Key Concepts: Orbital Motion

Orbits

• An orbit occurs when gravity provides exactly the centripetal force needed for circular motion: GMm/r² = mv²/r.
• Orbital speed: v = √(GM/r) — depends on the central mass M and orbital radius r, not on the orbiting mass.

Orbital Period

• T = 2πr/v = 2π√(r³/GM).
• Higher orbit → slower speed but longer period.

Types of Orbits

• Circular orbit: Constant radius, constant speed.
• Elliptical orbit: Varying radius and speed (Kepler's first law).
• Geostationary orbit: T = 24 hours, stays above the same point on Earth. r ≈ 42,164 km from Earth's center.