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  • 24-05-2011 9:27am
    Registered Users Posts: 10,917 ✭✭✭✭ GT_TDI_150

    one that has had friend of our going for years:

    a truck driver gets stopped and put on the weigh-bridge, he hows as being over his tonnage. his defense is that he is carrying birds, when he drives the fly round in the lorry thus lessening the weight of the lorry.



  • Registered Users Posts: 68,318 ✭✭✭✭ seamus

    You can see why a flying bird would appear to "weigh" less than a standing one.

    However in order for a bird to fly, it needs to exert downforce on the air which is greater than their body weight. So, assuming that the trailer is airtight, then total downforce applied to the air in the trailer by all of the birds will be greater than or equal to their collective weight, and this downforce will likewise apply to the floor of the trailer, registering on the scales.

    However, if the trailer is open (think wire grill along either side), then the pressure in the trailer is equalised with the pressure outside and as such much (or most) of the energy applied by the flying birds is lost to the outside, and the trailer will indeed appear to weigh less.

    In effect, with a standard closed trailer, one hundred birds flying around will likely appear heavier than 100 birds standing still, because of the additional force used to propel themselves upwards.

  • Closed Accounts Posts: 89 ✭✭ naoise80

    Surely if the trailor was airtight, all the birds would die.

  • Registered Users Posts: 68,318 ✭✭✭✭ seamus

    Airtight, not a vacuum :)

    They'd be OK for a few hours.