4.6 Pulley:
A pulley is a wheel on an axle or shaft that is designed
to support movement and change of direction of a cable
or belt along its circumference. Pulleys are used in a
variety of ways to lift loads, apply forces, and to transmit
power. In nautical contexts, the assembly of wheel, axle,
and supporting shell is referred to as a "block. "A pulley
may also be called a sheave or drum and may have a
groove between two flanges around its circumference.