The pulley is a machine which in its simplest form consists of a grooved wheel, made of wood, brass, or iron, with a rope or chain passing over it, fixed in a framework, and free to revolve. As the type of pulley shown in Fig. 17 turns on an axle fixed in one place it is called a fixed pulley.

Such a device makes it easier for a man standing on the floor to raise a weight by pulling on the end of the cord at P than if he pulled the weight straight up by the cord without any pulley, or carried the weight up a flight of stairs.

A pulley may be considered as a rotating lever which is used simply to change the direction of a force. The belt or rope does the work, not the wheel. There is no leverage in a single fixed pulley, and if the weight is 50 lbs., it takes a pull of 50 lbs. at P (ignoring the slight friction of the wheel axle) to raise it. In Fig. 17 the lever arms in the pulley are equal to the radius and the fulcrum is at the center; that is, in a pulley 16 in. in diameter one arm would be 8 in. on one side and the other 8 in. on the other side of the fulcrum.