Looking out across cities under construction today and you’ll see hundreds of huge cranes of all shapes and sizes reaching high into the air and helping builders make ever taller constructions. But these things just didn’t exist in the past, so how did the Romans build their huge buildings, like the Colosseum?
Manpower, I hear you say! Well, that’s certainly true that they may not have had to pay quite as much per worker as the modern contractors, and slaves were easy to come by, but that’s not quite the answer either.
They had all sorts of clever contraptions to minimise effort when raising the huge stones, and one of the key concepts is pulleys. Pulleys involve a rotating disc with a free-running rope or wire running around them, and can significantly decrease the amount of work you have to put in to lift something off the ground.
Now, with a simple pulley system, to lift the weight, you have to use the same force as the weight itself. But with more complex systems, you start to only require a fraction of the force you needed before
Here, the force required has dropped to a third of the original weight
And here, it's dropped to a fifth!
So you can see, that if you can divide the weight of the stone by five, what was one a 500kg block that you would need a LOT of manpower to move, suddenly becomes something that four or five men could wrestle into place. And that makes a big difference when you’re making something that’s so big, it’s still impressive today – even though later inhabitants have been using it as their very own ready-made-bricks store!
One final note – it’s important to think about the assumptions we’ve made too, and here we’ve neglected any friction in the pulleys. This is probably not an insignificant amount of force, especially if we’re dealing with huge stones, but it’s easier to see the point without such things muddying the waters!