Sharing the load
How Do Pulleys Help?
Add more rope sections and each one carries less of the weight.
-
Step 1 of 5
One pulley just turns the direction.
A single fixed wheel lets you pull down instead of lifting up. Handy, because your body weight helps — but the force needed is unchanged.
-
Step 2 of 5
Two rope sections halve the force.
Loop the rope under a moving pulley attached to the load. Now two lengths of rope hold it, and each only has to carry half the weight.
-
Step 3 of 5
More sections, less force.
Every extra section of rope supporting the load takes another share. Four sections and you pull a quarter. Ship cranes use eight or more.
-
Step 4 of 5
The rope pays for it.
Halve the force and you must pull twice as much rope. To lift the load one metre with two sections you haul in two metres.
-
Step 5 of 5
It is the same bargain as every machine.
Levers, ramps and pulleys all do it: make the force smaller by making the distance bigger. The total work never changes.
Cranes and lifts are full of pulley blocks — that is how one small motor lifts a car.
The short version
A pulley system shares the load across several rope sections, trading a lighter pull for a longer one.
Try it yourself
Loop a belt under a bag's handle and hold both ends. Lifting suddenly feels half as heavy.
Topics