Meaning and mechanism
When multiple pulleys support movement, the weight of an object is divided among the individual pulleys. It is assumed that in an ideal single pulley, the tension is equal.
If the range where support is provided is large, then when the height is raised, the length of each segment within that range must decrease simultaneously. Consequently, the total distance that can be manually pulled increases.
Relationships and conditions
When the model has n or more vertical support segments, and F=mg/n and h=s/n, the work done by a force Fs is equal to the increase in potential energy of the object, mgh.
In a real pulley system, there is friction within the pulley, and the mass of the rope and pulleys themselves necessitate a greater input force. It is important to distinguish between the number of simple pulleys used and the actual length of the supported rope.
What to observe in the simulation
Force falls by the strand count while required pull distance grows.
Rope and pulley mass, friction and stretch are ignored. Rope drawing is scaled to explain displacement ratios. Do not reproduce real load tests.
Key relationships and symbols
F = mg/n · h = s/n · Fs = mgh
| Symbol | Meaning | Unit |
|---|---|---|
| n | Number of rope segments supporting the load | items |
| s, h | Rope length pulled and lifting height | m |
Worked example
What is conserved in an ideal pulley?
Input work equals potential energy · Force falls by the strand count while required pull distance grows.
Common misconceptions
- Simply stating that a pulley was pulled “more” does not explain why the same ratio of force gain consistently occurs each time.