Physics Question for you.....

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  • #31
    Originally posted by LCHIEN View Post
    The orignal problem states the plane moves. And that the
    conveyor belt moves equal speed in the opposite direction.

    Those who say the plane stay motionless w.r.t. the ground are seriously in error.
    Yeah, I think Loring's interpretation most closely matches the original problem, after re-reading it. I.e. if the plane is moving 100mph forward, relative to the ground, then the conveyor is only moving 100mph in the opposite direction, not as fast as it takes to keep the plane motionless wrt the ground.

    The landing gear moving at 100mph (freewheeling, essentially) will not provide enough force to counter the jet engines, and the plane will continue to pick up speed relative to the ground (and air), until it has enough airspeed to take off.

    Regards,
    Tom

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    • #32
      OK, this is what I get for not reading it all.

      I change my answer, plane DOES take off, due to the fact that thrust is not pushing against the ground (conveyor belt), but rather thrust is pushing against the air. This is an interesting picture to wrap the brain around.
      The voices made me do it.

      Comment


      • #33
        I just copied the question to have it in front of me

        "A plane is standing on runway that can move (some sort of band conveyer).

        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 opposite direction)."



        Let's make an experiment.
        Instead of the airplane put your car on the conveyer and just for the experiment lets stick a pole near by. Take your hand out of the window and hold the pole.

        Now, "step on it", the car will accelerate to 100mph, but the conveyer will also accelerate to 100mph to the opposite direction at the same time. The relative velocity to the ground and to the air is - zero. your hand will still hold the pole.

        Second experiment
        Go to the gim and stand on the conveyer and start to walk on it.
        Slowly slowly increase the conveyer speed until you run at your maximum running speed.
        Are you advancing relatively to the ground? - no, you are running at the same place.
        Do you feel any wind on your face (which indicates relative velocity between you and the air)? - I don't think so, you are not advancing relative to the air around you.

        niki

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        • #34
          What is it about equal speed in the opposite direction that is confusing?
          The experiment is exactly like a tread mill. When you try to run faster and faster forward, the conveyer runs just as fast the other way to effectively keep you stationary with absolutely no wind in your hair. Why do you guys still imply that the plane keeps on moving forward. Its stated in the beginning that its forward speed is negated by the conveyor. It really ain't going anywhere.
          Lee

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          • #35
            let me explain it one other way for you doubters:
            The treadmil examples the feet are coupled by friction to the treadmill.

            In the airplane/conveyor its a different problem. The airplane is not coupled to the conveyor at all, except by low, assumed zero friction of the wheel bearings. THerefore when the engines fire, the plane will accelerate w.r.t the air and eventually take off when it reaches sufficient air velocity. The conveyor is completely a red herring. The conveyor, by definition of the problem, must move in an equal and opposite direction to the plane, but it cannot hold the plane back, or motionless w.r.t. the air that supports the airplane's flight.
            Loring in Katy, TX USA
            If your only tool is a hammer, you tend to treat all problems as if they were nails.
            BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions

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            • #36
              Overestimating thrust.

              Originally posted by LCHIEN View Post
              let me explain it one other way for you doubters:
              The treadmil examples the feet are coupled by friction to the treadmill.

              In the airplane/conveyor its a different problem. The airplane is not coupled to the conveyor at all, except by low, assumed zero friction of the wheel bearings. THerefore when the engines fire, the plane will accelerate w.r.t the air and eventually take off when it reaches sufficient air velocity. The conveyor is completely a red herring. The conveyor, by definition of the problem, must move in an equal and opposite direction to the plane, but it cannot hold the plane back, or motionless w.r.t. the air that supports the airplane's flight.
              Loring,
              1. An aircrafts thrust builds slowly, unless catapulted (aircraft carrier). As a former jet engine mechanic I used to run up near mach capable jets (A-7's) to full thrust (called "military") while the front landing gear was tethered to the ground by a single cable (yes it was 3 inches thick) and with the wheels chocked. The small landing gear had NO problem handling the strain BECAUSE the thrust was not instantaneous. Pilots do not increase thrust as they travel the runway in order to increase speed, they hit full thrust very quickly and it takes the length of the runway (the larger the aircraft the longer the runway needed) to gain the necessary airspeed to acheive lift...NO LIFT - NO FLIGHT. THis is also why aircraft carriers will wheel around to face into the wind in order to aid in acheiving lift sooner.
              2.) The problem states that the aircraft's forward motion is being equally countered by the conveyor belt. If the conditions are as stated, and conveyor belt friction does not disable the mechanism and the aircraft wheels and bearings (though only casting will still build up heat via friction, it would take some amazingly impossible engineering!), the aircraft will NEVER acheive the necessary airspeed for lift-off...in fact it will run out of fuel and roll backwards.
              3.) Your statement "airplane is not coupled to the conveyor at all" is in error, as ALL objects on this planet are "coupled" to the planet's surface by gravity, and will remain so until acted upon with sufficient force to defeat gravity.
              4.) To settle this definitively, my wife works for Boeing for a senior Vice-President who has a degree in physics/aeronautics and she has agreed to present this question to him. I will post his answer. Now, don't we all have Christmas/Hannakah(spelling?) presents to either make or buy?

