Lesson Notes By Weeks and Term v4 - SHS 1
FORCES ACTING ON SUBSTANCES AND MECHANISMS
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FORCES ACTING ON SUBSTANCES AND MECHANISMS
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: General Science
Class: SHS 1
Term: 2nd Term
Week: 15
Grade code: 3.3.2.LI.2
Strand code: 3
Sub-strand code: 2
Content standard code: 3.3.2.CS.1
Indicator code: 3.3.2.LI.2
Theme: VIGOUR BEHIND LIFE
Subtheme: FORCES ACTING ON SUBSTANCES AND MECHANISMS
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This lesson explores the physics of collisions, which are fundamental interactions that happen all around us every day. From a tro-tro moving on the streets of Accra, to a footballer heading a ball in Kumasi, to playing a game of ampe with friends, collisions govern how moving objects interact. Understanding the different types of collisions helps us appreciate the principles of road safety, design better machines, and explain many natural phenomena. By studying collisions, we learn about two crucial physical quantities: momentum and kinetic energy, and how they are conserved or changed during an interaction.
This section breaks down the core scientific principles needed to understand collisions. A. Fundamental Definitions Collision: In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. The objects do not need to physically touch. For example, the repulsion of two magnets is also a type of collision. *Examples: A car hitting a wall, a bat hitting a cricket ball, two marbles rolling into each other.* Momentum (p): This is a measure of an object's "quantity of motion." It depends on both the object's mass and its velocity. A heavy object moving slowly can have the same momentum as a light object moving quickly. Formula: Momentum (p) = mass (m) × velocity (v) p = mv Unit: kilogram-meter per second (kg m/s). Contextual Example: An `okada` (motorcycle) with a mass of 150 kg moving at 20 m/s has a momentum of 150 kg × 20 m/s = 3000 kg m/s. A small car with a mass of 1000 kg moving at just 3 m/s also has a momentum of 1000 kg × 3 m/s = 3000 kg m/s. They both have the same "quantity of motion." Kinetic Energy (KE): This is the energy an object possesses due to its motion. Formula: Kinetic Energy (KE) = ½ × mass (m) × (velocity (v))² KE = ½mv² Unit: Joules (J). B. The Principle of Conservation of Linear Momentum
This is the most important rule governing all collisions. The Principle states: In the absence of external forces (like friction), the total momentum of a system of colliding objects before the collision is equal to the total momentum of the system after the collision. In simpler terms: Momentum is always conserved in any collision. Formula for two objects: `m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂` Where: `m₁`, `m₂` = masses of object 1 and object 2 `u₁`, `u₂` = initial velocities of object 1 and object 2 (before collision) `v₁`, `v₂` = final velocities of object 1 and object 2 (after collision) C. Types of Collisions: The Main Difference
The key difference between collision types lies in what happens to the kinetic energy. While momentum is *always* conserved, kinetic energy is not. Elastic Collision Definition: An elastic collision is one in which both momentum and total kinetic energy are conserved. Characteristics: The objects bounce off each other perfectly after the collision. No kinetic energy is lost to sound, heat, or deformation (change of shape). Kinetic Energy Rule: Total KE before = Total KE after `½m₁u₁² + ½m₂u₂² = ½m₁v₁² + ½m₂v₂²` Real-world Examples: Collisions between billiard balls or marbles are very close to being perfectly elastic. At the atomic level, collisions between gas particles are considered elastic. *Perfectly elastic collisions are an idealisation; in reality, some energy is always lost.* Inelastic Collision Definition: An inelastic collision is one in which momentum is conserved, but total kinetic energy is NOT conserved. Characteristics: Some of the initial kinetic energy is converted into other forms, such as heat, sound, and the work done to permanently change the shape of the objects (e.g., denting a car). The objects may stick together or move apart, but they will be deformed or generate sound/heat. Kinetic Energy Rule: Total KE before > Total KE after. Special Case: Perfectly Inelastic Collision: This is the most extreme inelastic collision, where the objects stick together after impact and move with a single, common final velocity. The conservation of momentum formula simplifies to: `m₁u₁ + m₂u₂ = (m₁ + m₂)v` where `v` is the common final velocity of the combined mass. Real-world Examples: A car crash where the vehicles crumple and lock together. A ball of kenkey dropping onto the floor and not bouncing. A tackler and a football player moving together after a tackle. Summary Table
| Feature | Elastic Collision | Inelastic Collision | | :--- | :--- | :--- | | Momentum | Conserved | Conserved | | Kinetic Energy | Conserved | Not Conserved (decreases) | | Objects' Behaviour | Bounce off each other | May stick together or deform | | Energy Conversion | No conversion of KE to other forms | KE is converted to heat, sound, etc. | | Example | Billiard balls colliding | A car crash; clay balls sticking |