Manipulate does not accept constant (i.e. just number) as upper limit for variable
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In the code below, If I chenage (4/(\[Zeta]*\[Omega])) to, say 20, nothing is plotted. Why?
If I remove the two sliders at the beginning, nothing is plotted 


    ClearAll[\[Zeta], \[Omega]]
    {Slider[ Dynamic[\[Zeta]], {0.1, 1.4, 0.1}], Dynamic[\[Zeta]]}
    {Slider[ Dynamic[\[Omega]], {1, 5, 0.1}], Dynamic[\[Omega]]}
    tf[\[Omega]_, \[Zeta]_] := 
     TransferFunctionModel[\[Omega]^2/(s^2 + 
         2 \[Zeta] \[Omega] s + \[Omega]^2), s]
    f[t_] = OutputResponse[tf[\[Omega], \[Zeta]], UnitStep[t], t];
    Manipulate[
     Plot[f[t], {t, 0, (4/(\[Zeta]*\[Omega]))}, 
      PlotRange -> {{0, (4/(\[Zeta]*\[Omega]))}, {0, 2}}], {{\[Zeta], 
       0.2}, 0.1, 1.4}, {{\[Omega], 1}, 0.5, 4}
     ]