Question

How do pulleys change the amount of mechanical force needed to move an object?

Answer 1

Pulleys change how much force you must apply by redistributing the load across rope segments and/or changing the direction of the applied force. Key points:

- Fixed pulley: it only changes direction (e.g., pull down to lift up). It does not reduce the magnitude of force needed. Effort = load (ignoring friction).

- Movable pulley: the pulley moves with the load. The rope supports the load from two segments, so each segment carries part of the load and the required effort is reduced. For an ideal (frictionless, massless) movable pulley, Effort = Load/2. You must pull twice as much rope as the load rises.

- Block-and-tackle / compound systems: combine fixed and movable pulleys. In an ideal system the mechanical advantage (MA) equals the number of rope segments supporting the load. Effort = Load / MA. For example, with 4 supporting segments, Effort = Load/4, but you must pull 4 times the distance the load rises.

- Conservation of energy: lowering the required force increases the distance you must pull (work in = work out, ignoring losses). So less force × more distance = same (ideal) work.

- Real systems: friction in pulleys and rope stretch add losses, so actual force needed is slightly higher than the ideal calculation.

Simple formulas:
- MA (ideal) = number of rope segments supporting the load
- Effort = Load / MA
- Distance pulled = MA × load rise

Examples:
- Lift 100 N with a single fixed pulley → need ~100 N.
- With one movable pulley (MA = 2) → need ~50 N but pull twice the rope.
- With MA = 4 (block and tackle) → need ~25 N but pull 4× the distance.

That’s how pulleys let you trade force for distance and reduce the effort needed to move an object.