Ch 2 · 6.1.2 The Solar System · O Level Physics 5054
Topic 2 of 5 in Space Physics — O Level Physics 5054 — 44 lessons.
Introduction
The Sun holds a family of eight planets, hundreds of moons, millions of asteroids and countless comets in orbit by its gravity. Each planet is very different, and exam questions often give you a table of planetary data to interpret. (Syllabus section 6.1.2.)
Key ideas — What is in the Solar System
🧑🏫 Teacher's explanation
Think of the Solar System like a massive, spinning dance floor. The Sun is the heavy, central DJ who holds everyone's attention. Because the Sun is so incredibly massive, its gravitational pull is the 'glue' that keeps every planet, asteroid, and comet from flying off into the dark void of interstellar space.
📖 Definitions
🔍 Worked example — Easy
List the eight planets in order starting from the Sun.
🔍 Worked example — Standard
A planet has a mass of M and a radius of R. If the distance from the center of the planet to a satellite is doubled to 2R, how does the gravitational field strength change?
🔍 Worked example — Exam-style
A planet's surface gravitational field strength is g_p = 10 m/s^2 and its mass is M_p. Another planet has a mass of 2M_p and the same radius. Calculate the new surface gravitational field strength.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State the force that keeps the planets in stable orbits around the Sun. [1] (b) Explain why the orbital speed of Neptune is much lower than the orbital speed of Earth. [2] [3 marks]
Key ideas — Gravitational field strength
🧑🏫 Teacher's explanation
Imagine you are holding a heavy magnet. If you hold it right against a piece of metal, the pull is incredibly strong. If you move that magnet just a few centimeters away, the pull becomes much weaker. Gravitational field strength works in a very similar way. It is a measure of how much 'pull' a mass exerts on objects around it.
📖 Definitions
🔍 Worked example — Standard
A planet has a surface gravitational field strength of 12.0 N/kg at a radius of 4.0 × 10^6 m. If the radius increases to 8.0 × 10^6 m but the mass remains the same, what is the new field strength?
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — The Sun's gravity
🧑🏫 Teacher's explanation
Imagine you are spinning a ball on a string around your head. The string provides the pull that keeps the ball from flying away. In the Solar System, the Sun acts like the center of that rotation, and gravity acts like the string. Because the Sun is incredibly massive, it has a massive 'pull' that dominates everything else.
📖 Definitions
🔍 Worked example — Easy
A small asteroid is located at a distance r from the Sun. If the gravitational field strength at that distance is 0.006 m/s^2, and the distance is doubled to 2r, calculate the new gravitational field strength.
🔍 Worked example — Standard
The Sun contains approximately 99.8\% of the mass in the Solar System. If the total mass of the Solar System is 2.0 × 10^30 kg, calculate the mass of the Sun.
🔍 Worked example — Exam-style
A planet orbits the Sun at a distance of 1.5 × 10^11 m with an orbital speed of 2.9 × 10^4 m/s. Calculate the gravitational acceleration (field strength) provided by the Sun at this distance, assuming a circular orbit.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Interpreting planetary data
(Rounded values.) Patterns to look for:
🧑🏫 Teacher's explanation
Imagine the Solar System as a giant cosmic dance floor. The Sun is the heavy, massive center of attention, and its gravity acts like an invisible tether, pulling everything toward it. The further a planet is from this center, the 'lazier' its dance becomes—it takes much longer to complete one full circle (orbital period) and it moves much slower along its path (orbital speed).
📖 Definitions
🔍 Worked example — Easy
Using the provided table, identify the relationship between the distance from the Sun and the orbital period for the first four planets (Mercury to Mars).
🔍 Worked example — Exam-style
A student claims that because Saturn is further from the Sun than Earth, it must be colder. Using the data, explain why this general trend might be challenged by Venus.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the relationship between a planet's distance from the Sun and its orbital period. [1] (b) Using the table, compare the surface gravitational field strength of Jupiter and Earth. State which is greater and by what approximate factor. [2] [3 marks]
Worked example
Using the table, describe and explain the relationship between a planet's distance from the Sun and its orbital period.
🔍 Worked example — Exam-style
A planet's distance from the Sun is 4 times greater than that of another planet. If the Sun's gravitational field strength at the first planet is g_1, explain qualitatively how the field strength g_2 at the second planet compares to g_1.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. (2) Between Mars and Jupiter. (3) The gravitational attraction of the Sun. (4) The Sun's gravitational field is weaker further from the Sun. (5) Venus: it is further from the Sun than Mercury but hotter, because its thick atmosphere traps energy.