Geostationary satellites must orbit in the equatorial plane because their orbital motion requires a balance between gravitational and centripetal forces, which can only occur directly above the equator. If a satellite is in a non-equatorial orbit, it will move north and south relative to the equator, making it impossible to maintain a fixed position over a single point on the Earth's surface. The gravitational force acts towards the center of the Earth, while the centripetal force required for circular motion must be directed towards the center of the orbit, which complicates the dynamics for inclined orbits. Additionally, any inclination would necessitate a figure-eight motion rather than a stationary position. Understanding these principles is crucial for grasping satellite motion and orbital mechanics.