ContourPlot[0.5==10/7*9.8/1.83^2*(x/100)*y*0.3048,0.71==10/7*9.8/1.83^2*(x/100)*y*0.3048,1==10/7*9.8/1.83^2*(x/100)*y*0.3048, {x, 1, 17.63}, {y, 4, 15}]

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