Acertain metal wire has a cross sectional area of 1 mm2 and is 1 m long. when it is hung from the ceiling and a 10 kg mass is hung from the bottom of it, the wire stretches 1 mm. treat the wire as a single macroscopic spring. what is its effective spring constant?

From the Hooke's law , the extension force of an elastic material is directly proportional to the extension. 
That is, F = k e, where F is the force , k is the constant and e is the extension
 F = 10 × 10 = 100 N
e = 1mm or 0.001 m
Hence, k = F/e
                = 100 N/ 0.001
                = 100000 N/m or 100 N/mm

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