you were trying in the first place.

                            Comment

                            • Anna
                              Senior Member
                              • Feb 2006
                              • 728
                              • CA, USA.
                              • BT3100

                              Originally posted by Alex Franke
                              Yeah, I think I get the whole friction part. Regardless of how that plane is connected to the ground, there's some about of friction, no matter how trifling. Once the engines overcome this friction, it'll take off -- unless the conveyor can somehow add more friction to compensate (e.g. plane on skids and conveyor getting more and more rubbery).

                              But once the initial friction is overcome by thrust, you can't add more drag just by moving the conveyor faster.

                              Is that right?

                              So it's physically impossible for any conveyor (unless it can dynamically increase the friction) to make a plane appear stationary to the tower. Right?
                              Yep, that's pretty much it. The part about the plane's speed is also moot because the treadmill's speed, whether it matches the air speed or ground speed, is immaterial.

                              I made the same assumptions with respect to being stationary as seen from the tower because my point of reference are cars and people on treadmills. Now, I think I now just a little bit more about planes.

                              P.S. I know this is making me a better woodworker at some level. I just know it!
                              I sure hope it's making me one. Lord knows I spent enough time on this riddle over the weekend. Maybe that should be the next "physics" question: How does answering BT3Central's physics questions make you a better woodworker?

                              Comment

                              • Russianwolf
                                Veteran Member
                                • Jan 2004
                                • 3152
                                • Martinsburg, WV, USA.
                                • One of them there Toy saws

                                Originally posted by Anna
                                How does answering BT3Central's physics questions make you a better woodworker?

                                The best woodworkers I've ever seen have a knack for thinking through the problems that the wood is throwing at them.

                                Anybody can slap a couple pieces of wood together and call it a box.

                                The better woodworkers take into account the grain orientation and how the joinery is going to be affected over the life of the box.

                                So thinking through things and thinking outside the box can indeed help you be a better woodworker.

                                As an example. Below is a picture of a box (not mine). How do you open it?
                                Last edited by Russianwolf; 02-04-2008, 03:49 PM.
                                Mike
                                Lakota's Dad

                                If at first you don't succeed, deny you were trying in the first place.

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