Here's a brainteaser: a driver with a cargo of birds approaches a bridge with a limit of 5,000kg. His lorry weighs 4,800kg and his load 400kg. The driver then has an idea: he strikes the side of the lorry, frightening the birds into flight, and quickly drives across while they are airborne. Does the ploy work? This question is the stuff of late-night bar discussions among physics students. The answer is that physics is not fooled. A bird's flapping produces a downdraft that pushes on the lorry's floor with a force, on average, the same as the bird's weight.
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