what is the spring constant of a typical 1/2 inch bungee cord?

Question

I need to know k, if I had one I could figure it out, but I'm trying to figure it out before hand.

Answer 1

Spring constant k equals the unit-length stiffness S (my term) of the cord divided by its length L. You can find S for a particular cord type by measuring k (force/deflection) of a cord of length L1 and multiplying by L1. (S is in units of force.) Then you can compute k for any length L cord of that type as k = S/L.
Note that you can't use the Young's modulus for the cord's elastic material to find k, since that is a material-only property and would only apply if the cord was made of nothing but that material. I couldn't find any website giving S for a particular type of bungee cord. However the ref. gives two examples in its theory section relating force to k. In one case k = 20 N/m with L = 20 m, so S = 400 N. In the other k = 10.37 N/m with L = 45 m so S = 467 N. I have no idea what thickness cord that was, but it's probably the type used by bungee-jumpers.