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Re: Solving for Stiffness using a plot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99971] Re: Solving for Stiffness using a plot
  • From: dh <dh at metrohm.com>
  • Date: Wed, 20 May 2009 05:02:43 -0400 (EDT)
  • References: <guu2cm$65p$1@smc.vnet.net>


Hello,

your problem is not well posed. As you write it, k is a function of the 

difference of x1 and x2: k[x1[t]-x2[t]].

But you only give equations for x1 and x2, your system is under defined.

Further, k would be a functional and you do no more have an ordinary 

differential equation.

On the other hand, if k is a constant and you mean: k (x1[t]-x2[t]), 

there is still an equation for k missing.

Daniel



twoseat 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!

> 




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