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Where can I find the explanation for the symbology used for equations in

I'm trying to understand the answers given to the question: How long it will take for a tree to fall on a ground?

For example how are the following to be interpreted $ $$ \ { }

and what are: frac mgh mh mathcal partial left right

I seem unable to find any sort of "introduction" to physics.stackexchange. I'd appreciate being pointed in the right direction.

I tried to ask this question on "meta" as directed - but found I need "five reputation" - & can find nothing telling what this is and how to earn them.

Also please advise of all the faux pas made herein

Thanks, petercone