## Block and Tackle

Suppose you have a rope and a pulley, and you want to pull a log six feet across the ground to the base of a tree. Anchor the pulley to the base of the tree and run the line through it with one end tied to the log. To pull the log six feet, you have to pull the rope six feet. The mechanical advantage is zero and you might as well just drag the log with the rope and forget the pulley. All the pulley did is change the direction of the force.

Now, tie one end of the rope to the tree, tie the pulley to the log, and run the line through it. Thus arranged, to move the log six feet, you'll have to pull the rope twelve feet. In exchange for the double distance, you'll have double strength. One hundred pounds of pull can move two hundred pounds of load.

When you're skidding a timber, you can stand anywhere and pull. Lifting a timber, though, you probably want to stay on the ground, so you add one more pulley at the top of the gin pole to change the direction of the force. Now you have that 2:1 advantage again, and you can pull the rope with all your weight.

Rule of thumb: pull from the anchored end and your advantage equals the number of passing ropes; pull from the moving end and you subtract one, because the last pulley just changes the direction of the force.