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For my story, I need a cliff face with a slab sticking out of the middle of it, like an inflexible diving board. I need it to be 5 feet wide (exact) and 3-to-6 inches thick (thinner is better). I need it to stick straight out from the middle of the cliff face and for most humans to be able to stand on it without it noticeably flexing (meaning the person on it feels like they’re standing on a rigid plane when they step onto the unsupported end of it).

At 3” thick, what is the longest such slab I can plausibly have with current material science? What material is the slab made of? How much longer do I get if I allow more thickness? I’m hoping for something measured in meters.

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    $\begingroup$ You might get better answers to this on Engineering SE. I presume you don't mind if your platform is not a homogenous solid? It's much easier to support 100kg/m of steel lattice and a 70kg person than 1000kg/m of solid steel and a 70kg person. $\endgroup$
    – g s
    Commented Jun 8 at 15:43
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    $\begingroup$ It would also help if you numerically define "noticeable flex" and specify whether the cliff-board intersection is fixed and indestructible, or if not, what it's made of. $\endgroup$
    – g s
    Commented Jun 8 at 15:50
  • $\begingroup$ A simple strut would bend. You could make a strut that does not deflect at all if it has a cable below the upper surface which is actively tensioned to compensate for the weight upon it. $\endgroup$ Commented Jun 8 at 16:03
  • $\begingroup$ Does it need to be flat? I'm thinking of a structure like a metal tape-measure shape, with a material stressed like that too - would that fit in with your requirements? $\endgroup$ Commented Jun 8 at 16:40
  • $\begingroup$ I agree with @gs. It will take specialized knowledge to answer this because we humans don't have a reason to build such an extension. We do build bridges, and while I can't find the longest unsupported span, I've personally seen spans of hundreds of feet. Keep in mind that you need to stick into the cliff +/- one third of the external extension depending on the weight and stability of the mass above your extension. And frankly, you could likely make quite a long length out of carbon nanotubes with internal I-beam reinforcement. However... (*Continued*) $\endgroup$
    – JBH
    Commented Jun 9 at 2:29

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