x[t_, n_, \[Omega]0_, \[Phi]_, A_] :=
Assuming[{t > 0, n > 0, \[Omega]0 > 0, \[Phi] > 0, A > 0},
A*Exp[-n*t]*Sin[Sqrt[\[Omega]0^2 - n^2]*t + \[Phi]]]
Plot[x[t, \[Pi], 100 \[Pi], \[Pi]/8, 10], {t, 0, 1}]

x[t_, n_, \[Omega]0_, \[Phi]_, A_] :=
Assuming[{t > 0, n > 0, \[Omega]0 > 0, \[Phi] > 0, A > 0},
A*Exp[-n*t]*Sin[Sqrt[\[Omega]0^2 - n^2]*t + \[Phi]]]
Plot[x[t, \[Pi], 100 \[Pi], \[Pi]/8, 10], {t, 0, 1},
PlotPoints -> 100]