Key Terms
- Orbit: The curved path of an object around a central body due to gravity
- Centripetal force: The resultant force directed towards the centre of a circular orbit โ provided by gravity
- Geostationary orbit: An orbit at 35,786 km altitude with a 24-hour period โ satellite stays above the same point on Earth
- Low Earth Orbit (LEO): An orbit at 200โ2000 km altitude with ~90-minute period
- Orbital period: The time taken for one complete orbit
- Orbital speed: The speed of an orbiting object; given by v = 2ฯr / T (Higher tier)
Key Equation
- v = 2ฯr / T (Higher tier)
- v = orbital speed (m/s), r = orbital radius (m), T = orbital period (s)
- Closer orbit โ smaller r โ faster v needed
- Further orbit โ larger r โ slower v and longer T
Must-Know Facts
- Gravity provides the centripetal force for all orbits
- An orbiting object is constantly accelerating (direction changing) even at constant speed
- Closer orbit โ faster orbital speed; further orbit โ slower orbital speed
- Closer orbit โ shorter orbital period; further orbit โ longer orbital period
- In an elliptical orbit: fastest at closest point (perigee), slowest at furthest point (apogee)
- Geostationary satellites: 35,786 km, 24 h period, used for TV and communications
- LEO satellites: 200โ2000 km, ~90 min, used for ISS and Earth observation
Exam Tips
- Gravity always pulls TOWARDS the central body โ it never "pushes" an orbiting object
- Closer orbit = FASTER speed (stronger gravity must be balanced by greater centripetal motion)
- Geostationary = 24 hours = stays above same point โ 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 โ it provides the centripetal force pulling the satellite inward; there is no outward "centrifugal force" in reality
- Thinking higher orbit means faster satellite: Higher orbits have weaker gravity โ 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 โ 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 โ gravity provides all the force needed for circular motion
- Misidentifying geostationary orbit altitude: Geostationary satellites orbit at ~35,800 km altitude with a 24-hour period directly above the equator โ LEO satellites are much lower (~200-2000 km)
This topic summary covers Knowledge Organiser: Orbits within Orbits for GCSE Physics. Revise Orbits in Space Physics for GCSE Physics with 14 exam-style questions and 10 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 8 of 8 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Orbits
What is a protostar?
Explain why a main sequence star remains stable (in equilibrium) for billions of years.
Quick recall flashcards
What is a polar orbit used for?
Mapping - the satellite orbits at ~800 km altitude covering the whole Earth as the planet rotates beneath it
What is Low Earth Orbit (LEO)?
Orbit at 200-2000 km altitude with ~90 minute orbital period. Used for ISS, imaging, and Earth observation