The physics of sailing arises from a balance of forces between the wind powering the sailing craft as it passes over its sails and the resistance by the sailing craft against being blown off course, which is provided in the water by the keel, rudder, underwater foils and other elements of the underbody of a sailboat, on ice by the runners of an ice boat, or on land by the wheels of a sail-powered land vehicle.
Forces on sails depend on wind speed and direction and the speed and direction of the craft. The speed of the craft at a given point of sail to the “apparent wind” —the wind speed and direction as measured on the moving craft. The apparent wind on the sail creates a total aerodynamic force, which may be resolved into drag — the force component in the direction of the apparent wind — and lift — the force component normal (90 °) to the apparent wind. Depending on the alignment of the sail with the apparent wind (angle of attack), lift or drag may be the predominant propulsive component. Depending on the angle of attack of a set of sails with respect to the apparent wind, each sail is providing motive force to the sailing craft either from lift-dominant attached flow or drag-dominant separated flow. Also, sails may interact with one another to create forces that are different from the sum of the individual contribute of each sail, when used alone.