The Pulley

The pulley is a wheel that turns around an axle. A groove in the edge of the wheel allows a rope to pass over it or under it without slipping off. There are two kinds of pulleys fixed pulleys and movable pulleys. In a fixed pulley, the wheel turns but the arm holding the wheel is fixed in one place. A movable pulley moves up or down while the wheel turns. 

Sometimes the two kinds of pulleys are combined. The drawing at the left on the next page shows one fixed pulley and one movable pulley, used together to lift a load. The load is easier to lift this way than with a single fixed pulley. In fact, you can overcome a resistance of 50 N in this way with an effort of only 25 N. A block and tackle uses two fixed pulleys and two movable pulleys. With this combination, you can overcome a resistance of 100 N with an effort of 25 N.

The Wheel and Axle

 A wheel and axle machine is just that a wheel fixed to an axle, like the steering wheel of a car. When a steering wheel is turned, the axle, or shaft, turns with it. It would be impossible to turn the steering axle by grasping it in your hands. The wheel that is attached to the top of the axle makes steering possible. By grasping the rim of the wheel, the driver can easily apply enough force to turn the axle. The doorknob is another common example of the wheel and axle machine. As with the pulley and the lever, the advantage of greater force is matched by a disadvantage of increased distance.