Physics · 6.1 Earth & the Solar System
Solar System. Orbit it.
Watch the planets orbit the Sun. The further out a planet is, the slower it moves and the longer its year. Gravity provides the centripetal force, and orbital speed is v = 2πr / T. Comets follow stretched ellipses and speed up near the Sun.
0625 Topic 6.1 — Solar System
Orbital speed v = 2πr/T
Gravity = centripetal force
Inspect a body
—
Orbital data
Orbital radius (AU)
1.00
Orbital period (years)
1.00
Orbital speed (km/s)
29.8
g at this distance
weaker further out
Gravitational attraction from the Sun provides the centripetal force keeping each planet in orbit. Further out → weaker pull → slower orbit → longer year.
📋 The Solar System (Cambridge)
- Eight planets orbit the Sun: Mercury, Venus, Earth, Mars (rocky, inner), then Jupiter, Saturn, Uranus, Neptune (gas/ice giants, outer).
- Also: dwarf planets, moons (orbit planets), asteroids (between Mars and Jupiter), and comets (icy, very elliptical orbits).
- Orbital speed v = 2πr / T. Outer planets have a larger orbit and a longer period, so a smaller average speed.
- The Sun's gravitational field provides the centripetal force; the field is stronger closer to the Sun.
- A comet speeds up as it nears the Sun (strong pull, small r) and slows far away — its kinetic and gravitational potential energy interchange.
🎯 Syllabus reference (0625)
- 6.1.1 The Solar System — describe the Solar System; state the order of the planets; describe the orbits as elliptical with the Sun at one focus.
- 6.1.2 Orbits — recall and use v = 2πr/T; explain that gravitational attraction provides the centripetal force; relate orbital radius to period and speed; describe how a comet's speed varies in its orbit.