Book excerpt: New analysis shows how Newton and Galileo provide a straightforward and useful explanation of sailing. According to Newton, the sail re-directs a mass of air of the apparent wind backwards to create a backward force (Force = ma). The reactive equal and opposite forward force pushes the sailboat ahead. According to Galileo a boat can sail downwind faster than the wind for the same reasons that it can sail upwind faster than the wind. Each tack is simply the mirror image of the other one. This is because in both situations the sailboat experiences a headwind, and the true wind is moving backwards relative to the boat. When sailing both upwind and downwind, the sail extracts momentum and energy from the true wind by slowing it down. That's it.