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MathGroup Archive 2004

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Re: DSOLVE in 5.01 ??

  • To: mathgroup at smc.vnet.net
  • Subject: [mg47584] Re: DSOLVE in 5.01 ??
  • From: drbob at bigfoot.com (Bobby R. Treat)
  • Date: Fri, 16 Apr 2004 05:21:46 -0400 (EDT)
  • References: <c5j82s$r3h$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Here's a solution (for real constants):

Block[{v, result, eq},
  eq = m g - c v[t]^2 == m v'[t];
  v[t_] = Sqrt@u[t];
  result = First@DSolve[eq, u[t], t];
  Clear[v];
  v[t_] = Sqrt@u[t] /. result;
  Simplify[PowerExpand /@ {v[t], eq}]
  ]

Bobby

turbo cyx <turbocyx at gmx.net> wrote in message news:<c5j82s$r3h$1 at smc.vnet.net>...
> Hi
> The following sequnece worked fine in all Mathematica versions including 4.2
> 
> dgl1 =  m g  -  c v[t]^2 ==  m v'[t]
> dgl2 = s'[t] == -v[t]
> DSolve[{dgl1, dgl2, s[0] == h0,v[0] == 0}, {s[t], v[t]}, t]
> 
> But since 5.0 (and now in 5.01) there are only a error messages ? 
> Whats wrong here, why can't 5.01  solve this thing when 4.2 can???
> OTOH: DSolve without the boundary conditions works fine (DSolve[{dgl1,
> dgl2}, {s[t], v[t]}, t]), and calculationg the Constants C[1], C[2] by
> "hand" (i.e. with Solve) also...
> Thanks for help
> Cyx


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