(*  <listing>   *)

(* Adapted from Feagin's Appendix *)

(* Projectile motion without resistance *)

x := vo t Cos[a]
y := -g/2 t^2 + vo t Sin[a]

flow = {x,y} /.{ g-> 9.8, vo -> 10, a->15 Degree } //N

flow1 = {x,y} /.{ g-> 9.8, vo -> 10, a->60 Degree } //N
flow2 = {x,y} /.{ g-> 9.8, vo -> 10, a->45 Degree } //N
flow3 = {x,y} /.{ g-> 9.8, vo -> 10, a->30 Degree } //N


(*
AspectRatio, Equal scales:
The above picture does not preserve the angles! The only way(??) I easily found is to define 
equal ranges for x and y explicitely. This is done by    PlotRange->{{0,10},{-2,8}}

and by   AspectRatio->1    *)

flowPlot1 := ParametricPlot[ flow1, {t, 0, 3},PlotRange->{{0,10},{-2,8}},
				AxesLabel -> {" x","y"},AspectRatio->1 ,
				DisplayFunction->Identity ];
flowPlot2 := ParametricPlot[ flow2, {t, 0, 3},PlotRange->{{0,10},{-2,8}},
				AxesLabel -> {" x","y"},AspectRatio->1 ,
				DisplayFunction->Identity ];
flowPlot3 := ParametricPlot[ flow3, {t, 0, 3},PlotRange->{{0,10},{-2,8}},
				AxesLabel -> {" x","y"},AspectRatio->1,
				DisplayFunction->Identity ];	
Show[flowPlot1,flowPlot2,flowPlot3,DisplayFunction -> $DisplayFunction]							

</listing> 

<IMG SRC="projectile.gif">