ContourPlot[0.5==10/7*9.8/1.83^2*tan(x/180*pi)*y*0.3048,0.71==10/7*9.8/1.83^2*tan(x/180*pi)*y*0.3048,1==10/7*9.8/1.83^2*tan(x/180*pi)*y*0.3048, {x, 1, 10}, {y, 4, 15}]

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