Solar System And Orbital Motion
Overview
Somewhere on the wall of a building near you there is a satellite dish, bolted into the brickwork, pointing at an empty patch of sky. Nobody adjusts it. Nobody tracks anything with it. It was aimed once, on the day it was fitted, and it has not moved since, and the pictures keep arriving. That only works because a machine the size of a minibus, thirty six thousand kilometres above the equator, is circling the planet at a speed chosen so precisely that from the ground it appears to be nailed in place.
This lesson is about why that speed had to be what it is, and why the engineers had no freedom to pick a different one. You will take an inventory of the solar system and learn to separate a planet from a moon, a dwarf planet from an asteroid and either of those from a comet, on size and on the shape of the path they follow. You will find out what actually holds a planet in its orbit, why an object going round a circle at a steady speed is nevertheless accelerating every second of the journey, and why a satellite in a low orbit is racing while one far out is dawdling. By the end you will be able to look at a job a satellite has to do and say how high it must fly and how fast it must travel to do it.
Objectives
- The Earth is one of eight planets orbiting the Sun (a medium sized star), which together with other smaller objects (asteroids, dwarf planets, comets) and moons orbiting several planets, make up the solar system. Students should be able to describe the principal differences between planets, moons, the Sun, comets and asteroids in terms of relative size and motion.
- Our universe is made up of: thousands of millions of galaxies that are each made up of thousands of millions of stars; our Sun is one of thousands of millions of stars in our galaxy called the Milky Way.
- Planets orbit the Sun and a moon is a natural satellite of a planet. Artificial satellites orbit the Earth and can be in geostationary or low polar orbits.
- Gravity provides the centripetal force that keeps planets and satellites (both natural and artificial) in orbit.
- The force of gravity acts towards the centre of the orbit. This unbalanced force causes acceleration towards the centre of the orbit, changing the direction of motion of the body (its velocity) but not its speed. The equation for calculating centripetal force is not required.
- The centripetal force due to gravity decreases as the separation of orbiting masses increases, resulting in lower orbital speeds.
- At a particular separation of the masses, the centripetal force results in a particular orbital speed. To stay in a stable orbit at a particular distance, the planet or satellite moves at a particular speed. A change in orbital speed results in a change in orbital radius. Students should be able to explain the motion of moons and artificial satellites and be able to apply this to the design of satellite placing where the speed will determine the radius of the satellite’s final position.
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Lesson Evaluation
Congratulations on completing the lesson on Solar System And Orbital Motion. Now that you've explored the key concepts and ideas, it's time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.
You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.
Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.
- Which of these objects orbits a planet rather than orbiting the Sun directly? A. A comet B. An asteroid C. A moon D. A dwarf planet Answer: C
- What provides the centripetal force that keeps the Earth in orbit around the Sun? A. Friction B. Gravity C. Magnetism D. Air resistance Answer: B
- A satellite moves around a circular orbit at a constant speed. Which quantity is changing? A. Its mass B. Its speed C. Its velocity D. Its distance from the centre of the orbit Answer: C
- Satellite X orbits the Earth at a radius of 7.0 x 10^6 m and satellite Y at a radius of 4.2 x 10^7 m. Which statement is correct? A. X travels faster than Y and takes less time for one orbit B. Y travels faster than X and takes less time for one orbit C. X and Y travel at the same speed D. X travels more slowly than Y and takes more time for one orbit Answer: A
- How does the orbit of a comet differ from the orbit of a planet? A. A comet's orbit is nearly circular and a planet's is highly elliptical B. A comet's orbit is highly elliptical and a planet's is nearly circular C. Both orbits are perfect circles D. A comet orbits a planet and a planet orbits the Sun Answer: B
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Recommended Books
Further learning from open sources
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Textbook section
Satellites and Kepler’s Laws: An Argument for Simplicity
Examples of gravitational orbits abound. Hundreds of artificial satellites orbit Earth together with thousands of pieces of debris. The Moon’s orbit about Earth has intrigued humans from time immemorial. The orbits of planets, asteroids, meteors, and comets about the Sun are no less interesting. If
"College Physics: Satellites and Kepler’s Laws: An Argument for Simplicity" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/6-6-satellites-and-keplers-laws-an-argument-for-simplicity. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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Textbook section
Centripetal Force
Any force or combination of forces can cause a centripetal or radial acceleration. Just a few examples are the tension in the rope on a tether ball, the force of Earth’s gravity on the Moon, friction between roller skates and a rink floor, a banked roadway’s force on a car, and forces on the tube of
"College Physics: Centripetal Force" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/6-3-centripetal-force. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
Published by other organisations under open licences, credited as they ask. Links open their websites.
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