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.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 moves beyond our everyday understanding of weight to explore the fascinating concept of weightlessness. We often see videos of astronauts floating inside their spacecraft and might think there is no gravity in space. This is a common misconception! In this lesson, we will uncover the true physics behind weightlessness, distinguishing between true weight and 'apparent weight'. We will connect this scientific principle to the human experience of space travel, exploring how astronauts from different backgrounds and genders collaborate on missions like the International Space Station (ISS).
(Estimated Time: 30 minutes) Part 1: The Physics of Weightlessness
A. Recap: Mass vs. Weight Before we discuss weightlessness, we must be very clear on the difference between 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). An object's mass is constant no matter where it is in the universe. Your mass is the same on Earth, on the Moon, and in deep space. Weight (W): This is the force of gravity acting on an object's mass. It is a vector quantity, as it always acts downwards towards the centre of the gravitational body (like Earth). The formula is: W = mg where `g` is the acceleration due to gravity (on Earth, approximately 9.8 m/s² or 10 m/s² for calculations). Weight is measured in Newtons (N).
B. Introducing 'Apparent Weight' What you *feel* as your weight is not actually the force of gravity itself. It is the normal contact force (or normal reaction force) that a surface (like the floor or a chair) exerts on you to prevent you from falling through it. This feeling is what physicists call apparent weight. When you stand still on the ground, the upward normal force from the ground is equal and opposite to the downward force of gravity (your true weight). In this case, Apparent Weight = True Weight (mg). Let's consider an elevator (or 'lift') to understand how apparent weight can change.
C. The Elevator Thought Experiment Imagine a person of mass `m` standing on a weighing scale inside an elevator. The scale reading shows the normal contact force, `R`, which is the apparent weight. Case 1: Elevator is at rest or moving at a constant velocity. The forces are balanced. Upward force (`R`) = Downward force (`mg`). `R = mg` The apparent weight is equal to the true weight. You feel your normal weight. Case 2: Elevator accelerates upwards (a). To accelerate you upwards, the upward force must be greater than the downward force. Using Newton's Second Law (`F_net = ma`): `R - mg = ma` `R = mg + ma = m(g + a)` The apparent weight (`R`) is greater than the true weight (`mg`). You feel heavier. Case 3: Elevator accelerates downwards (a). Now, the downward force of gravity is greater than the upward support force. Using Newton's Second Law (`F_net = ma`, taking downward as positive for this case): `mg - R = ma` `R = mg - ma = m(g - a)` The apparent weight (`R`) is less than the true weight (`mg`). You feel lighter. Case 4: The elevator cable snaps! (Freefall) If the cable snaps, the elevator and everything inside it accelerates downwards at `a = g`. Let's use the formula from Case 3: `R = m(g - a)` Since `a = g`, `R = m(g - g) = m(0)` `R = 0 N` The normal force is zero. The scale reads zero. This is weightlessness.