mechanical · 30 minutes · Official OETT Aligned
Pulley Systems
Use less pull by moving more rope
What to know
- fixed pulley: stays in place
- moving pulley: rises with the load
- rope sections that hold up the load
- less force means more rope travel
Count the rope sections holding up the moving load to find how much pull you need.
The core rule
Without friction, pull needed = load ÷ the number of supporting rope sections.
A fixed pulley changes the direction you pull. A pulley that moves with the load can let more rope sections share its weight. Count only tight rope sections that help lift the moving load. Two supporting sections cut the pull in half, but you must pull 2 feet of rope to lift the load 1 foot. This force savings is called mechanical advantage.
Worked examples
Example 1: easy
A fixed pulley changes pull direction but gives about 1:1 force.
- The pulley stays fixed in place. One rope section holds the load.
- Without friction, the pull needed equals the load.
This pulley changes pull direction without reducing the needed force.
Example 2: medium
Two supporting segments lift 240 lb with 120 lb ideal effort.
- Count two rope sections holding up the moving 240-lb load.
- Divide: 240 ÷ 2 = 120 lb of pull.
The two sections share the weight equally in this example.
Example 3: hard
Four support segments require 4 ft of rope travel to lift a load 1 ft.
- Four rope sections each shorten by 1 ft when the load rises 1 ft.
- Pull in 4 × 1 = 4 ft of rope.
Using one-quarter of the force requires four times the rope travel.
Common mistakes
Every pulley wheel adds the same amount of lifting help.
Count the supporting rope sections, not the wheels. A wheel that only changes pull direction adds no force savings.
Put it into practice
Use guided explanations, check your answers, and try a new variation.