Naked Science Forum
Non Life Sciences => Technology => Topic started by: Karsten on 07/01/2009 13:38:47
-
The original problem:
____________________________________________________________________________
A box truck loaded with 2000 pigeons arrives at a bridge. The bridge is limited to slightly less than the truck weighs at the moment. The driver gets out with a stick, pounds on the outside of the truck to scare the pigeons and get them flying. He then drives the truck quickly across the bridge while the pigeons are still airborne. The truck got lighter because the pigeons are flying inside the truck. Does that work?
___________________________________________________________________________
Please assume: a) the truck is not sealed, b) the birds are small and have room to fly around, c) the birds are able and willing to stay airborne for a while, d) it takes only a short time to cross the bridge, e) it does not matter how much more the truck weighs before or after the bridge crossing.
After a night of sleeping and thinking I have to take back my initial excitement thinking I had finally found the answer posted by Graham D.:
"If the truck is a reasonably sealed container (it can have leaks and airflow to allow the birds to breathe) the truck's weight will fluctuate but the average will stay the same. If you can devise any scheme which involves a sealed container but somehow can change its weight (for a lengthy period) by some event inside the box, you would make a huge amount of money!"
A) It IS (!) possible to lighten a container that is NOT completely sealed with an event happening inside. All you have to do is put the air holes at the bottom of the container and light a camping stove inside. If you do this well, the container even gets airborne. It would be called a hot air balloon. Not much money is to be made with this these days. If there is one exception that fits the premises, there might be others. The truck is not sealed, the birds use energy to lift themselves, it works only until energy for flight runs out.
B) I am not worried about lightening the truck permanently. The driver wants the truck to be lighter for the time necessary to cross the bridge. It does not matter if it is heavier before or after. Say, the truck needs to be lighter for 1 minute. This phenomenon might be just a fluctuation, but if any fluctuation in weight is possible, the truck can get lighter for the time it takes to cross the bridge.
C) If a object falls freely from the ceiling to the ground inside a truck, the truck would be lighter for the time of the fall. Would it be lighter LONGER if the objects were attached to a parachute? If I could jump up for one minute, the truck would be lighter for one minute (even if it was heavier before and after the jump). Does powered flight result in pressure on the ground equal to the weight of the flying object?
Now, I hope you are still reading this even though I thought I was proven wrong and publicly accepted that the truck will not get lighter. The answer seemed to obvious yesterday. Sorry if this is annoying.
Karsten
-
This sort of thought experiment is a bit like a role playing game. If the original rules aren't specified down to the last detail, the exercise has limited validity.
The thread chased its tale - as they all do. It was useful, though, if it made people think. The 'right' answer doesn't exist but does that matter?
-
Well, it matters to the driver of the truck who does not want to crash through the bridge and perish due to having come to wrong conclusions. :)
Maybe I will have one day the opportunity to put a large cage on a sensitive scale and just measure it. Maybe a cage/box with bats since they are able to fly in a small enclosure. I will post the results here of course.
Karsten
-
Assuming each pigeon weighs approximately 1KG, if the truck has ventilation top & bottom the downdraught from the 2000 flying pigeons will have the same affect as a helicopter or hovercraft. The truck will get 2 tonnes lighter but the bridge will still be overloaded by the added 2 tonne air pressure. However if the bridge deck is of open construction, the downdraught will pass through and the pigeon lorry can pass safely across [;)]
-
Do birds in flight create the same downdraft as helicopters or hovercraft? Does a fixed-wing aircraft create downdraft as well? Do birds fly more like helicopters or more like fixed-wing aircraft.
Boy, the questions never end.
Karsten
-
Q1: Do birds in flight create the same downdraft as helicopters or hovercraft?
A1: Only when hovering
Q2: Does a fixed-wing aircraft create downdraft as well?
A2: A pressure wave rather than down draft
Q3: Do birds fly more like helicopters or more like fixed-wing aircraft.
A3: See answer 1
The pigeons may not perform too well in the race following the exhausting trauma of crossing the bridge. Could they be released prior to crossing? This might solve both problems. [:-\]
-
And a helicopter produces much more downdraft AND is much less efficient when it is hovering.
-
OK, so an airplanes (and presumably birds in forward flight) create a pressure wave. May I assume further, that if the birds or airplane are far above the ground this pressure wave does not reach the ground, or cannot be measured on the ground? If that is so, I will assume even further that this is due to internal friction of air. I will bravely conclude that if energy does not go away, it is transferred into heat in this case. Now, going really far out on a limb, if my birds (who I do not want to release even if they get tired and stressed due to repeated banging on the truck each time there is a bridge) are in forward flight not close to the floor, does my truck get lighter and warmer as long as the birds are in the air? Even if it is completely sealed (hypothetically speaking, since the birds will have to breathe)?
This is getting complicated and rather hypothetical. Still amuses me though.
Karsten
-
Think I've hit on the answer: If the pigeons all grip the their perches with their feet and have the ventilation reconfigured so that the air intake is on top of the truck and the exhaust to the rear. The reduced pressure above the truck will provide lift thus satisfying the first objective of reducing weight. The air flow diverted rearwards will provide thrust and assist fuel economy without loading the bridge deck with downdraft. This could be a real environmental winner!