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Adding assumptions to SOLVE

  • To: mathgroup at smc.vnet.net
  • Subject: [mg90908] Adding assumptions to SOLVE
  • From: "Joe Hays" <joehays at vt.edu>
  • Date: Tue, 29 Jul 2008 01:38:33 -0400 (EDT)

Newbie question.

SOLVE documentation indicates I have to include assumptions to solve this (I
thought) simple problem:

<snip>
r[t] = {x[t] - L*Sin[q[t]], L*Cos[q[t]]};
r'[t] = D[r[t], t];
r''[t] = D[r'[t], t];

Eq1 =  Mc*x''[t] - u + N
Eq2 = Mp*({1, 0}.r''[t]) - N
Eq3 =  Mp*({0, 1}.r''[t]) - P + Mp*g
Eq4 = J*q''[t] - P*L*Sin[q[t]] - N*L*Cos[q[t]]

Solve[{Eq1 == 0, Eq2 == 0, Eq3 == 0, Eq4 == 0}, {x''[t], q''[t]}]
</snip>

My assumptions are:

   - N > 0
   - P > 0
   - g > 0
   - L > 0
   - Mc > 0
   - Mp > 0

Yet, I can't figure out how to get Solve to return the sybolic equations for
x''[t] and q''[t]. What am I missing?

Thanks in advance.

Joe



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