Human beings aren’t exceptionally strong by animal standards. Using our heads as much as our hands has been key to realizing global dominance as a species. Simple machines, items so simple to make and so important to our survival, were the most impressive technology of the ancients and remain important today for many tasks. In simplicity and effectiveness however, none is so deceiving as the pulley.
The Pulley Explained
A pulley is a fairly simple piece of equipment. They can be made of wood or metal, and they don’t require an advanced mechanism to make them (within tolerable limits). Coupled with rope and used in tandem, they become the “block and tackle,” a pivotal technology once invented and adopted.
It works in a deceivingly simple way. I’ll use the example of lifting a block of gold for illustrative purposes. Imagine you had a block of gold, weighing in at over 700 lbs. It is unlikely that you or anyone you know could lift that alone, and even as a team, that is a tough load to lift. If you needed to get that gold onto a truck for delivery, you might be out of luck.
Fortunately, you have two pulleys and an overhead beam that could (theoretically) handle the entire 700 lbs. To begin, you would tie one pulley to the gold, and another to the beam. These pulleys are going to magnify our strength (at the expense of our speed/distance). Now you would tie one end of the rope to the beam. Finally, you would loop the free end down through the gold’s pulley, back up through the beam’s pulley, and bring it back down to you.
Notice that you needed three times as much rope as the distance from you to the beam. This is critical. You can think of it as “spreading out the work” of lifting the gold over the rope, and the more rope involved in your pulley network, the further it gets spread. With enough pulley loops, back and forth to the beam and a pulley on the gold, even a hamster wheel could power the lifting of the gold. On the other hand, it would take a very long time because it would need to reel in an excessive length of rope.
In the scenario outlined above, with just 2 pulleys, the amount of force you would need to exert on the rope to lift the 700 pound block of gold would be a more manageable 350 lbs. Add another pair of pulleys and it would be down to 175 lbs.
The History of the Pulley
Pulleys date back to at least Archimedes. Of all of the ancients, none understood machinery and mechanical advantage like Archimedes, and Plutarch recorded that Archimedes once moved a laden warship using just his own strength. While this is probably an exaggeration, since he certainly wouldn’t be able to move a warship far without an excessive amount of rope (think about the hamster above), he most likely did work with pulleys in many of his documented feats.
The pulley is at the heart of the concept of a crane, from the ancient wooden cranes to the modern ones. Cranes can act as the beam in the above example, and oftentimes their motion is also dependent upon pulleys.
Physics of a Pulley
The magic of pulleys is fully explained by mechanical laws of physics. The work you do to lift an object is “fixed” by the weight (technically the mass) of the object and how far you want to lift it. The rope and pulley system spreads the work out over a longer distance, allowing your body, capable of a small amount of peak work, to work on lifting it a bit at a time. With each loop of rope between pulleys, you increase the amount of rope you will need to pull in to lift the load. The more rope it takes, the less force you will need to be able to pull the weight in, and viola, given enough rope and pulleys, even a tiny hamster wheel could pull the Titanic ashore.
Wehner, David: Archimedes Biography