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Weight of a vehicle

  1. Jul 10, 2012 #1
    Hey

    I was watching a TV show called QI on BBC and there was a question which went along the lines of:

    If you weight a lorry full of chickens. Then all the chickens jump off the ground at the same instant, in that lorry, the lorry doesn't loose any weight.

    Then it went on to say, if the lorry was open top, and the chickens jumped, it will only lose weight if the chickens jumped higher than the lorry's height - but any lower and the lorry's weight doesn't go down.

    But to me I don't see how this can be possible. If all the chickens are airborne how are they adding to the weight of the lorry?

    I tried to find the clip on youtube, no such luck but its on BBC Iplayer who those who have access to watch the episode:
    http://www.bbc.co.uk/iplayer/episode/b00vs4h4/QI_Series_H_Hypothetical/

    Hypothetical is the episode in which they mention it.

    For me I don't see how it can't lose weight, can some one explain it in a more scientific way ?
     
    Last edited: Jul 10, 2012
  2. jcsd
  3. Jul 10, 2012 #2
    EDIT: Below is a totally different scenario...see Berkeman's post!
    I would imagine that this has to do with the displaced air by the chickens. In an open lorry, if the chickens jump high enough, it is possible that some of the air they displace gets directed away from the lorry. This results in a reduced effective weight on the lorry. If the lorry is closed, then for the birds to move up in the air, the air must be displaced downwards, and the net weight on the lorry at any given time is the same.

    An analogous example of this is a closed plane filled with birds to carrying capacity. When the birds fly up, the plane cannot rise up because to fly, the birds displace air downwards. The plane is still experiencing the same weight. However, if we open the windows, now the plane can rise because air can escape through the windows.
     
    Last edited: Jul 10, 2012
  4. Jul 10, 2012 #3

    berkeman

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    There is a difference between whether they are hovering flying in the lorry in a steady-state way, versus if they jump up and fall back down to the lorry floor without using their wings at all.

    If they are hovering flying, then the lift they are using to hover results in an equivalent force down on the floor of the lorry.

    But if they jump up and then come back down, that is a dynamic situation where the effective weight of the lorry first goes up as they exert the extra jumping force, then goes down to just the weight of the lorry, and then spikes higher again as they land on the lorry floor. It's the same as if you stand on a scale to weigh yourself, and jump up and then land back on the scale...
     
  5. Jul 10, 2012 #4
    Well according to the TV show they specifically focussed on jumping more than hover/flying. And said the weight doesn't change =/
     
  6. Jul 10, 2012 #5

    berkeman

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    Well, if you have a bathroom scale at home, it's easy to see what really happens, eh?
     
  7. Jul 10, 2012 #6
    Yeh but they were saying its not the case with a lorry because it acts like a "system" - they didn't get very scientific with it, but they are usually good at doing their research. Wish i could find the part of the episode cos it would explain better than I :P

    But their info did seem incorrect to me :P
     
  8. Jul 10, 2012 #7
    The scale is an example of the open lorry with the chickens jumping a large height. Imagine if the whole floor of the bathroom were a scale and you were really really really REALLY BIG and imagine the walls were totally sealed off. Now, the scale would read the same at all times, jumping or otherwise.

    EDIT: above is false :P
     
    Last edited: Jul 10, 2012
  9. Jul 10, 2012 #8

    berkeman

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    They probably just did not explain it very well. If they were saying that the weight of the lorry didn't change, almost certainly they were referring to the birds flying. After all, there's no reason for the animals to be birds in the example, unless their action involves flying. Might just as well have used frogs if the point was jumping. :smile:
     
  10. Jul 10, 2012 #9

    berkeman

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    Not true. An impulse generates an extra force, no matter how big the scale is.
     
  11. Jul 10, 2012 #10
    Well let's say we have a glass jar, with a frog and some air in it. If I put this jar on a scale, whether the frog jumps in the jar or not, it weighs the same on the scale. If we poke holes in the jar at the top, and the air isn't too dense, the jar will still register roughly the same weight. If we poke holes in the jar near where the frog's legs are, the jar will register as lighter when the frog jumps. I think this is the picture the show was trying to illustrate.

    EDIT: Nevermind, the act of jumping is different than steady state hovering, as Berkeman pointed out. My example above only makes sense if I replace "frog" with "fly" and replace "jump" with "fly!" :P
     
    Last edited: Jul 10, 2012
  12. Jul 10, 2012 #11
    oh...ok nevermind then...I guess I didn't think about it enough. Sorry.

    Yeah the jumping is different from flying. I wasn't thinking about that.
     
    Last edited: Jul 10, 2012
  13. Jul 10, 2012 #12
    Are you sure this is true?? I would have thought if the frog jumps, the scales would show that it would gain weight then lose weight, then gain weight on landing then return to starting weight ?

    Otherwise there is no force being measured from the frogs jump..?
     
  14. Jul 10, 2012 #13

    berkeman

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    Yeah, he said nevermind. I think we're all on the same page now. :smile:
     
  15. Jul 10, 2012 #14
    It works on a similar principle to how rear wing on a Formula 1 car adds weight, by generating downforce. It's not really weight in the physics sense (mass*gravitational acceleration), but if you had an F1 car on a set of scales while it was generating downforce, it would weigh more without any change in the vehicles mass.

    A rear wing on an F1 car generates high pressure above the wing and low pressure below the wing, the pressure of air effectively exerts a downward force on the car.

    The example of the chickens is similar assuming the lorry is a sealed unit, the chickens flying exert a downward pressure on the lorry much in the same way a wing on a car exerts a downward force due to pressure.

    If you think about it, the chickens lifting off must exert an equal and opposite force in order to lift of, as they must at least match their body weight in lift to fly. The exact same force must act in the opposite direction for this to happen.

    If the chickens are not in a sealed container, as the fly above the lorry, the equal and opposite force is not all directed to the lorries bed, so is dissipated to the surroundings and the net weight of the lorry will drop.
     
  16. Jul 11, 2012 #15

    A.T.

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    When chickens "jump", they mostly use their wings to create lift. I guess that is what they meant. But I agree that is a ambiguous scenario. Usually a fly hovering in a jar, versus sitting at the bottom of it is used.

    Also: The weight of truck + chickens is of course constant and given by m*g. What might change due to accelerations is the normal force exerted on the scale (measured weight).
     
  17. Jul 11, 2012 #16

    berkeman

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    I need to close this thread now that it's run its course. There are some deleted posts that y'all can't see -- the thread has become a magnet for misinformation. I think we've covered the physics okay.
     
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