Solving for Stiffness using a plot

*To*: mathgroup at smc.vnet.net*Subject*: [mg99896] Solving for Stiffness using a plot*From*: twoseat <caseyktimmons at gmail.com>*Date*: Tue, 19 May 2009 06:41:30 -0400 (EDT)

Hello I am trying to use mathematica to solve for the stiffness of a spring system but I cannot figure out how to plot my equations correctly. I have: m1=220; m2=300; v0=12; k; (*temp. I am solving for this*) a = NDSolve[{k[x1[t] - x2[t]]*(x1[t] - x2[t]) - m1*x1''[t] == 0, m1*x1''[t] - m2*x2''[t] == 0, x1[0] == 0, x2[0] == 0, x1'[0] == v0, x2'[0] == 0}, {x1[t], x2[t]}, {t, -10, 20}] dis = Plot[Evaluate[{x1[t] - x2[t]} /. a], {t, -10, 20}] This gives me multiple errors and wont plot. I am going to try to plot k[x1[t]-x2[t]]*(x1[t]-x2[t]) vs (x1[t]-x2[t]) to solve for k but I can't determine how to set up my equations to plot. Any help or recommendations would be appreciated!