Firstly, [tex]F=N(kb)T(\frac{x+X}{X^2}-\frac{X}{(X+x)^2})[/tex] where (kb) is the boltzmann constant, T the temperature, X is the original length and x is the total length of the rubber after the weight is added.

The second equation is k=(3Nk

_{b}T)/X

^{2}where k is the spring constant of the spring. I have totally no clue where this equation is derived from.

Thanks