The Shuttle was off vertical after the first mile. Air density decays in an almost exponential manner, which is why the most efficient ("synergistic" in missileer's vernacular) trajectory to orbit or suborbit looks a lot like a rectangular hyperbola. It's really a problem of integral calculus.
As for the force of gravity, it's sensitive to altitude. Even at a low exoatmospheric altitude of 250 km we're already at the 9m/s² mark. cn