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MathGroup Archive 2004

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Re: DSOLVE in 5.01 ??

  • To: mathgroup at smc.vnet.net
  • Subject: [mg47598] Re: DSOLVE in 5.01 ??
  • From: turbo cyx <turbocyx at gmx.net>
  • Date: Sat, 17 Apr 2004 02:31:39 -0400 (EDT)
  • References: <c5j82s$r3h$1@smc.vnet.net> <c5oaui$1fl$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Why should I bother with this?? I am not willing to use workarounds
for Wolfram bugs (I am coauthor of a widely used courseware in
austria. Now I have to write this workaround for all the Lessons wich
use DSolve and boundary conditions, in a few month when 6.0 finally is
available I can undo this again ....)
Mathematica 4.2 (and all previous version) has been able to solve this system,
so I think 5.01 should be also (BTW: 6.0 Alpha can solve this system
again)
Wolfram where is a bugfix
Cyx
 
On Fri, 16 Apr 2004 10:01:54 +0000 (UTC), drbob at bigfoot.com (Bobby R.
Treat) wrote:

>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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