Perhaps those forces are affecting the Earth to a degree, but, since the Earth is infinite, the effects are too small to be noticeable. You cannot bend the edges of the world to form a sphere because the world is infinite, and thus has no edges. I think Tom is trying to say that certain forces, when applied to a single pebble, may destroy said pebble, but the same amount of force, applied the same way, would not destroy an entire mountain. Thus, the forces do not "act" the same way with mountains.