m=.15
c=.25*.07^2
cm=c/m
g=9.8
v0=30
theta=50.*Pi/180.
solution=NDSolve[{
x''[t]==-cm Sqrt[x'[t]^2+ y'[t]^2] x'[t]  ,
y''[t]==-g -cm Sqrt[x'[t]^2+ y'[t]^2] y'[t] ,
  x[0]==0,
  y[0]==0,
x'[0]==v0 Cos[theta],
y'[0]==v0 Sin[theta]},
{x,y},{t,0,10}]

ParametricPlot[Evaluate[{x[t],y[t]}/.First[solution]],{t,0,8}]
Plot[Tan[theta]x-.5 g x^2/(v0 Cos[theta])^2,{x,0,90}]
Show[%%,%]
