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: 4
Grade code: 3.1.1.LI.4
Strand code: 1
Sub-strand code: 1
Content standard code: 3.1.1.CS.2
Indicator code: 3.1.1.LI.4
Theme: MECHANICS AND MATTER
Subtheme: BASIC PHYSICS
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This lesson explores the fascinating concept of "weightlessness." We often see images of astronauts floating effortlessly inside their spacecraft and think there is no gravity in space. This is a common misconception! This lesson will demystify weightlessness, showing that it is not about the absence of gravity, but rather the absence of a "support force." We will connect this advanced concept to everyday experiences here in Ghana, like the feeling you get in a fast elevator in a tall building in Accra or even when a tro-tro goes quickly over a hump in the road. Understanding this topic is key to grasping fundamental principles of forces and motion as described by Newton's Laws.
Part 1: Revisiting Mass and Weight Mass (m): This is the amount of 'stuff' or matter in an object. It is a scalar quantity and is measured in kilograms (kg). Your mass is the same whether you are on Earth, on the Moon, or in space. It is a fundamental property of an object. Weight (W): This is the force of gravity acting on an object's mass. It is a vector quantity (it always acts downwards, towards the centre of the Earth). It is measured in Newtons (N). The formula for weight is: W = m × g Where: W is the Weight in Newtons (N) m is the mass in kilograms (kg) g is the acceleration due to gravity (approximately 9.8 m/s² near the Earth's surface)
Example: A student, Adjoa, has a mass of 60 kg. What is her weight on Earth? W = m × g W = 60 kg × 9.8 m/s² W = 588 N Part 2: Apparent Weight - The Force We "Feel"
When you stand on a bathroom scale, what are you actually measuring? You are not directly measuring the force of gravity pulling you down. You are measuring the upward support force that the scale exerts on you to prevent you from falling through it. This support force is called the Normal Reaction Force (R). Apparent Weight: This is the force an object exerts on its support. It is equal to the normal reaction force. In many situations, it is equal to the true weight (mg), but not always! When you are standing still on the ground, the upward normal force from the ground perfectly balances the downward force of gravity. By Newton's Third Law, the force you exert on the ground is equal and opposite. This is what you feel as your weight. At rest: Apparent Weight (R) = True Weight (mg) Part 3: The Elevator Thought Experiment
Imagine you are standing on a weighing scale inside an elevator. Let's analyse what happens to your apparent weight in different situations. Let your mass be 'm'.