Final answer:
A movable pulley is a system where at least one wheel is attached to the object to be moved, and combinations of such pulleys can multiply the input force. The mechanical advantage is calculated by counting the cables pulling directly upward on the system.
Step-by-step explanation:
A movable pulley is one in which at least one wheel is attached to the object that is to be moved. An ordinary pulley has a mechanical advantage (MA) of 1; it only changes the direction of the force and not its magnitude. However, combinations of pulleys are used to multiply force. In a friction-free environment, the force output of a pulley system is approximately an integral multiple of the tension in the cable. The number of cables pulling directly upward on the system indicates the MA of that pulley system. Each cable applies an external force in approximately the same direction as the others, resulting in a total force that is nearly an integral multiple of the input force T. Simple machines like pulleys are often part of the mechanism in devices such as construction cranes and are used for raising flags or window blinds.