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  1. #81  
    Quote Originally Posted by aprasad View Post
    I another one:

    You are on an airplane. Lunch is served. After everyone has eaten and the visited the restrooms etc, is the plane heavier, the same, or lighter than before lunch was served?

    Assuming no in-air discharge of waste material -- I would still say lighter -- as expended body energy has consumed some of the weight that the plane started with.

    As well, the plane is continually burning fuel and growing ever lighter.
    Last edited by gtwo; 12/13/2006 at 06:25 AM.
  2. vw2002's Avatar
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    #82  
    "Lets see what do others think" you say?

    Let us ask Tim the Enchanter, he knows much that is hidden....

    http://video.google.com/videoplay?do...ly+grail&hl=en
    Last edited by vw2002; 12/12/2006 at 10:06 PM.
    I gotta have more cowbell
  3.    #83  
    Quote Originally Posted by clulup View Post
    I would say, in the case of an air- and water-tight transatlantic jet with more than 100 people on board, about 2.5 tons lighter than before lunch was served. Correct?
    That's about how much jet fuel would be burned in 45 minutes by a 747.
  4.    #84  
    Quote Originally Posted by aprasad View Post
    I think: yes, while you are peeing (gravity acting on the pee stream).
    After you are finished, the weight returns back to normal.
    Well I just poured water into a pot while standing on a scale and noticed no effect on the scale. Inconclusive.
  5. #85  
    Quote Originally Posted by Big_OTR_Fan View Post



    click photo to see this fabulous video, if you haven't already seen it...
    `
    `
    `
    Any of the brits (or anybody watching BBC last night) see the royal variety last night?

    The sugababes had an act which reminded me a lot of the clip in the link above..
    <IMG WIDTH="200" HEIGHT="50" SRC=http://www.visorcentral.com/images/visorcentral.gif> (ex)VisorCentral Discussion Moderator
    Do files get embarrassed when they get unzipped?
  6. #86  
    Quote Originally Posted by Perry Holden View Post
    Assuming no in-air discharge of waste material -- I would still say lighter -- as expended body energy has consumed some of the weight that the plane started with.
    Energy conversion does not lead to a change in mass and hence weight (not counting nuclear power). In biological systems, it is more or less only a matter of electrons, H and O atoms changing places, the mass stays constant.
    As well, the plane is continually burning fuel and growing ever lighter.
    True. About 2.5 to 3 tons per hour.
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  7. #87  
    Quote Originally Posted by clulup View Post
    Energy conversion does not lead to a change in mass and hence weight (not counting nuclear power). In biological systems, it is more or less only a matter of electrons, H and O atoms changing places, the mass stays constant.
    I'm not sure thats true. A less energetic system should weigh marginally less than a more energetic system, via E=MC2, no matter how the energy gets stored or generated.

    Surur
  8. #88  
    If the mass in both systems is the same, the total energy is the same. More of that energy might be kinetic in one of the systems, but that one would have less potential energy. In E=MC^2, E=total energy, M=mass, and C^2 is the speed of light squared; C^2 is constant, so if the two Ms are equal, the two Es are equal.
    "Yeah, he can talk. It's gettin' him to shut up that's the trick!"
    -Shrek
  9.    #89  
    Not at all sure about this one, but a couple of thoughts...

    Food is burned, so mass is converted to heat energy, which escapes the plane.

    Also, heat expands the plane, displacing more air and causing the plane to weigh less.

    ?
  10. #90  
    Quote Originally Posted by samkim View Post
    Not at all sure about this one, but a couple of thoughts...

    Food is burned, so mass is converted to heat energy, which escapes the plane.

    Also, heat expands the plane, displacing more air and causing the plane to weigh less.

    ?
    The first statement makes sense. The second is non-sense. Expanding a gas makes it less dense, but not lighter.
    Bob Meyer
    I'm out of my mind. But feel free to leave a message.
  11.    #91  
    Quote Originally Posted by meyerweb View Post
    The first statement makes sense. The second is non-sense. Expanding a gas makes it less dense, but not lighter.
    How much does a hot air balloon weigh?

    Mass doesn't go down, but weight does.
  12. #92  
    Quote Originally Posted by samkim View Post
    Not at all sure about this one, but a couple of thoughts...

    Food is burned, so mass is converted to heat energy, which escapes the plane.

    Also, heat expands the plane, displacing more air and causing the plane to weigh less.

