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  1.    #41  
    Quote Originally Posted by aprasad View Post
    What if you were peeing into a bucket that you were holding?
    Hmm. Wow.

    Initially, your weight would go down, like in the previous example. Then as the pee hit the bucket, the measured weight would increase, and then stay constant (assuming a steady stream). The falling pee accelerates due to gravity. Would its downward impact in the bucket increase your weight by more than the combination of the weight of the mid-air pee and your upward propulsion? That is, would your weight while peeing be greater than before peeing? It's arguably similar to continually dropping rocks on the scale, or suddenly squatting.

    I would guess, Yes, but I really don't know!
  2. #42  
    Quote Originally Posted by samkim View Post
    Because it's true, a car is able to easily overcome the force of friction to accelerate. Because it's true, you're able to overcome the force of friction to accelerate the car by pushing it.

    The fact that a car rolling at 50 mph with its engine shut off would slow down proves that the force of friction is greater than zero. But it doesn't change the fact that it's a trivial force relative to the power of the engine.


    You're confusing speed with acceleration. Force is proportional to acceleration, not speed. If you can hold a car on the treadmill at 2 mph, you can hold it at 50 mph. You'd be fighting roughly the same amount of friction. (Heat might affect the friction.) The bottom line is that you're stronger than axle grease friction.

    The more interesting question (which doesn't change the relative strengths of friction, you, and a plane engine) is whether you'd be able to offset the force if the treadmill were to accelerate really quickly from 2 mph to 50 mph. Well, that would depend on the level of acceleration. But regardless, the wheels would begin to spin before you're overcome, since you're stronger than axle grease friction.
    I'm a self-critical person so when I ran into a physicist yesterday night (Ph.D., internationally renowned university ), I told him about the problem at hand. He totally supported my version and the additional examples I brought up.

    IMHV, you don't seem to understand that the treadmill accelerates, too, when the plane accelerates, and you don't seem to accept some basic action=reaction principles (e.g. holding car on a treadmill rolling at -x mph vs. pushing car on ground with x mph), but what can I do? Case closed.
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  3.    #43  
    Quote Originally Posted by clulup View Post
    I'm a self-critical person so when I ran into a physicist yesterday night (Ph.D., internationally renowned university ), I told him about the problem at hand. He totally supported my version and the additional examples I brought up.
    You should ask your friend what grade he or she got in high school physics. I bet it wasn't an A+ (or the equivalent).

    IMHV, you don't seem to understand that the treadmill accelerates, too, when the plane accelerates, and you don't seem to accept some basic action=reaction principles (e.g. holding car on a treadmill rolling at -x mph vs. pushing car on ground with x mph), but what can I do? Case closed.
    There's nothing H about your V.

    Earlier, I implied that the frictional force on a car's axles would vary with the acceleration of the treadmill, but I was wrong. It stays constant. Whether the treadmill is moving at 2 mph, 50 mph, or accelerating quickly from 2 mph to 50 mph, the force of friction on the axles will be the same (aside from the effects of heat).

    Kinetic friction is generally directly proportional to the weight of an object (which determines how hard two surfaces press against each other). It's normally unrelated to horizontal speed or acceleration. For example, as tires skid on the ground, the frictional force slowing down the vehicle stays constant (aside from variations in the road surface and tire tread).

    In the case of axles on a car or plane, horizontal acceleration of the vehicle becomes a factor because it increases the pressure between the axle and the wheel. (When a car is standing still or moving at constant velocity, the force of the weight is straight down. When the car accelerates, the axle force increases and points slightly to the rear.) But an accelerating treadmill does nothing to change the pressure - it just spins the wheels faster - so the friction doesn't increase.

    Your case gets even worse.

    As a plane accelerates, it already experiences an elevated level of axle friction (still a trivial amount relative to the force of the engines). The treadmill will spin the wheels faster, but will add ZERO to the force of friction that already exists. So the plane will move exactly the same as if it were traveling on solid ground.

    Case closed.

    If there is anything you didn't understand, please feel free to ask your physicist friend.
  4. #44  
    Quote Originally Posted by samkim View Post
    You should ask your friend what grade he or she got in high school physics. I bet it wasn't an A+ (or the equivalent).

    There's nothing H about your V.

    Earlier, I implied that the frictional force on a car's axles would vary with the acceleration of the treadmill, but I was wrong. It stays constant. Whether the treadmill is moving at 2 mph, 50 mph, or accelerating quickly from 2 mph to 50 mph, the force of friction on the axles will be the same (aside from the effects of heat).

