If a lorry is carrying a load of birds, does it weigh more, less or the same if the birds are flying?
by 
cambsman
ANSWERS: 1
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The weight of the lorry would vary a little, but the average weight would stay the same. Consider a thought experiment where there is vacuum in the lorry, and I'm inside in a space suit. The lorry is suspended from a very accurate spring balance. I flap my arms. When I excert an accelerating force on my arms to move them downwards, I exert less force on the base of the lorry and the lorry appears to weigh a little lighter. The opposite happens when I accelerate my arms upwards. The average force I exert on the lorry must be equal to my weight, or the net force would mean I was accelerating through the ceiling or floor. The bird example is more complex because the forces are transmitted through pressure waves in the air, but the result is the same.
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