Wednesday, 14 November 2007 Flight of fancy A truck driver approaches a bridge that has a weight limit of 5000 kilograms. He and his truck weigh 4950 kilograms so he would be able to cross it were it not for his 100-kilogram cargo ? a flock of pigeons loose in the back of the truck. He has the bright idea of banging on the side of the truck to scare all the birds into taking flight and then he quickly drives across the bridge. Does it work? Assume each pigeon is 1kg. it needs to provide 9.8 N of force the counteract gravity. Most of this air will probably be forced agianst the floorspace of the lorry. Thus the net effect will be nearly the same as if they were on the floor. Some will escape, but in the problem, it would require half or the force to be allowed out as escaping air. I dont know how much would actually go out, through cracks, so I suppose that the success of the solution depends on how airtight the vehicle in question is. It also depends how lazy the pigeons are, they may not neccessarily continue in flight for the duration of the crossing. I seriously doubt that the health and safety people give people no leeway as to the exact weight of their load. I'm absolutely sure that if the birds are in flight they are weightless as far the weight of the truck is concerned. Even in an