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[?] Deal with "Encountered non-numeric value for a derivative at t == 0" ?

L GonGa
Posted 6 years ago

Hi, new in mathematica and im facing a problem. Im trying to solve a diferencial equation with NDSolve and but the error "Encountered non-numeric value for a derivative at t == 0" shows. I understand the nature of the error but I don't know how to manage it. The function im obtaining is defined in the small interval t around 0 to 1. Here the code:

V=NDSolve[{v'[t]==-g +(a*ve)/(mr+mw*Exp[-ve*t]) +(mw*ve*Exp[-ve*t])/(mr+mw*Exp[-ve*t])*v[t]-(b/(mr+mw*Exp[-ve*t]))*v[t]^2 ,v[0]==0.01},v[t],{t,0.01,0.4}]

Here is an screenshot: enter image description here Thank you!
POSTED BY: L GonGa
1 Reply
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Difficult to say without the full code. You are using numerical integration so, you will have to define the parameters before integrating. This here, for example, works:

g = -9.81; a = 0.4; ve = 4.3; mr = 0.2; mw = 0.5; b = 0.1;

V = NDSolve[{v'[
     t] == -g + (a*ve)/(mr + 
        mw*Exp[-ve*t]) + (mw*ve*Exp[-ve*t])/(mr + mw*Exp[-ve*t])*
      v[t] - (b/(mr + mw*Exp[-ve*t]))*v[t]^2, v[0] == 0.01}, 
  v[t], {t, 0.01, 0.4}] 

Plot[v[t] /. V, {t, 0.01, 0.4}]

enter image description here

Cheers,

Marco
POSTED BY: Marco Thiel
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