    Kinetic friction is generally directly proportional to the weight of an object (which determines how hard two surfaces press against each other). It's normally unrelated to horizontal speed or acceleration. For example, as tires skid on the ground, the frictional force slowing down the vehicle stays constant (aside from variations in the road surface and tire tread).

    In the case of axles on a car or plane, horizontal acceleration of the vehicle becomes a factor because it increases the pressure between the axle and the wheel. (When a car is standing still or moving at constant velocity, the force of the weight is straight down. When the car accelerates, the axle force increases and points slightly to the rear.) But an accelerating treadmill does nothing to change the pressure - it just spins the wheels faster - so the friction doesn't increase.

    Your case gets even worse.

    As a plane accelerates, it already experiences an elevated level of axle friction (still a trivial amount relative to the force of the engines). The treadmill will spin the wheels faster, but will add ZERO to the force of friction that already exists. So the plane will move exactly the same as if it were traveling on solid ground.

    Case closed.

    If there is anything you didn't understand, please feel free to ask your physicist friend.
    I'll admit that I was wrong when I said case closed. I also say you are right about your corrected version of rolling friction being independent of speed/acceleration (even if it doesn't change anything).

    You still don't take into consideration the nature of the treadmill in case 1 described in post 23 in this thread - think about it again: the treadmill moves backwards at the speed of the plane relative to the treadmill.

    Think of a plane facing north tied with a rope to a pole standing past the north end of the treadmill. The plane is standing precisely at the equator. The treadmill is rolling south at 5 mph. The plane does not move relative to the equator because it is tied to the (extended) rope. What is the speed of the airplane relative to the treadmill, in your view? My view: it is 5 mph. This is consistent to the "law of the treadmill in case 1".

    For any movement towards north, the plane would have to move FASTER than the speed of the treadmill in the opposite direction. This is in violation of how the treadmill works in this case, so it is NOT possible in this scenario!!! If the plane would roll north at 5 mph, this would mean it rolls 10 mph relative to the treadmill which rolls south at only 5 mph, which is not possible according to the basic setting... do you really dispute that?

    As mentioned in post 23, a plane CAN take off on other treadmills (e.g. in case 2 described above, treadmill moving in opposite direction at airplane speed relative to ground), but not in the special case of the case 1 treadmill.
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  5. #45  
    Three possibilities:

    1) Treadmill surface moves (relative to the ground) at the same speed but opposite direction as the airplane moves relative to the ground. Plane takes off, no problem, wheels spinning at 2x normal RPM.

    2) Treadmill surface moves (relative to plane) at the same speed but opposite direction as the plane moves relative to the treadmill surface. Since nothing is "indexed to" the ground, the plane takes off; the treadmill surface in this case can be moving at any speed relative to the ground in either direction, or even stationary, since the speed of the plane and treadmill are relative to each other. An observer on the treadmill surface will see the plane go by at v, and an observer in the plane will see the treadmill surface go by at -v, no matter how fast or slow (or which direction) the treadmill surface is moving.

    3) Treadmill surface moves (relative to ground) at the same speed but opposite direction as the plane moves relative to the treadmill surface. The plane will remain stationary relative to the ground, and any change in the speed of the treadmill will have to be initiated by the treadmill, since for the plane to move relative to the ground, its speed along the treadmill will have to be greater than the speed of the treadmill relative to the ground.

    Plane full of birds: Plane doesn't weigh any less if the birds are flying compared to standing on the seats, provided it's a normal (enclosed) airplane. That had the people at airliners.net joking about an airplane made of chicken wire, so if any of you happen across that site, you may see references to a chicken-wire airplane full of birds taking off from a conveyor belt.
    Last edited by Tom LaPrise; 12/11/2006 at 07:53 AM.
    "Yeah, he can talk. It's gettin' him to shut up that's the trick!"
    -Shrek
  6. vw2002's Avatar
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    #46  
    really, folks. this thread reminds me of the famous "coconut" debate from monty python's holy grail....


    king arthur to the castle guard:

    " we are on an quest by god in a search for the holy grail. WHOSE castle is this?!!!"


    castle guard:
    "wait a minute...those aren't horses you're riding, you've got coconuts and you're bangin' em together!

    arthur: "SO?"

    castle guard: " well coconuts are tropical - this is a temperate zone!!!"

    arthur: " it doesn't matter. as I was saying..."

    castle guard interrupts: "where'd ya get the coconuts?"

    arthur: " BE QUIET!!"....

