the birds and the surface would allow energy losses through turbulence and heat to reduce the force sufficiently to save the bridge. I believe that actually when a birds wings are on the upstroke, they are still generating lift, due to some clever aerodynamics, so, even on the upstroke, there will be a net force on the surface beneath the pigeon. This reminds me of a similar problem which I heard - a man wants to carry two gold bricks across a flimsy rope bridge. The bridge is only strong enough to support the weight of himself plus one gold brick. I was told that there was ingeneous way he could manage this in one trip, and having thought about it for a long time, I felt certain that it couldn't be done. I was horrified the learn that the supposed solution was to juggle the bricks whilst walking across the bridge, because this would be even more dangerous than simply carrying both bricks across. Here's why - according to newton's third law of motion, whenever anything exerts a force on an object, that object exerts an equal force back on whatever is exerting that force. Now, consider what this means when the man is standing on the bridge with both bricks, throwing one into the air. In order to do that, the man must exert an upward force on the brick greater than the weight of the brick. By newton's third law, as this happens, the brick exerts a