In[]:=
sol=NDSolve[{r''[t]==-r[t]-Sign[r'[t]]/10,r[0]==0,r'[0]==1},r,{t,0,Pi}]
Out[]=
rInterpolatingFunction
Domain: {{0.,3.14}}
Output: scalar

In[]:=
sol2=DSolver''[t]==-r[t]-
1
10
,r[0]==0,r'[0]==1,r[t],t,Assumptions->0<=t<=
Pi
2
//Expand
Out[]=
r[t]-
1
10
+
Cos[t]
10
+Sin[t]
In[]:=
Plot[{(r[t]/.sol),r[t]/.sol2},{t,0,Pi},PlotLabels{"Numeric","Copilot Google"}](*Onlyfor:0<t<Pi/2*)
Out[]=