    Second castle guard: "an african swallow maybe? he could grip it by the husk...."

    castle guard: " its not a question of where he grips it, its a simple question of weight ratios... a four ounce bird could not possibly carry a one pound coconut......"

    arthur: " BE QUIET!!! I ORDER YOU TO BE QUIET!!!...."

    second castle guard: "but what if two swallows carried it?"

    castle guard: "ahhh, yes I see... but then of course.... etc.

    arthur "rides" away....
    I gotta have more cowbell
  7.    #47  
    As I said in post #11:
    Quote Originally Posted by samkim View Post
    Interpreting the speed of the plane as being relative to the moving surface of the treadmill doesn't work. When the plane's engines start, the plane will begin moving; the treadmill is not capable of offsetting the force of the engines. And then you'll be trying to set (the speed of the treadmill) = (the speed of the plane relative to the treadmill). But that equals (the speed of the treadmill) + (the speed of the plane relative to the ground).
    a=a+b, where b>0
    Can't happen.
    Once the engines are turned on, the plane will start moving, and the conditions of this interpretation of the question will be violated. So this interpretation is impossible.

    If you'd prefer, you can adopt this interpretation and say that the pilot is somehow unable to turn on the engines. But once the engines turn on, it's game over.
    Last edited by samkim; 12/11/2006 at 10:53 AM.
  8. #48  
    "Are you trying to tell me coconuts migrate?"
    "Yeah, he can talk. It's gettin' him to shut up that's the trick!"
    -Shrek
  9. #49  
    Quote Originally Posted by samkim View Post
    As I said in post #11:Once the engines are turned on, the plane will start moving, and the conditions of this interpretation of the question will be violated. So this interpretation is impossible.

    If you'd prefer, you can adopt this interpretation and say that the pilot is somehow unable to turn on the engines. But once the engines turn on, it's game over.
    Wrong again. The pilot turns the engines on, the plane accelerates with the acceleration a, the treadmill accelerates, too, but with the acceleration -a. That's an innate feature of the problem, not something you can argue against. The result is that the airplane stays where it is relative to the air/ground, the treadmill rolls backwards at the speed the airplane would have on normal ground. That's what I have been trying to explain for ages now. Thank goodness we finally got there!
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  10. #50  
    For the plane to stay fixed relative to the ground, Newton would require that the rear-ward force on the plane (from the treadmill) be equal to the forward thrust from the jet engines.

    Not possible.
    --
    Aloke
    Cingular GSM
    Software:Treo650-1.17-CNG
    Firmware:01.51 Hardware:A
  11. #51  
    Quote Originally Posted by aprasad View Post
    For the plane to stay fixed relative to the ground, Newton would require that the rear-ward force on the plane (from the treadmill) be equal to the forward thrust from the jet engines.

    Not possible.
    Why not? It's perfectly feasible to have a plane rolling at 5 mph on a treadmill which is rolling at 5 mph into the other direction. Or at 50 or 500 mph if you like. Of course you can always say "but there are no treadmills of that size, or they can't accelerate lika a plane, etc.", but would that be any fun?
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  12. #52  
    Because there is only so much rolling friction at the treadmill interface and in the axle of the plane. The engines are much more powerful.

    Will your scenario break down if this was a treadmill with very low coeff of friction (coated with ice) and the plane had jetski's instead of wheels? Yes.

    My scenario will break down if the friction between the tires and treadmill was infinite (rigidly attached).

    Reality is that of a wheel rolling on a surface and roller bearings at the wheel-plane axle. In that mode, the rear-ward force on the plane will NEVER equal the engines at full thrust.
    --
    Aloke
    Cingular GSM
    Software:Treo650-1.17-CNG
    Firmware:01.51 Hardware:A
  13.    #53  
    Quote Originally Posted by clulup View Post
    Wrong again. The pilot turns the engines on, the plane accelerates with the acceleration a, the treadmill accelerates, too, but with the acceleration -a. That's an innate feature of the problem, not something you can argue against. The result is that the airplane stays where it is relative to the air/ground, the treadmill rolls backwards at the speed the airplane would have on normal ground. That's what I have been trying to explain for ages now. Thank goodness we finally got there!
    *Sigh* I thought we were finally coming to an agreement.

    The horizontal force transferred by the treadmill is due to friction, and will stay small no matter how fast you move or accelerate the treadmill.

    If a racecar were driving by you at 50 mph, how fast would you have to rub sandpaper on it to stop it? What if you were to accelerate the sandpaper from 0 to 20,000 mph? Doesn't matter; the friction force stays the same.
  14.    #54  
    New question: Is there a difference in how much exercise you get running on a treadmill versus running on the ground?

