You would be surprised at the different things that can be used to implement logic - crabs, slime mold, water droplets, and now pulleys. Anyone want to build a mechanical computer using pulleys?
The principles of logic can be realized in so many systems that it makes you understand how they really are universal. All you need is something that takes an input and produces the opposite output - a NOT gate; something that take two inputs and produces an output when any one of the inputs is active - an OR gate; and something that takes two inputs and produces an output only when both inputs are active - an AND gate.
Once you have AND, OR and NOT you can build any other logic gate you need. They form a universal basis for logic. You can even do better - a single NAND, i.e. a NOT AND gate, is universal. Yes, you can make any other logic gate by simply combining NAND gates.
This sounds powerful but you also need to keep in mind that you can use logic gates to build a fully functioning computer.
All you need to build a computer is lots of NAND gate.
Now take a look a the video where Alex Gorischek demonstrates how a set of ropes and pulleys can be used to implement the basic gates. To understand what is going on notice that binary values correspond to zero or one as written on the board - i.e. up is 1 and down is 0. For example, you immediately get a NOT gate with a single piece of rope. When one end is up the other is down and vice versa - it is an inverter.
Of course there really was no need to go on to the more complex and less common gates as once you have shown that you can build a NOT, OR and AND your job is done. In fact we could just have stopped after the NAND or the XOR gate, both of which are universal. But there is a fascination on seeing how these mechanical contraptions can actually be put together.
Any practical use?
It is doubtful that it is worth trying to build a pulley-based computer but you can imagine that you could use the same idea to implement logic locks in a game. The drawbridge is only opened when the correct combination of ropes is pulled.
Any other ideas gratefully received.
Alex also has an Amazon page where he lists all of the parts you need to build a pulley based logic gate.
My only real complaint is why did a chessboard have to suffer in the making of this video?!