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So I have not been able to find any examples of the Manipulate feature for two ODE's. I would like to plot a graph of x[t] vs. y[t] and manipulate the initial conditions, v and theta. Thank you very much for any help you can offer.

k=.313
g=9.81;
NDSolve[{x''[t] == -k x'[t] Sqrt[y'[t]^2 + x'[t]^2], 
y''[t] == -k y'[t] Sqrt[y'[t]^2 + x'[t]^2] - g, x[0] == 0, 
y[0] == 0, x'[0] == v Cos[theta], y'[0] == v Sin[theta]}, {x, 
y}, {t, 0, 10}]
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How is this?

sol[v_?NumberQ, theta_?NumberQ] := Module[{}, 
sol1[t_] = ({x[t], y[t]} /. NDSolve[{x''[t] == -k x'[t] Sqrt[y'[t]^2 + x'[t]^2], y''[t] == -k y'[t] Sqrt[y'[t]^2 + x'[t]^2] - g, x[0] == 0, y[0] == 0, x'[0] == v Cos[theta], y'[0] == v Sin[theta]}, {x, y}, {t, 0, 10}])[[1]];
ParametricPlot[Evaluate[sol1[t]], {t, 0, 10}, AspectRatio -> 1]
]

Manipulate[sol[v, theta], {v, 0, 1}, {theta, 0, 1}]
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