How heavy is the truck?

[Bonzo 2000]

If a truck is carrying a load of live birds, and the birds all take flight within the truck at the same time, does the truck get lighter?

[WaldoMcFroog]

The truck weighs the same regardless of its cargo. If it had three tonnes of gold on the back, the weight of the truck itself is unaffected.

[Bonzo 2000]

...but the gold would be resting on the truck floor

[Insane Wally]

yes, the truck would be lighter

[Potatoman]

this does sound a bit like one of thos trick questions! but if it isnt, the truck would be lighter

[Clanad]

No, the truck would weigh the same. As the birds take flight their weight is tranferred to the air through which they are passing, which in turn is "passed on" to the truck...

[LeMarchand]

The birds' weight would not "be transferred through the air"; you can check this by placing something light (so you don't break your equipment) on a pair of scales, then lifting it up and dropping it onto the scales - it will only register the weight when the item touches the scales (or possibly fractionally before due to air pressure).

To sum up the agruments above: the truck remains the same weight, but if you were to weigh the truck and the birds and compare it to the weight of the truck with the birds in flight, the second weight would be lighter.

[Bonzo 2000]

to clarify, its not meant to be a trick question. Of course the weight of the truck itself is always the same.  I wonder about the combined weight of the truck and its load, or the weight the truck tyres transfer to the road - will this reduce as the birds take off?

[Dagman]

This is an old one! The birds in flight would create a downdraft which would be pushing down on the truck. The weight of truck and cargo does not change if the trailer is sealed. However, if the sides were open then some of that downdraft energy will be lost and there would be a loss in some weight. It's all about containing energy!

[Normski]

Thats true, to an extent. The molecules of gasses being moved by the birds' wingstroke would exert some pressure on the floor of the container, but this would dissapate relative to the distance the wings are from the floor. some molecules would change direction enough to be going horizontally and would exert their pressure on the sides of the container so their 'weight' would be pusing outwards rather than downwards.

 

None of this takes account of the period of time that the truck weighs more because of the birds take offs/landings.

 

So the answer is that the truck would be marginally lighter with the birds in flight than with the birds at rest varying dependant upon the height of each individual bird in flight, it's position in it's wingstroke and the volume of gasses that they are conatined within and the randonmess of the directions that the gas molecules are taking when they make contact with the container flor/walls/roof as a result of the wing movement (note the walls, like already said, if it was open sided then there would be a greater degree of randomness to the 'weight'.