Lesson Notes By Weeks and Term v4 - SHS 3
BASIC PHYSICS
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BASIC PHYSICS
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Subject: Physics
Class: SHS 3
Term: 1st Term
Week: 5
Grade code: 3.1.1.LI.3
Strand code: 1
Sub-strand code: 1
Content standard code: 3.1.1.CS.2
Indicator code: 3.1.1.LI.3
Theme: MECHANICS AND MATTER
Subtheme: BASIC PHYSICS
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This lesson explores the physics behind satellites, objects that are central to our modern lives. From the DSTV we watch, to the GPS that guides us, and the weather forecasts from the Ghana Meteorological Agency, satellites play a crucial role. We will investigate how a satellite stays in orbit, behaving much like the Moon orbits the Earth. By applying our previous knowledge of circular motion and Newton's Law of Universal Gravitation, we will deduce a powerful formula to calculate how long it takes for a satellite to complete one full journey around a celestial body – its period.
This section breaks down the topic into manageable parts. We will build from concepts you already know. Part A: The Condition for a Stable Orbit
First, let's define what a satellite is. A satellite is any object that orbits or revolves around another larger object (a primary body) due to gravity. Natural Satellites: These are celestial bodies that orbit a planet. Example: The Moon is a natural satellite of the Earth. Artificial Satellites: These are human-made objects intentionally placed into orbit. Example: The satellites used by MultiChoice (DSTV) for broadcasting, or GhanaSat-1 for Earth observation.
The Key Question: Why doesn't a satellite fall back to Earth or fly off into space?
The answer lies in a perfect balance of two forces we have studied before: Gravitational Force (Fg): This is the force of attraction between the satellite and the primary body (e.g., Earth). It always pulls the satellite *towards* the centre of the Earth. From Newton's Law of Universal Gravitation: `Fg = (G * M * m) / r²` Where: `G` = Universal Gravitational Constant (6.67 x 10⁻¹¹ Nm²/kg²) `M` = Mass of the primary body (e.g., Mass of Earth, `M_E`) `m` = Mass of the satellite `r` = Orbital radius (distance from the centre of the Earth to the satellite) Centripetal Force (Fc): This is not a new force, but the *net force* required to keep any object moving in a circle. For an orbiting satellite, this required force is directed towards the centre of the circle (the centre of the Earth). We know its formula: `Fc = (m * v²) / r` Where: `m` = Mass of the satellite `v` = Orbital speed of the satellite `r` = Orbital radius