    How about walking up ten steps on stairs versus ten steps on an escalator?
  15. #55  
    Quote Originally Posted by samkim View Post
    The horizontal force transferred by the treadmill is due to friction, and will stay small no matter how fast you move or accelerate the treadmill.

    If a racecar were driving by you at 50 mph, how fast would you have to rub sandpaper on it to stop it? What if you were to accelerate the sandpaper from 0 to 20,000 mph? Doesn't matter; the friction force stays the same.
    Yes, exactly, it is not possible for an airplane to roll at speed x on a treadmill rolling at speed -x. The slightest push forward would create a relativistic instability leading to the formation of a local black whole which swallows the airplane AND the treadmill.

    As if this had anything to do with whether one can stop a speeding racecar with sandpaper. Either you don't want to get the basic principle describing the bahaviour of the particular treadmill in the particular version of the problem, or you don't want to get it. Never mind, it's ok.
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  16. #56  
    Quote Originally Posted by samkim View Post
    New question: Is there a difference in how much exercise you get running on a treadmill versus running on the ground?
    Running on a treadmill is not possible because you cannot stop a speeding racecar with sandpaper, unless you are an airplane, of course.
    How about walking up ten steps on stairs versus ten steps on an escalator?
    It is easier on an escalator because while you walk up, the escalator carries you higher where the gravitational force is smaller.
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  17. vw2002's Avatar
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    #57  
    "Are you trying to tell me coconuts migrate?"

    they could be carried. the swan flies south for the winter,as does the swallow north in the spring, yet these are not strangers to our lands....
    I gotta have more cowbell
  18.    #58  
    Quote Originally Posted by clulup View Post
    As if this had anything to do with whether one can stop a speeding racecar with sandpaper. Either you don't want to get the basic principle describing the bahaviour of the particular treadmill in the particular version of the problem, or you don't want to get it. Never mind, it's ok.
    It's exactly like stopping a race car with sandpaper. It's exactly like a plane with skis on ice. It's exactly like wearing rollerblades on a treadmill and pulling yourself forward. Understanding that is necessary to understanding the original problem.

    Quote Originally Posted by clulup
    I'm a self-critical person
    Not enough!
  19. #59  
    Quote Originally Posted by samkim View Post
    It's exactly like stopping a race car with sandpaper. It's exactly like a plane with skis on ice. It's exactly like wearing rollerblades on a treadmill and pulling yourself forward.
    Just for the record: you say it is impossible that an airplane rolls at 5 mph relative to the surface of a treadmill that rolls at 5 mph in the opposite direction?
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  20. vw2002's Avatar
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    #60  
    SOLDIER

    Are you suggesting coconuts migrate?

    ARTHUR

    Not at all. They could be carried.

    SOLDIER

    What? A swallow carrying a coconut?

    ARTHUR

    Why not?

    SOLDIER

    I'll tell you why not ... because a swallow is about eight

    inches long and weighs five ounces, and you'd be lucky

    to find a coconut under a pound.


    ARTHUR

    It could grip it by the husk ...


    SOLDIER

    It's not a question of where he grips it, It's a simple

    matter of weight - ratios ... A five-ounce bird could not

    hold a a one pound coconut.


    ARTHUR

    Well, it doesn't matter. Go and tell your master that

    Arthur from the Court of Camelot is here.



    A Slight pause. Swirling mist. Silence.



    SOLDIER

    Look! To maintain airspeed Velocity, a swallow needs to beat

    its wings four hundred and ninety three times every

    second. right?

    ARTHUR

    (irritated)

    Please!

    SOLDIER

    Am I right?

    ARTHUR

    I'm not interested.

    SECOND SOLDIER

    (who has loomed up on the battlements)

    It could be carried by an African swallow!

    FIRST SOLDIER

    Oh yes! An African swallow maybe ... but not a European

    swallow. that's my point.

    SECOND SOLDIER

    Oh yes, I agree there ...

    ARTHUR

    (losing patience)

    Will you ASK your master if he wants to join the Knights

    of Camelot?!

    FIRST SOLDIER

    But then of course African swallows are non-migratory.



    SECOND SOLDIER

    Oh yes.



    ARTHUR raises his eyes heavenwards and nods to PATSY. They turn

    and go off into the mist.

    FIRST SOLDIER

    So they wouldn't be able to bring a coconut back anyway.

    SECOND SOLDIER

    Wait a minute! Suppose two swallows carried it together?

    FIRST SOLDIER

    No, they'd have to have it on a line.


    Stillness. Silence again.
    I gotta have more cowbell
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