An elastic cord is 65 cm long when a wieght of 75N hangs from it but is 85 cm long when a weight of 180 N hangs from it. What is the "spring" constant k of this elastic cord.
I know that k= f/x. Separately, than gives you: 75N/0 .65m = 115.4N/m and 180N/.85m= 211.8N/m. But I figured k would be the same. It's obviously not and now I don't know what else to do. Any help is appreciated.Thank you.

The actual formula is F= k \Delta x. Here you don't know what the starting length of the elastic cord is. What you do know is:


75 = k (65-x_0) \\
180 = k (85-x_0)


Now solve for k.