Knowledge Organiser: Orbits
Key Terms
- Orbit: The curved path of an object around a central body due to gravity
- Force towards the centre: The force that points towards the centre of a circular orbit, provided by gravity
- Geostationary orbit: An orbit at 35,786 km altitude with a 24-hour period — satellite stays above the same point on Earth
- Low Earth Orbit (LEO): An orbit at 200–2000 km altitude with ~90-minute period
- Orbital period: The time taken for one complete orbit
- Orbital speed: The speed of an orbiting object. For a stable orbit, the radius must change if the speed changes
Key Ideas
- Closer orbit → smaller r → faster v needed
- Further orbit → larger r → slower v and longer T
Must-Know Facts
- Gravity is the force that holds all orbits
- An orbiting object is constantly accelerating (direction changing) even at constant speed
- Closer orbit → faster orbital speed; further orbit → slower orbital speed
- Closer orbit → shorter orbital period; further orbit → longer orbital period
- In an elliptical orbit: fastest at the closest point to the Earth, slowest at the furthest point
- Geostationary satellites: 35,786 km, 24 h period, used for TV and communications
- LEO satellites: 200–2000 km, ~90 min, used for ISS and Earth observation
Exam Tips
- Gravity always pulls TOWARDS the central body — it never "pushes" an orbiting object
- Closer orbit = FASTER speed (stronger gravity needs a greater speed to stay in orbit)
- Geostationary = 24 hours = stays above same point — vital for satellite TV dishes
- For circular orbits: speed is constant but velocity changes (direction changes)
- For elliptical orbits: speed AND direction change throughout the orbit
Common Mistakes
- Saying gravity pushes satellites outward: Gravity always acts towards the central body — it pulls the satellite inward; there is no outward force pushing it away
- Thinking higher orbit means faster satellite: Higher orbits have weaker gravity — satellites travel more slowly and take longer to complete one orbit
- Confusing speed and velocity in circular orbits: Speed is constant in a circular orbit but velocity is constantly changing — velocity is a vector and direction changes continuously
- Saying satellites need engines to stay in orbit: Once in a stable orbit, no thrust is needed — gravity provides all the force needed for circular motion
- Saying there is no gravity in orbit: Gravity is still pulling on the satellite. It is gravity that keeps it in orbit, because gravity pulls the satellite towards the centre of its orbit.
- Misidentifying geostationary orbit altitude: Geostationary satellites orbit at ~35,800 km altitude with a 24-hour period directly above the equator — LEO satellites are much lower (~200-2000 km)