"Sample code 0: Simulate a single SHM pendulum "
theta0=15Degree;L=0.01;g=9.81;
omega=Sqrt[g/L];
T=2Pi/omega;
theta[t_]:=theta0*Cos[omega*t];
x[t_]:=L*Sin[theta[t]];y[t_]:=-L*Cos[theta[t]];
pp[t_]:=ListPlot[{{x[t],y[t]}},PlotRange->{{-L,L},{0,-1.1L}},PlotMarkers->{"●",Medium}];
line[t_]:=Graphics[Line[{{0,0},{x[t],y[t]}}],PlotRange->{{-L,L},{0,-1.1L}}];
Manipulate[Show[{pp[t],line[t]}],{t,0,10T,0.001T}]

"Sample code 0: Simulate a single SHM pendulum "
theta0=15Degree;L=2.01;g=9.81;
omega=Sqrt[g/L];
T=2Pi/omega;
theta[t_]:=theta0*Cos[omega*t];
x[t_]:=L*Sin[theta[t]];y[t_]:=-L*Cos[theta[t]];

Manipulate[
line:=Line[{{0,0},{x[t],y[t]}}];
Graphics[
{Point[{x[t],y[t]}],
line},PlotRange->{{-5,5},{-5,0}},Frame->True],{t,0,2T,0.05T}]