Assume the earth IS a flat disc
> my current latitude is 42.7 degrees north
> gravity acts towards an object's center of mass (I.e. The disc's center)
> if I drop a ball on the floor, it should roll north
Go ahead, put a ball on the ground. Does it roll north? If not
> the forces acting on the ball are perpendicular to the surface it is on
> the only shape for which all central facing vectors are perpendicular to the surface is a sphere
So there you have it, the earth is roughly a sphere.
also there is strong photographic evidence