Mechanical Movement 12 — Simple Fixed Pulley
Why does a simple pulley not make lifting easier?
Mechanism No. 12 from 507 Mechanical Movements: a single fixed pulley hung from a hook. The rope tension is the same on both sides, so to hold the load you must pull with a force equal to its weight; to lift it you pull a little more to beat friction in the bearing and rope. What the pulley gives you is a change of direction, not a smaller force. The hook, meanwhile, carries both rope pulls. Simulation: 180 mm pulley, rope pulled at 35° from vertical.
How it works, step by step
1. Holding still
The rope tension equals the load: 245 N on both sides. Your pull must also be 245 N.
2. Start lifting
Pull a little harder than the weight, 261 N, to overcome friction in the bearing and rope.
3. Rope in = load up
Every metre of rope you pull raises the load one metre. Mechanical advantage 0.94, never above 1.
4. Lowering
Going down, friction helps you: only 231 N is needed to let 245 N down slowly.
The key idea
F ≥ W (MA ≤ 1) — A fixed pulley only turns the pull around; friction makes the pull a little larger than the load.
A single fixed pulley trades nothing: effort ≈ load. It only changes the direction you pull, which is often easier.
Model and assumptions
Ideal fixed pulley: effort = load, rope pulled = height lifted, mechanical advantage 1. With efficiency η, lifting needs W/η and lowering needs only η·W. Hook force = vector sum of the two rope tensions: 2T·cos(φ/2), where φ is the angle between the rope runs (35° here).
Frequently asked questions
Does a fixed pulley make lifting easier?
Not in force: the pull equals the load (a little more with friction). It only lets you pull in a more convenient direction.
What does the hook carry?
Both rope runs: nearly twice the load when the rope hangs straight down.
What is its mechanical advantage?
1 ideally, slightly less in practice.
Part of the 507 Mechanical Movements series
This is Movement No. 12 from Henry T. Brown’s 507 Mechanical Movements (1868), redrawn and simulated by BCRUZ.
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