CoachA pulley is a wheel with a groove for a rope. A fixed pulley hangs from a beam and only changes the pull’s direction: haul down, the load goes up. A movable pulley rides on the rope with the load hooked to it: the load then hangs from two strands of rope, and each strand carries only half the weight.
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Example 31.4 (Pulleys at work)
The flag rises up its pole while your hands pull comfortably down: a fixed pulley at the top — direction changed, effort unchanged, but hauling down lets your own weight help. The builder’s block-and-tackle chains several pulleys so the load hangs from four or six strands: each strand carries a quarter or a sixth of the weight, and one worker raises a piano. Watch the worker, though: metre after metre of rope races through their hands, while the piano creeps upward slowly.
Example 31.9 (Checking the rule)
The movable pulley: your hand pulls with half the force, and to raise the load you haul in of rope — both strands must shorten by a metre. The crowbar: your end sweeps a long arc, the boulder barely stirs. The ramp: the crate reaches the same height, but you pushed it along the whole slope. Every saving paid for, precisely.