Why does pulling down lift the load?
At a flagpole, pulling the rope down raises the flag. A fixed pulley bends the rope into a convenient direction. Ignoring friction and lifting slowly, it does not reduce the needed force.
A moving pulley rises with its load. Two rope sections support it from above, sharing the weight and reducing the force needed at your hand.
There is a trade-off. With two supporting sections, your hand pulls twice as much rope as the load rises. Count the sections directly supporting the moving load, then compare how far the hand and load travel.
Observe a clothesline and a flagpole
Be a detective with no tools at all:
- Watch a flag raised. The puller hauls downward and the flag climbs. Find the pulley at the top and see where the rope bends. Also note: as the flag rises, how much rope passes through the puller’s hands? (Fixed pulley: exactly as much.)
- Watch a clothesline. Many clotheslines carry a small pulley at each end, looped into one ring. Pull one side gently and the hangers glide to the other. Follow the rope and find where the force turns.
- Build one. With a thread spool (or a disc lid), thick string and a water bottle: run the string under the spool and hang the bottle—that is a movable pulley; drape it over a chair back and you have added a fixed one. Lift with each rig and compare the feel.
Predict first: switching the bottle from “hanging at the rope’s end” to “hanging under a movable pulley”—will the pull shrink, and by how much? Say your answer, then pull once and check.
What do the savings cost?
More: a closer explanation
Watch a flag ceremony: we pull the rope downward while the flag climbs. The little wheel at the top—a fixed pulley—bends the rope’s path. The direction changes, the force does not: it works like an equal-arm lever bent around, so with negligible friction, at rest or constant speed, your pull equals the load. Hang two equal sand buckets on the two ends and the rope balances.
To actually save force, let more rope strands share the weight. Hang the pulley into the rope together with the load (a movable pulley) and two strands hold it, each carrying half; chain fixed and movable pulleys into a block and tackle and the load can hang from three, four, even six strands.
Counting n is something anyone can do: starting from the load (or the pulley carrying it), count the rope strands reaching up to it directly—however many there are, that is n. Count every strand pulling directly upward on the load or movable block. If the free end leads upward from that block to your hand, it counts too. A downward strand beyond an upper fixed pulley is not an extra support.
- Fixed pulley: n = 1, pull = weight. No savings—only a comfortable downward direction.
- Movable pulley: n = 2, pull = half the weight.
- Block and tackle: the bigger n, the less each strand carries; cranes lifting multi-tonne beams run on this arithmetic.
Open: where one-nth comes from
The load hangs from n strands; in an ideal rope the tension is the same everywhere, so each strand carries T. At rest, the upward n·T balances the downward weight W:
The pull shrinks, but you must haul more rope: raising the load by h shortens every one of the n strands by h, so your hand takes in n·h:
Open: work conservation, F·s = W·h
Multiply force by distance:
Pulleys create no energy; they only redistribute it—less force means more distance, with the product unchanged. This is “simple machines save force, not work”—OpenStax’s Simple Machines and Static Equilibrium chapters derive this shared conclusion for every simple machine, and NASA’s STEmonstrations videos demonstrated pulleys and levers aboard the space station.
Limits of the model: friction and rope weight
This experiment uses ideal pulleys: no friction, and no weight for the rope or the pulleys themselves. Real axles have friction, and a movable pulley with long rope weighs something, so the actual pull exceeds W/n—the savings are discounted. That is why real rigging never adds strands without limit: too many, and friction plus rope weight eat the savings.
Next time you pass a construction site, look up at the tackle on the crane hook: count the strands of steel rope sharing the load, and you can estimate the force each one carries.