1.  12/09/2006, 09:47 PM Originally Posted by aprasad 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.  12/10/2006, 02:39 PM Originally Posted by samkim 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. ****)
6.  12/11/2006, 09:29 AM 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.  12/11/2006, 11:41 AM As I said in post #11: Originally Posted by samkim 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 11:53 AM.
8.  12/11/2006, 12:30 PM "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.  12/11/2006, 02:35 PM Originally Posted by samkim 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.  12/11/2006, 02:51 PM 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.  12/11/2006, 03:40 PM Originally Posted by aprasad 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.  12/11/2006, 04:07 PM 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.  12/11/2006, 04:53 PM Originally Posted by clulup 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.  12/11/2006, 04:59 PM 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.  12/11/2006, 05:20 PM Originally Posted by samkim 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.  12/11/2006, 05:26 PM Originally Posted by samkim 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.  12/11/2006, 05:36 PM "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.  12/11/2006, 06:49 PM Originally Posted by clulup 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. Originally Posted by clulup I'm a self-critical person Not enough!
19.  12/12/2006, 01:31 AM Originally Posted by samkim 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.  12/12/2006, 01:45 AM 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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