(* ------------------------------------------------------------- *)(*   a618-curves2.nb  18 February 2013   [Richard Hall]          *)
(* ------------------------------------------------------------- *)

(*    a618 Q(1.4)     u = g = 1    *)

x[t_,a_] := Cos[a] t

y[t_,a_] :=- t^2/2 + Sin[a]t

(*  curve-plotting function for given  a    *)

plota[a_]:=ParametricPlot[{x[t,a],y[t,a]},{t,0,Pi},PlotRange->{{0,1},{0,1/2}}]

(* try plota  *)

plota[1]

(* trajectories by use of a Dynamic Slider for a    *)
splota:=DynamicModule[{a=1},Column[{
Slider[Dynamic[a],{0,2}],
"a = " Dynamic[a],
Dynamic[plota[a]]
}]]

(* Now we can choose a and plot the graph by moving the slider with  the mouse  *)

splota