              Comment


              • #37
                Thurst has NOTHING to do with it FLYING in our earth atmosphere!
                Runway/treadmill Speed has NOTHING to do with it FLYING in our earth atmosphere.

                Thrust in this case only keeps it in the same place as the treadmill runway moves.

                (HINT: Think - What are WINGS for? What is the purpose of wings? Wings lift. Wings need wind speed. Not wheels, not thrust, wind speed!)

                THRUST does NOT make the plane fly! The PURPOSE of Thrust is to get enough speed up so that there is a fast flowing of wind/air OVER the WINGS. It is WIND Speed over the Wings that create lift for flying.

                Thrust will not make it "fly" until there is enough airspeed (wind) over the wings.

                Think - Generate Lift, Generate Lift, Generate Lift, - then the plane will fly. Generateing lift is what Wings are for. If you think you will fly before you have "180 MPH wind speed" over the wings of a 747 then you will not need wings at all. So take the wings off.

                For those who think it would fly once 200 mph treadmill speed is reached are thinking in terms of Star Trek space travel where Thurst is what drives it and wings are not needed.
                Hank Lee

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

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                • #38
                  Wings need thrust

                  In response to:

                  "THRUST does NOT make the plane fly! The PURPOSE of Thrust is to get enough speed up so that there is a fast flowing of wind/air OVER the WINGS. It is WIND Speed over the Wings that create lift for flying.

                  Thrust will not make it "fly" until there is enough airspeed (wind) over the wings.

                  Think - Generate Lift, Generate Lift, Generate Lift, - then the plane will fly. Generateing lift is what Wings are for. If you think you will fly before you have "180 MPH wind speed" over the wings of a 747 then you will not need wings at all. So take the wings off.

                  For those who think it would fly once 200 mph treadmill speed is reached are thinking in terms of Star Trek space travel where Thurst is what drives it and wings are not needed."

                  Of course, but wings without thrust are just nifty sun-shades. Wings cannot lift off by themselves without an outside force, Wind. Wings, properly balanced can glide if given sufficient airspeed to generate lift, but they won't do it from the ground by themselves.

                  Since the natural winds are hardly controlable, nor convenient to our flight requirements, artificial wind must be generated, BY THRUST, to acheive sufficient airspeed.

                  OK, sorry to belabor the subject. Have a wonderful holiday season!

                  Comment


                  • #39
                    OK, now that I've changed my mind three times, I'm ready to commit: the plane WILL take off, and it will be in a shorter distance than if on a normal runway.

                    If you were walking on a treadmill, or driving a car on a conveyor, your motion relative to the air around you would be zero. The conveyor belt/treadmill would in effect be providing a frictionless surface, and no progress would be made (think of running or spinning your wheels on ice). But this only applies where forward motion is dependent upon the friction between the wheels and the ground.

                    In the case of an airplane (or jet), forward progress is based on the backward thrust from the propeller or engine (equal and opposite reaction) relative to the surrounding air, not the ground. The plane WILL move forward, and because of the conveyor belt (no friction between wheels and ground), the plane will reach takeoff speed in a shorter distance (IMHO).

                    Please don't make me change my mind again....
                    Ken W.
                    _____________________
                    "If you can't fix it right, fix it so no one else can fix it right."

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                    • #40
                      Guys.... Think back to your basic high school physics/mechanics and draw up a force diagram.

                      Force = Mass x Acceleration (F=MA)

                      Lets assume the wheels are the famous massless frictionless bearing surface (these come from the same catalog as massless inextensible strings, frictionless blocks, pulleys, and surfaces, etc.)

                      In our thought experiment the airplane is sitting quietly on the conveyer with nothing moving. Now without turning on the engines, start the treadmill. The wheels will start spining, but plane STILL won't be moving because you have not exerted any lateral force on it F=MA, with no force, you have no acceleration.

