"Sample code 1: 2D projectile with fixed angle and initial speed "
x0=0;y0=0;g=-9.81;
v0=0.5;theta=56Degree;
T=-2(y0+v0*Sin[theta])/g;
x[t_]:=x0+v0*Cos[theta]*t;
y[t_]:=y0+v0*Sin[theta]*t+0.5g*t^2;
pp=ParametricPlot[{x[t],y[t]},{t,0,1.2T}];
Print[pp]

"Sample code 2: 2D projectiles with fixed initial speed but different launching angles "
x0=0;y0=0;g=-9.81;
vx0:=v0*Sin[theta];vy0:=v0*Cos[theta];

x[t_]:=x0+v0*Cos[theta]*t;
y[t_]:=y0+v0*Sin[theta]*t+0.5g*t^2;

v0=1.0;theta=45Degree;T=-2(y0+v0*Sin[theta])/g;
pp1=ParametricPlot[{x[t],y[t]},{t,0,1T},PlotRange->All,PlotStyle->Blue];

v0=1.0;theta=55Degree;
T=-2(y0+v0*Sin[theta])/g;
pp2=ParametricPlot[{x[t],y[t]},{t,0,1T},PlotRange->All,PlotStyle->Red];
Show[{pp1,pp2}]

"Sample code 3: Modify sample code 2. It is not as flexible. "
x0=0;y0=0;g=-9.81;
x[t_,theta_,v0_]:=x0+v0*Cos[theta]*t;
y[t_,theta_,v0_]:=y0+v0*Sin[theta]*t+0.5g*t^2;
T[theta_,v0_]:=-2(y0+v0*Sin[theta])/g;
pp[theta_,v0_]:=ParametricPlot[{x[t,theta,v0],y[t,theta,v0]},{t,0,1T[theta,v0]},PlotRange->All];
Show[{pp[30Degree,1.0],pp[45Degree,1.0],pp[60Degree,1.0],pp[74Degree,1.0]}]