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Lunghezza finale $\small l_1= l_0\left(1+\dfrac{30}{100}\right) = 25×1,3 = 32,5\,cm\;(\approx{33}\,cm);$
allungamento $\small \delta= l_1-l_0= 32,5-25 = 7,5\,cm;$
costante elastica $\small k= \dfrac{F\,(N)}{\delta\,(m)} = \dfrac{50}{7,5×10^{-2}} = \dfrac{50}{0,075}= 666,\overline6\,N/m\;(\approx{6,7×10^2}\,N/m).$