A fixed pulley changes direction.
A pulley is a wheel that guides a rope or similar flexible member. In an ideal fixed-pulley arrangement, pulling down can lift a load up, but the required rope force equals the load’s weight.
The direction is more convenient; the force has not been reduced. The rope moves the same distance as the load.
A moving pulley can share the load.
When a pulley travels with the load and two vertical rope segments support it, each segment contributes an upward tension. With ideal equal tensions, each provides half the total supporting force.
This model assumes a massless rope and pulley, no friction and no acceleration. The real moving pulley’s weight and other effects must also be supported.
Less force means more rope travel.
- LoadAn ideal 1,000 N load rises 1 m.
- Effort500 N applied over 2 m of rope travel.
- Work1,000 joules on each side.
Work is force multiplied by distance in the force’s direction. Halving the force while doubling the travel keeps the work the same.
Count supporting segments, not wheels.
The mechanical advantage depends on the arrangement. Extra fixed pulleys may only redirect the rope and add losses; simply counting every wheel gives the wrong answer.
If the supporting segments are angled, only their upward force components support the load. The simple two-segment example assumes they are vertical.
Roping changes the relationship to the machine.
In a simple 2:1 lift roping arrangement, the rope travels twice as far as the car. The machine’s rope speed therefore differs from the car speed.
Roping ratio is one part of the design, alongside counterweight balance, traction and moving masses. The example explains the ratio; it does not determine a real lift’s rope tensions or motor size.
A closer look
Real systems need more input work than the useful output work because bearings, rope bending and other effects introduce losses. Mechanical advantage trades force for distance; it never removes the energy requirement.
Check your understanding
Does adding one fixed pulley always halve the effort?
Reveal the explanation
No. A fixed pulley can simply change the direction of the effort. Mechanical advantage depends on how the moving load is supported.