Question

What is the spring constant for a rod of length l and youngs modulus y

Answer 1

Y = stress / strain   definition of Young's modulus

Y = (F/A) / (x/L)   where x is the elongation of the rod

Y = (F * L) / (A * x)

F = (Y A  / L) * x = k x      ignoring the minus sign

k = (Y A) / L    

units:    (dynes / cm^2)  * cm^2 / cm = dynes/cm

For a spring F = k x    and k has units of F/x = dynes/cm