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Overview
Orbital motion is the motion of one object around another under the influence of gravity. Planets orbit stars, moons orbit planets, artificial satellites orbit Earth, and many small bodies orbit larger astronomical objects. An orbit is not motion without gravity. It is motion shaped continuously by gravity.
Falling Around a Body
A useful way to understand orbit is to imagine an object moving sideways while falling. Gravity pulls it inward, but its sideways velocity keeps it from falling straight into the central body. The result can be a curved path around the body.
Gravity as Centripetal Force
In many simple orbital models, gravity provides the inward net force required for circular or nearly circular motion. The gravitational force points toward the central body and changes the direction of the orbiting object's velocity.
Orbital Speed
Orbital speed depends on the mass of the central body and the orbital distance. A closer circular orbit generally requires a higher speed than a farther circular orbit around the same body.
Orbit as Free Fall
Objects in orbit are in continuous free fall. Astronauts appear weightless not because gravity is absent, but because they and their spacecraft are falling together around Earth.
Orbit Shapes
Orbits can be circular, elliptical, parabolic, or hyperbolic depending on energy and speed. Many introductory treatments begin with circular orbits because they are mathematically simpler.
Energy in Orbits
Orbital motion involves kinetic energy from motion and gravitational potential energy from position in a gravitational field. Changing orbit usually requires changing the object's energy.
Stable and Unstable Orbits
A stable orbit persists when the object's speed, distance, and gravitational environment support repeated motion around the central body. Disturbances, drag, or gravitational interactions with other bodies can change or destabilize orbits.
Common Mistakes
A common mistake is thinking an orbiting object has escaped gravity. Another is assuming satellites need a continuous engine push to stay in orbit. In an ideal orbit, gravity and inertia maintain the motion without continuous thrust.
Why This Matters in Physics
Orbital motion connects gravitation, circular motion, energy, momentum, astronomy, satellites, spaceflight, planetary science, and navigation technology.