restart;
with(LinearAlgebra):
with(VariationalCalculus):
# Energie cinetique/potentiel
T := 0.5*m*(diff(r(t),t)^2+r(t)^2*diff(theta(t),t)^2)+(1/6)*m[0]*l^2*diff(theta(t),t)^2;
Ve := 0.5*k*(r(t)-r[o])^2;
Vg := -m*g*r(t)*cos(theta(t)) - 0.5*m[o]*g*l*cos(theta(t));
# Lagrangien
L := T - (Ve + Vg);
E := EulerLagrange(L,t,[theta(t),r(t)]);
EQ1:=simplify(E[1]);
EQ2:=simplify(E[2]);
dsolve({EQ1,EQ2,D(theta)(0)=0,D(r)(0)=0,theta(0)=0,r(0)=10},{r(t),theta(t)});
dsolve({EQ1,EQ2},{r(t),theta(t)});

Hello ! 

I'm currently trying to learn some advanced mechanics with the Lagrangian method. I'm ok with the physics part. I can solve small problems by hand but when I try it on maple, it won't work.

The equations I'm using have no mistakes, I'm following a book step by step. 

What I'm stuck with is to get a formula of theta and r that are only function of time (t) to then be able to plot it. And I need dsolve to work to get that.