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I'm trying to plot a phase portrait for the equation:

(d^2/dt^2)y + b * (dy/dt)^2 = A, A > 0, b < 0

The first thing I did was changing dy/dt to p[y], so the equation is:

p'[y] + b * (p[y])^2 = A, A > 0, b < 0

However, when I make a stream plot and substitute for A and b, I see a blank plot. How can I fix this?

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  • $\begingroup$ Can you post the exact command that gives you a blank graph? $\endgroup$ Commented Dec 24, 2017 at 20:45
  • $\begingroup$ Manipulate[StreamPlot[{1, - b * (p[y])^2 + A}/. A->k /. b->m,{y,-3,3},{p,-3,3}],{k,0.1,5},{m,-5,-0.1}] $\endgroup$ Commented Dec 24, 2017 at 20:51
  • $\begingroup$ Possible duplicate: mathematica.stackexchange.com/questions/14160/… $\endgroup$ Commented Dec 25, 2017 at 0:05

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You should change p[y] to p in your Manipulate-command 

Manipulate[StreamPlot[{1, - b * (p(*[y] *))^2 + A}/. A->k /. b->m,{y,-3,3},{p,-3,3}],{k,0.1,5},{m,-5,-0.1}]
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