Gravity and orbits · Grade 11
Gravity keeps the Moon in orbit around Earth and the planets in orbit around the Sun. An orbiting object is constantly falling towards the body it orbits, but moving sideways fast enough to keep missing it. Satellites in different orbits do different jobs: low orbits for imaging and the ISS, geostationary orbits for TV and weather.
Key facts
- Gravity provides the centripetal force that keeps satellites and planets in orbit.
- Orbital speed v = 2πr ÷ T, where r is measured from the centre of the planet.
- Lower orbits are faster and have shorter periods: v = √(GM ÷ r), with G = 6.67 × 10⁻¹¹ N m²/kg².
- A geostationary satellite orbits above the equator once every 24 hours, so it stays over one place.
- Kepler’s third law: T² ∝ r³ for everything orbiting the same body.
- Astronauts float because they are in free fall, not because there is no gravity.