Janka hardness test understand which woods are harder than others. Janka hardness test The Janka hardness test (from the Austrian-born emigrant Gabriel Janka, 1864-1932) measures the resistance of a sample of wood to denting and wear. It measures the force required to embed an diameter steel ball halfway into a sample of wood. A common use of Janka hardness ratings is to determine whether a species is suitable for use as flooring. The hardness of wood varies with the direction of the wood grain. Testing on the surface of a plank, perpendicular to the grain, is said to be of "side hardness". Testing
NP-hardness nor "Undecidable". For instance, the language of True quantified Boolean formulas is decidable in polynomial space, but not in non-deterministic polynomial time (unless NP = PSPACE). NP-hard problems do not have to be elements of the complexity class NP. As NP plays a central role in computational complexity, it is used as the basis of several classes: NP-hard problems are often tackled with rules-based languages in areas including: NP-hardness NP-hardness (non-deterministic polynomial-time hardness), in computational complexity theory, is the defining property of a class of problems that are, informally, "at least as hard as the hardest problems in NP". A