I'm fairly fascinated by the idea of mechanical advantage using pulley systems. Using this pulley system http://www.swe.org/iac/images/plly_071.jpg [Broken] as an example I read that this provides a mechanical advantage of 4. If I'm only putting say 20N force in and the block weights 80N where is the other 60N coming from? I read that Archimedes claimed he could move a ship with enough pulleys and the king challenged Archimedes to put his claim to the test and Archimedes actually managed to move a ship with pulleys. What kinda pulley system would he have used in that scenario?