Symbolic differential equation problem

*upd: all code attached

I`m trying to input function F(z) into diff.equation with substitution

but after that dsolve doesn`t identify my equation as differential

% F was calculated in upper section of code
% F = F(z) and it will change, so it`s impossible to write it "by hand"
eq = m * diff(z,t,2) - C * (z-z0) == - F;
dsolve(eq)%crash

what`s the problem can be?

F = (39*((130430*z*exp(-3/20))/3 - (130430*z)/3 + 1)^2)/(160*(z(t) - 3/2)^2) - (39*((130430*z)/3 - (130430*z*exp(-3/20))/3 + 1)^2)/(160*(z(t) + 3/2)^2)

p.s. then i need to build phase portrait, if you know good ways to do it, please tell me

[darova]

Please attach the whole code

[Andrew Bobylkov]

done

Answer 1

[darova]

The problem is in z_c variable

z_c =
z - z*exp(-t/r)

z(t) should be instead of z

try to change this line

z_c = subs(z_c,[r 'z'],[T1 z])