It just plots a blank
y[x_, \[Alpha]_, v_] = x*Tan[\[Alpha]] - 0.5*g*(x/(v*Cos[\[Alpha]]))^2
Manipulate[ Plot[y[x, \[Alpha], v] , {x , 0 , 10 }], {\[Alpha] , 0 , Pi/2 } , {v , 0 , 10} ] /. g -> 9.8
Honestly not sure why...maybe I'm just too tired to see the mistake.

Manipulate[ Plot[y[x, \[Alpha], v] /. g -> 9.8, {x, 0, 10}], {\[Alpha], 0, Pi/2}, {v, 0.0000001, 10}]$\endgroup$