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Pulley system, demonstrating how pulleys facilitate the lifting of heavy weights (2004)
(upload.wikimedia.org)
A gun tackle has a single pulley in both the fixed and moving blocks with 2 rope parts dividing the load of 100N. The mechanical advantage is 2, requiring a force of only 50N to lift the load.
Author: César Rincón
Okay, I've found a nice picture for it. Whoever sad 150 N (with the 180° angle, less with other angles) was right, and the rest of us were wrong :)