From the hydrostatic approximation, we know that pressure will decrease more rapidly in a dense column of air than in a less dense column of air. Dr. Van Domelen at Ohio State University gives a fairly lucid explanation of the Coriolis Force. The following figures were borrowed from Dr. Van Domelen's website. If not in contact with the Earth, this object will maintain its eastward speed as it moves poleward. In the upper atmosphere, where friction is negligible, the pressure gradient force and the Coriolis force balance to produce a geostrophic wind.
Geostrophic winds flow parallel to isobars.
The steeper the pressure gradient, the stronger the wind. Friction Frictional forces act in the opposite direction of the flow and represents molecular interactions between the air and surface. As a result of friction, the balance between the pressure gradient force and the Coriolis force is disrupted. Friction slows surface wind speeds, weakening the Coriolis force. A balance will be reestablished and now the winds cross the isobars.