                      In a world (like ours) with friction, the plane will in fact start moving back due to friction on the wheels, but not at the same speed as the treadmill, and the smoother the bearings, the less movement there will be. So, there would be a small force vector pointing toward the back of the plane (a small amount of force in the negative direction).

                      Now lets look at the props or the jet engine. It's forcing mass (air) toward the back of the plane. Thanks to convervation of momentum and our buddy Newton's laws of motion we know that an equal and opposite force will be pushing the plane forward at whatever amount of thrust the engine generates. So now we have a large amount of thrust (the normal takeoff thrust) in the positive direction.

                      F=MA if the sum of the small amount of negative force and the large amount of positive force is positive then we have a net positive force and MUST have the airplane accelerating.

                      Since veloctiy = acceleration * time, and we've just shown that the plane is accellerating its velocity is increasing when it reaches takeoff speed, the plane will in fact take off.

                      The reason these aren't used on aircraft carriers is because as you just saw above it takes just as long (time and distance) to reach that velocity, the tredmill only serves to put extra mileage on the landing gear.

                      Kristofor.

                      Comment


                      • #41
                        Originally posted by niki View Post
                        I just copied the question to have it in front of me

                        "A plane is standing on runway that can move (some sort of band conveyer).

                        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 opposite direction)."



                        Let's make an experiment.
                        Instead of the airplane put your car on the conveyer and just for the experiment lets stick a pole near by. Take your hand out of the window and hold the pole.

                        Now, "step on it", the car will accelerate to 100mph, but the conveyer will also accelerate to 100mph to the opposite direction at the same time. The relative velocity to the ground and to the air is - zero. your hand will still hold the pole.

                        Second experiment
                        Go to the gim and stand on the conveyer and start to walk on it.
                        Slowly slowly increase the conveyer speed until you run at your maximum running speed.
                        Are you advancing relatively to the ground? - no, you are running at the same place.
                        Do you feel any wind on your face (which indicates relative velocity between you and the air)? - I don't think so, you are not advancing relative to the air around you.

                        niki
                        The airplane is pushing against the air, your feet are pushing against the conveyor, and there is a big difference.
                        The voices made me do it.

                        Comment


                        • #42
                          Originally posted by Stytooner View Post
                          What is it about equal speed in the opposite direction that is confusing?
                          The experiment is exactly like a tread mill. When you try to run faster and faster forward, the conveyer runs just as fast the other way to effectively keep you stationary with absolutely no wind in your hair. Why do you guys still imply that the plane keeps on moving forward. Its stated in the beginning that its forward speed is negated by the conveyor. It really ain't going anywhere.
                          Again,

                          walking on a treadmill: feet impart thrust by pushing against treadmill

                          airplane engine on treadmill: engines impart thrust against AIR

                          if the treadmill runway is not powered by it's OWN engine/motor, then I would be surprised if the belt even moved.

                          example, picture an unpowered treadmill backed up to a wall

                          walk on treadmill, treadmill moves due to the thrust created by your feet, in the opposite direction, and you remain relatively motionless to the stationary wall

                          now stand on a powered treadmill (the runway) wearing roller skates (landing gear) and push off from the wall.
                          The voices made me do it.

                          Comment


                          • #43
                            Okay, my two cents since everybody else has replied in this thread...

                            Walking on the treadmill/conveyor belt: you can go forwards or backwards depending on your walking speed relative to the belt. Because the Force in the "F=ma" equation to make you move occurs between your feet and the belt (traction/friction) the relative speed of the belt combines with your foot speed to create the total speed your body moves. Now replace the belt with a sheet of ice... your feet move a ton but you go nowhere (except downwards onto your butt) because friction/traction are nil. If somebody gave you a shove though you'd move across the ice. Even if you didn't move your feet. If the force is EXTERNAL to the belt (ice) then the belt has no effect.

                            Now stand on a conveyor belt again, with roller skates on. If the belt starts rolling, your skate wheels will go backwards. If there were no friction in the skate bearings you'd stay put. Now if somebody shoves you in the back, you WILL go forwards. And your skate wheels will be spinning at a rate that doesn't match the rate you're moving forwards. Their spin rate corresponds to the sum of your rate plus the belt rate. This is exactly what happens to the airplane too. The applied force is acting on your body or the airplane body - NOT on the part in contact with the moving ground/conveyor belt.

