var x_velocity:Number = 25;
var y_velocity:Number = -80;
var a:Number = 10;
var dt:Number = 0.3;
var t:Number = 0;

ball_mc._x = 10;
ball_mc._y = 400;

ball_mc.onEnterFrame = function(){;
ball_mc._x += x_velocity*dt
ball_mc._y += y_velocity*dt + a*t*dt
t += dt;
};