I've been wondering something.
Say you were driving a lorry, and you get to a bridge. The bridge can only hold 10 tonnes. You know that you + the weight of the truck is exactly 10 tonnes, so you can get across ok. However, in the back of the truck are a bunch of birds, which make the total weight heavier than 10 tonnes. You get out of the truck, and whack on the side of it with a stick, causing all the birds to fly around inside the truck. If they stay flying for the amount of time it takes you to get over the bridge, will the bridge collapse or not?
My reasoning for the bridge collapsing would be that there's some kind of extra weight produced by the birds flapping their wings. Is this right?
PUT MY MIND AT REST, DiS!