A block and tackle is the simplest way to trade distance for force. By threading a rope back and forth between a fixed and a moving block, the load ends up hanging from several strands at once — and each strand carries an equal share. Support the load on four strands and you need only a quarter of the force to hold it: the mechanical advantage is four. Nothing comes free, though. To raise the load a metre you must reel in four metres of rope, because energy is conserved — and real pulleys skim a little off the top as friction.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: conservation of energy / simple-machine theory.
The pulley equations
The strand count is everything for the ideal case: count how many rope segments pull upward on the moving block. Efficiency η folds in the friction of the sheaves — multiply the ideal advantage by η to get what you actually feel. The rope length is fixed by geometry and energy: you always haul n times the lift distance, friction or not, so a high-advantage system means a lot of pulling.
Worked example — a 4:1 hoist
Scenario: Lift a 1,000 N load on a tackle with 4 supporting strands at 90% efficiency, raising it 2 m.
Friction turns the textbook 4:1 into a real 3.6:1, so you pull 278 N instead of 250 N — and reel in 8 m of rope to raise the load 2 m. Going to a 6-strand tackle would drop the effort to about 185 N, but you would then haul 12 m of rope and add two more sheaves of friction, illustrating the diminishing returns of simply stacking pulleys.
Frequently Asked Questions
The load hangs from n rope strands, each carrying 1/n of the weight, so you pull 1/n of the force — but n times the rope length.
Ideal MA = number of supporting strands. 4 strands → MA 4, so 1000 N needs 250 N ideal effort. Friction lowers it.
IMA = strand count (frictionless). AMA = load/effort, always lower. Efficiency = AMA/IMA, ~80–95% depending on sheaves.
MA × lift height. A 4:1 system needs 4 m of rope per 1 m of lift — you trade force for distance.
Only to a point — each sheave adds friction, so beyond ~4–6 strands gains shrink and you haul much more rope.