To model how satellites orbit Earth. Earth is often explained as a balance that is achieved when the outward-pulling centrifugal force of a revolving object is equal to the inward pull of gravity. However, if we examine Isaac Newton's First Law of Motion, we can see why this explanation is incomplete. According to this law, objects in motion remain in motion in a straight line unless acted upon by an unbalanced force. Because Earth-orbiting objects follow elliptical paths around Earth and not a straight line, forces cannot, by definition, be balanced.
Force is directional.
It is a push or a pull in a particular direction. At any one moment, the force of gravity on a satellite is exerted in the direction of a line connecting the center of mass of Earth to the center of mass of the satellite. Because the satellite is not stationary, the direction of this line, and consequently the direction of the force, is constantly changing. This is the unbalanced force that curves the path of the satellite. A second problem with the satellite orbit explanation is that centrifugal force is not an actual force but an effect.