2010
I have a problem with secret doors as portrayed in movies and videogames and such. When Batman goes into the Batcave and pulls a book on a bookshelf to move the bookcase aside, that's fine; Batman has access to electric motors and hydraulic rams. Presumably pulling the book just activates a system that does the necessary work to move the bookshelf. But when you're looking at Indiana Jones-esque "ancient traps and secret doors" supposedly found in medieval, classical or ancient tombs and ruins, I seriously doubt that screenwriters have ever heard of mechanical advantage.
Let's say your secret door is a bookcase. A medieval bookcase full of books has to weigh several hundred pounds. Or let's say your secret door is a section of stone wall, which has to weigh over a thousand pounds. And you propose to move it by pulling a small lever two inches. Medieval people didn't have electric motors or hydraulic rams, so unless there is a hidden system of donkeys which are fed, watered and bred through the ages just waiting for some intrepid explorer to pull the lever which somehow persuades them to start turning the crank to move the stone wall, the only thing that can move the said wall is the muscle power of the intrepid lever-operator.
Let's say the secret door is a stone wall six inches thick, three feet wide and seven feet high. Pretty small compared to some I've seen in movies. This will weigh about 1,320 lbs. If you pull the lever two inches, and the wall moves two feet, that means your mechanical advantage is negative. Let's assume the floor is wooden. The friction coefficient of concrete over wood is 0.62, it's on a flat surface, so the normal force is 5,873 newtons (mass x gravity), and the maximum static friction is 3,641 newtons. Add to that the 70,500 newtons required to move a 1,320lb stone wall over two feet using a two-inch pull, for a total of 74,141 newtons. The force exerted by a tug-of-war contestant is around 500 newtons. If you want to reduce that load to something more realistic for pulling a lever, then you have to move that lever over a mile. So if the intrepid explorer has to crank a cogwheel around and around and around to open the secret door very slowly, that's plausible. But the two-inch lever pull isn't.
You could do this with a fairly modest hydraulic ram. But I'm pretty sure that the ancients didn't have access to industrial-strength hydraulic rams. Even if they did, they would need a fairly sizeable electric motor to drive the thing. Like I said, this works fine if you're Bruce Wayne and you can just have a hydraulic ram and a large motor installed in your study and link them to an electrical switch triggered by piano keys or whatever, but for anything built before the 20th century (late 19th, if we stretch it) the devices needed to make this work just didn't exist.
There is no mechanical way around this. You can trade distance for force but you can't get something for nothing. A very light force exerted over a very short distance is never going to move a bookcase or a stone wall or a marble fireplace or whatever. This is a fairly minor point of bad movie physics, but it's one that jarrs my suspension of disbelief when I see it. The ancients did not have hydraulic rams. Find a more realistic way to hide your secret doors.
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