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: 3
Grade code: 3.1.1.LI.4
Strand code: 1
Sub-strand code: 1
Content standard code: 3.1.1.CS.1
Indicator code: 3.1.1.LI.4
Theme: MECHANICS AND MATTER
Subtheme: BASIC PHYSICS
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This lesson is about understanding the hidden forces that keep things stable and in one place. Every time you see a bridge over a gutter, a building standing firm, or even a simple wooden bench, there are invisible upward forces called 'reaction forces' at play. These forces, provided by supports like pillars, walls, or even your own shoulders when you carry something, prevent objects from falling down. In Ghana, we see this principle everywhere: from the grand Adomi Bridge supported by its towers to the local carpenter ensuring a table's legs can support its top.
This topic builds on your knowledge of forces and moments. Let's review and connect the key ideas needed to master this indicator. a. What is a Reaction Force? According to Newton's Third Law of Motion, for every action, there is an equal and opposite reaction. When a beam (or any object) rests on a support (like a pillar, a knife-edge, or a wall), its weight and any loads on it exert a downward force (the action) on the support. The support then pushes back with an equal and upward force on the beam. This upward push is called the reaction force.
If the support did not provide this reaction force, the beam would accelerate downwards and break through the support. b. Conditions for Static Equilibrium For an object like a bridge or a beam to be stable (not moving or rotating), it must be in static equilibrium. This requires two conditions to be met simultaneously: First Condition (Translational Equilibrium): The vector sum of all forces acting on the object must be zero. For vertical forces, this simplifies to: Sum of all Upward Forces = Sum of all Downward Forces (ΣF_up = ΣF_down) Second Condition (Rotational Equilibrium): The sum of all moments about *any* point (pivot) must be zero. This simplifies to: Sum of Clockwise Moments = Sum of Anticlockwise Moments (ΣM_clockwise = ΣM_anticlockwise)
A moment is the turning effect of a force. It is calculated as: Moment (τ) = Force (F) × Perpendicular distance from the pivot (d) c. Free-Body Diagrams (FBD) Before you can solve any problem, you must draw a simplified diagram showing only the object (the beam) and all the forces acting *on* it. This is a free-body diagram. Draw the beam as a simple line. Represent all downward forces (loads, weight of the beam) with arrows pointing down. Represent all upward forces (reactions from supports) with arrows pointing up. Label all forces and distances clearly. d. Calculating Reactions: A Step-by-Step Method
We will use the two conditions of equilibrium to find the unknown reaction forces.