                            You can mimic this whole test yourself with a plain old kitchen rolling pin and a sheet of cardboard or plank of wood. The rolling pin is the airplane wheels (or your skates) and the handles represent the airplane body (or the dude on skates)... Push the handles forwards... the rolling pin rolls. Whee, duh. Now have somebody pull on the cardboard - what happens? The rolling pin rolls at a different rate BUT the whole thing WILL still move forwards as long as you keep pushing the handles forwards.

                            My initial gut response to the original question was that the airplane would stay still, spinning its wheels. But that's not what happens because the movement is NOT dependent on motive force being transmitted to the ground - traction/friction are NOT required. The folks saying the airplane's wheels will be spinning furiously are right. Now if the original question was about a CAR instead of an airplane then the answer would be "car does not move"; the wheels spin furiously because the F=ma force is tied to the ground. Another way to look at the CAR version of this: the conveyor belt doesn't need to be there - just replace it with ice again. No grip = no car movement. If the tires can't bite and generate force the car won't accelerate. The engine may be spinning like mad too - high RPMs - but the horsepower will be very low actually since NO WORK is being done by the tires - the engine will be revving in neutral basically. Just like revving on ice - you don't need full throttle to rev up a lot when the wheels are just spinning uselessly with no traction; not doing any WORK.


                            An airplane sitting on a real runway uses engine thrust - action-reaction principle - to make the Force in the F=ma equation and get it moving. There is some rolling friction though... about 15% is typical... i.e. for every 100 pounds of vertical force on the wheel & tire there will be 15 pounds of rolling drag. That subtracts from the engine's thrust so net F is really equal to "Thrust - 0.15*weight" initially; as the aircraft accelerates the wings start lifting making "weight" seem to decrease. Rolling drag will thus go down... but drag of air over the whole airplane plus induced drag (drag due to lift, you can't make lift for free!) will increase. This 15% rolling drag makes the aircraft accelerate slower than it would given engine thrust alone in the F=ma equaiton. That 15% rolling friction is a 15% "traction" effectively giving the conveyor belt SOME input (drag or pull) to the airplane... but it's nowhere near enough to keep it from accelerating. It'll still take off, using just a little more actual runway length than normal.

                            mpc
                            Last edited by mpc; 12-09-2006, 11:50 PM.

                            Comment


                            • #44
                              I duck out to do flooring for the day and I come back to find 5 pages of responses? Sheesh.

                              Okay, I'll take one example that was used and try to model the situation. The treadmill. Take a matchbox car (with freespinning wheels) and put it on the treadmill. Now with your hand acelarate the car to 1 mph and turn the treadmill on to 1 MPH. Now the car is moving forward realative to items off the treadmill at 1 MPH and the wheels are spinning at 2 MPH.

                              The airplane will take off.

                              As was mentioned, the engines apply force to the air, not the conveyor. Therefor the aircraft will move in relation to items off the conveyor, which is what is needed to gain lift and airborn.
                              Mike
                              Lakota's Dad

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

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                              • #45
                                To make things simple we need to assume frictionless bearings on the wheels, frictionless conveyor belt (so it does not generate it's on wind speed), no wind (from mother nature).
                                The OP says the conveyor belt is moving in opposite direction of the plane.
                                I think we all agree the wheels of the plane do not make the plane move.
                                I think we all agree that the engine pushing against the air moves the plane forward.
                                I think we all agree that for a plane to take off air needs to pass across the wings generating lift, enough lift to raise plane off the ground.

                                The conveyor belt is moving opposite the speed of the plane, NOT the speed of the wheels, since they have no bearing on the speed of the plane. I think a few people are confused on this.

                                This planes engines push the plane forward, relative to a stationary post.
                                Since the plane is now moving forward, the conveyor belt is moving backwards, at the same speed, relative to the post.
                                The plane will keep moving forward at the same speed relative to the post, while the conveyor belt will move backwards at the same speed, relative to the post.
                                The wheels will freely move across the conveyor belt at twice the speed, Speed of plane relative to post + speed of conveyor belt relative to post.
                                Gravity makes little difference (other than the weight of the plane, so therefore less/more weight to overcome less/more lift needs to be generated).
                                As the speed of the plane (relative to the post) increases, the conveyor belt (relative to the post) increases, the wheels will spin twice as fast.
                                Air will move across the wings, since the plane is moving relative to the post. Once the air speed is great enough the plane will lift.

                                In the perfect world the plane will leave the ground at the SAME distance it normally would.

                                In the real world due to friction the plane would probably leave the ground about 5-10% further down the runway.
                                Ric

                                Plan for the worst, hope for the best!

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