       Re: Three masses and four springs

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• Subject: [mg132601] Re: Three masses and four springs
• From: Roland Franzius <roland.franzius at uos.de>
• Date: Thu, 17 Apr 2014 05:10:56 -0400 (EDT)
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• References: <lilc5f\$qlj\$1@smc.vnet.net>

```Am 16.04.2014 09:40, schrieb Robert Jenkins:
> The instruction
> DSolve[{-2*x1[t] + x2[t] == x1''[t], -2*x2[t] + x1[t] == x2''[t],
>    x1 == -1, x2 == 2, x1' == 0, x2' == 0}, {x1, x2}, t]
> produces a simple solution. But I am surprised to find the three-mass version produces a mass of complication. Have I made a mistake?
> DSolve[{-2*x1[t] + x2[t] == x1''[t], -2*x2[t] + x3[t] + x1[t] ==
>     x2''[t], -2*x3[t] + x2[t] == x3''[t], x1 == -1, x2 == 2,
>    x3 == -1, x1' == 0, x2' == 0, x3' == 0}, {x1, x2, x3},
>    t]
>

Its not that complicated but it involves a root of a third order
determinant for the eigenfrequency

In:= FullSimplify[{x1[t], x2[t], x3[t]} /.
DSolve[{-2*x1[t] + x2[t] == x1''[t], -2*x2[t] + x3[t] + x1[t] ==
x2''[t], -2*x3[t] + x2[t] == x3''[t], x1 == -1, x2 == 2,
x3 == -1, x1' == 0, x2' == 0, x3' == 0}, {x1[t],
x2[t], x3[t]}, t][]]

Out= {1/
2 ((-1 + Sqrt) Cos[Sqrt[2 - Sqrt] t] - (1 + Sqrt) Cos[
Sqrt[2 + Sqrt] t]),
1/2 (-(-2 + Sqrt) Cos[Sqrt[2 - Sqrt] t] + (2 + Sqrt) Cos[
Sqrt[2 + Sqrt] t]),
1/2 ((-1 + Sqrt) Cos[Sqrt[2 - Sqrt] t] - (1 + Sqrt) Cos[
Sqrt[2 + Sqrt] t])}

--

Roland Franzius

```

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