Wind power is used to drive machinery, which turns the millstones
(see video link below), sifts the meal and moves grain and flour around the mill. But other forces, such as the force of gravity, are used in mills. Having lifted grain to the top of the mill using wind power it is allowed to flow down again, from one machine to another, making use of gravity. Sometimes the two are combined; for example a steady flow of grain to the mills is achieved by allowing it to fall down a sloping chute assisted by vibration provided by the wind powered machinery.
Friction is used to stop the sails turning. An iron or timber band fitted around the main gear wheel, is tightened to act as a brake.
More about how windmills work (focusing on forces and motion) can be found on Page 6 of the Teachers' Notes 1.