Re: Solving for Stiffness using a plot

*To*: mathgroup at smc.vnet.net*Subject*: [mg99974] Re: Solving for Stiffness using a plot*From*: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>*Date*: Wed, 20 May 2009 05:03:16 -0400 (EDT)*References*: <guu2cm$65p$1@smc.vnet.net>

I'm not sure what you mean by saying that you solve for k, but I know that NDSolve does not want symbolic parameters. You have to assign a value to k. You also have to change the square brackets in k[x1[t] - x2[t]] to round ones: k(x1[t] - x2[t]). Additionally, you probably forgot a minus sign. And acceleration seems to depend on positions squared. For springs the relationship usually is m a = -k x. The following works: m1 = 220; m2 = 300; v0 = 12; k = 1000;(*temp.I am solving for this*) a = NDSolve[{-k (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}] Cheers -- Sjoerd On May 19, 12:41 pm, twoseat <caseyktimm... at gmail.com> wrote: > 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!