Description:
This sequence of three images shows the forces that are at play on a rotating planet such as Earth. The two forces that are at play are the force of gravitation, depicted in red, and the normal force, depicted in green. The blue arrow represents the resultant force of the two forces that are at play. The diagram displays a sideways view. The forces represented in the diagram account for the motion with respect to the inertial frame of reference that is co-moving with the center of mass of the Earth. Because of its rotation, the Earth is not spherical in shape, it is an oblate spheroid. The force of gravitation is (to a good approximation) directed towards the center of the Earth, and the normal force acts perpendicular to the local surface. On the poles and on the equator the two forces act exactly in opposite direction. On all other latitudes the angle between the two forces is not 180 degrees, so there is a resultant force. If the equatorial bulge is in equilibrium with the planet's rotation rate then at every latitude this resultant force exerts precisely the amount of centripetal force that is necessary to maintain an even thickness of the atmospheric layer. The solid Earth is ductile. Whenever the shape of the solid Earth is not quite in equilibrium with its rate of rotation, then there are shear stresses. These shear stresses then deform the solid Earth over a period of millions of years until the shear stresses are resolved. Depending on the circumstances, this diagram or the diagram of Image:Forces_oblate_spheroid.gif will be most useful. Created: 16 January 2006