Ch 1 · 6.1.1 The Earth · O Level Physics 5054
Topic 1 of 5 in Space Physics — O Level Physics 5054 — 23 lessons.
Introduction
Day and night, the months and the year are all set by motion in space: the Earth spinning, the Moon circling us, and the Earth circling the Sun. (Syllabus section 6.1.1.)
Key ideas — Key facts about the Earth, Moon and Sun
That last fact gives the distance to the Sun:
🧑🏫 Teacher's explanation
Think of the Earth as a spinning top that is also traveling in a large circle around a central lamp. The 'spinning' is the rotation on its axis, which creates our day and night cycle. The 'traveling' is the orbit around the Sun, which creates our year. Because the Earth's axis is tilted, different parts of the planet get more direct sunlight at different times of the year, causing seasons.
📖 Definitions
🔍 Worked example — Standard
A satellite orbits a planet at a radius of 7.0 × 10^6 m. It completes one orbit every 1.5 × 10^4 s. Calculate its average orbital speed.
🔍 Worked example — Exam-style
Light travels at 3.0 × 10^8 m/s. If it takes 500 s to reach Earth from the Sun, calculate the average distance between the Earth and the Sun.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Average orbital speed
For an object moving in a (nearly) circular orbit of average radius r, in one orbit it travels the circumference 2π r in one orbital period T:
🧑🏫 Teacher's explanation
Imagine you are running around a circular running track. If you want to find your average speed, you need to know the total distance you ran and how long it took you to finish one lap. In space, planets do the exact same thing. They travel along a path that is almost a circle, and the distance of that path is the circumference of the circle, which is 2π r.
📖 Definitions
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Example 1. The Earth's average orbital radius is 1.5 × 10^11 m and its period is 365 days. Find its average orbital speed.
🔍 Worked example — Easy
The Earth orbits the Sun at an average radius of 1.5 × 10^11 m. The orbital period is 365 days. Calculate the average orbital speed in m/s.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) (a) about 24 hours; (b) about 365 days; about one month. (2) An ellipse that is very nearly circular. (3) 3.0 × 10^8 × 500 = 1.5 × 10^11 m. (4) v = 2π × 7.0 × 10^6/5800 = 7.6 × 10^3 m/s.