       Re: DAE system

• To: mathgroup at smc.vnet.net
• Subject: [mg104230] Re: DAE system
• From: Szabolcs Horvát <szhorvat at gmail.com>
• Date: Sat, 24 Oct 2009 02:40:54 -0400 (EDT)
• References: <hbou1i\$lh6\$1@smc.vnet.net>

```On 2009.10.22. 8:28, rosenm wrote:
> Dear colleagues, can you help me to solve the following system of
> differential-algebraic equations, wich describes pendulum swinging:
>
> L = 1; m = 1; Te = 10; g = 9.81;
> sol = NDSolve[{m*x''[t] == -x[t]*F[t]/L ,
> m*y''[t] == -y[t]*F[t]/L - m*g, (x[t])^2 + (y[t])^2 - L^2 == 0,
> x == 0, x' == 1, y == -L, y' == 0, F == 10.81}, {x,
> y, F}, {t, 0, Te}];
>

Putting in extra forces is not really a workable way of handling
constraints.  Constraints really mean that the system can be described
by fewer parameters (in this case e.g. just an angle rather than x and
y).  See here for a derivation of the diff. eq. for the angle:

http://en.wikipedia.org/wiki/Pendulum_%28derivations%29

(Textbooks on analytical mechanics describe how to handle constraints in
a general way.)

P.S.  In your example F should be m g, i.e. 9.81 (not 10.81) for the
constraint to be satisfied.

```

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