       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,  x2 == 0,

>    x1' == v0, x2' == 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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