Pulleys redirect force — Engineering, 11–13
A pulley is a wheel with a groove for a rope. One pulley can make lifting feel more convenient; several pulleys can reduce the force needed, while increasing the distance of rope pulled.
What a pulley does
A pulley lets a rope bend around a wheel, so you can pull in a direction that suits you. Pulling down may be easier than lifting up, even when the load's weight has not changed. A fixed pulley mainly changes direction; a movable pulley can share the load between rope sections.
The problem of lifting
Lifting a heavy bucket straight up is tiring and may be impossible from the ground. People needed to raise sails, building stones and wells' buckets while standing somewhere safer or stronger. A pulley gives a better pulling direction, and a pulley system trades a smaller force for more rope movement.
Two rope sections
A 40-newton load hangs from a movable pulley supported by two equal rope sections. Step 1: share 40 newtons between 2 sections: 40 ÷ 2 = 20 newtons. Step 2: in a simple ideal system, the pull is about 20 newtons. Step 3: to raise the load 1 metre, pull about 2 metres of rope. Friction makes real values slightly larger.
The sensible mistake
It seems that a pulley system gives free force: if the pull is half as large, perhaps the load rises the same distance for free. The missing part is distance. You must pull twice as much rope, so the ideal work stays the same; real friction means you do slightly more work.
Where it appears
Cranes use pulley systems to lift construction materials. Sailors use them to raise and adjust sails, and rescue teams use them to move equipment or people with controlled effort. A simple flagpole pulley mainly changes the direction of the pull, so you can pull down while the flag goes up.
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