    ?
    Then it isn't a closed system... in a closed system, the heat would not escape the plane and the plane would not expand. The temperature of the air in the plane would increase, because the passengers and crew are trading some chemical potential energy for kinetic energy.
    "Yeah, he can talk. It's gettin' him to shut up that's the trick!"
    -Shrek
  13.    #93  
    Okay, it's not a closed system...
  14. #94  
    Quote Originally Posted by samkim View Post
    How much does a hot air balloon weigh?

    Mass doesn't go down, but weight does.
    Nope. Density goes down. The balloon, with the hot air it contains, is less dense that the surrounding air, which is why it rises. The balloon by itself is clearly heavier than air. 100s of pounds heavier. Filling one with air, no matter what temperature, doesn't make it lighter. That would imply that hot air has negative weight which is, of course, impossible. But the hot air, enclosed in the space of the balloon, is much less dense that the cold air around it. So it floats.

    Same is true of a helium ballon. Helium (or hot air), has a positive weight. Anything made of matter has a positive weight (in a gravity field, of course). And the rubber has weight: it doesn't float on its own. So the combination of rubber and helium weighs more than either alone. Adding helium to a rubber balloon can't be, by the laws of physics, lighter than either seperately, right? So it's NOT lighter weight that makes a ballon float. Can't be.

    It's the same reason a million pound naval vessel floats. It's not LIGHTER than the water, it's just less dense than the amount of water it displaces.
    Bob Meyer
    I'm out of my mind. But feel free to leave a message.
  15.    #95  
    Quote Originally Posted by meyerweb View Post
    Nope. Density goes down. The balloon, with the hot air it contains, is less dense that the surrounding air, which is why it rises. The balloon by itself is clearly heavier than air. 100s of pounds heavier. Filling one with air, no matter what temperature, doesn't make it lighter. That would imply that hot air has negative weight which is, of course, impossible. But the hot air, enclosed in the space of the balloon, is much less dense that the cold air around it. So it floats.

    Same is true of a helium ballon. Helium (or hot air), has a positive weight. Anything made of matter has a positive weight (in a gravity field, of course). And the rubber has weight: it doesn't float on its own. So the combination of rubber and helium weighs more than either alone. Adding helium to a rubber balloon can't be, by the laws of physics, lighter than either seperately, right? So it's NOT lighter weight that makes a ballon float. Can't be.

    It's the same reason a million pound naval vessel floats. It's not LIGHTER than the water, it's just less dense than the amount of water it displaces.
    You're right. I was sloppy in my use of the term 'weight.' I was defining it to be the net force on an object, including both gravity and buoyancy, rather than the force of gravity alone, but that's not right.

    As for the naval vessel, it is lighter than the equivalent volume of water, and equal in weight to the water it displaces.
  16. #96  
    I readily admit that I don't even pretend to have the knowledge of physics of aeronautics to address many of the points raised on this discussion with any claim of expertise......more just the average man's perceptions of the physics involved.

    I would imagine that ground speed is only important to gain air speed. A question could be posed. If a plane needs to be going 170 mph for lift off under normal conditions and there is a direct wind of 170 mph going from tail to nose.......can the plane lift off even though it's ground speed is 170 mph? Again, this question may be glaring of my ignorance....I just don't know.

    But it seems that the relative speed of the wheels is not the issue but the speed of the air going around the plane from nose to tail. If the treadmill is compensating for the speed of the wheels and the engines are able to provide enough thrust so that the the plane can move forward.....it would have to have enough thrust to overcompensate the speed that is reduced by the treadmill. I doubt the plane would have enough thrust to over compensate enough to gain the air speed it needs for lift off.
  17.    #97  
    Quote Originally Posted by HobbesIsReal View Post
    I would imagine that ground speed is only important to gain air speed. A question could be posed. If a plane needs to be going 170 mph for lift off under normal conditions and there is a direct wind of 170 mph going from tail to nose.......can the plane lift off even though it's ground speed is 170 mph? Again, this question may be glaring of my ignorance....I just don't know.
    You're correct that the key factor is the plane's air speed. So if both the air and the plane are moving in the same direction at the same speed, the relative speed of the plane to the air will be zero, and so it cannot take off.

    However, if the plane were to just turn around and sit still, in theory, the plane would get lift from the oncoming 170 mph wind.

    Note that aircraft carriers sail into the wind when planes take off.

    If the treadmill is compensating for the speed of the wheels...
    But the friction can't offset the force of the engines, and so it can't keep the plane still.
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