Lesson Notes By Weeks and Term v4 - SHS 1

Lesson Video

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Performance objectives

• Define elastic and inelastic collisions.
• Identify the key differences between them.
• Analyze real-life scenarios of collisions.
• Explain the importance of collisions in everyday Ghanaian life.

Lesson summary

Every day, we see objects bumping into each other. A football player tackles an opponent, a driver parks a car and gently taps the one behind, marbles clash in a game of 'chaskele', or unfortunately, two vehicles might collide on the Kumasi-Accra highway. These are all examples of collisions. In science, understanding what happens during these collisions is crucial for everything from road safety design to understanding the tiny particles that make up our universe. This lesson will help us classify and understand the two main types of collisions: elastic and inelastic.

Lesson notes

This topic relies on two fundamental quantities: Momentum and Kinetic Energy. Let's understand them first. A. Foundational Concepts Momentum (p) Definition: Momentum is the "quantity of motion" an object has. It is a measure of how hard it is to stop a moving object. It depends on both the object's mass and its velocity. Formula: Momentum = mass × velocity `p = m × v` Units: The S.I. unit for mass is kilograms (kg) and for velocity is metres per second (m/s). Therefore, the unit for momentum is kilogram-metres per second (kg·m/s). Key Idea: A heavy tro-tro moving slowly can have the same momentum as a light motorbike moving very fast. Momentum is a vector quantity, meaning it has both magnitude and direction. Kinetic Energy (KE) Definition: Kinetic energy is the energy an object possesses due to its motion. Formula: Kinetic Energy = ½ × mass × velocity² `KE = ½mv²` Units: The S.I. unit for energy is the Joule (J). Key Idea: Kinetic energy depends on the square of the velocity. This means if you double an object's speed, you quadruple (2²) its kinetic energy. This is why speeding is so dangerous. The Principle of Conservation of Linear Momentum This is the most important rule for ALL collisions. Statement: In a closed system (where no external forces like friction are acting), the total momentum of objects *before* a collision is equal to the total momentum of the objects *after* the collision. In simple terms: Momentum is never lost or gained in a collision; it is only transferred from one object to another. Formula: `m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂` `m₁`, `m₂` = masses of object 1 and object 2 `u₁`, `u₂` = initial velocities (before collision) of object 1 and object 2 `v₁`, `v₂` = final velocities (after collision) of object 1 and object 2

B. Types of Collisions

The key difference between the two types of collisions is what happens to the kinetic energy. Elastic Collisions Definition: An elastic collision is one in which the total kinetic energy of the system is the same *before* and *after* the collision. In other words, kinetic energy is conserved. Characteristics: Momentum is conserved. (This is true for all collisions). Kinetic Energy is conserved. Objects bounce off each other perfectly without any loss of speed, heat, or sound. No permanent deformation occurs. Real-world Examples: In the real world, perfectly elastic collisions are rare. However, some are very close: The collision between two billiard balls or marbles. The collision of air molecules or atoms (at the microscopic level). A high-quality bouncy ball hitting a hard surface. Inelastic Collisions Definition: An inelastic collision is one in which the total kinetic energy of the system is *not* conserved. Some kinetic energy is converted into other forms. Characteristics: Momentum is conserved. (Still true!). Kinetic Energy is NOT conserved. The "lost" kinetic energy is transformed into other forms, such as: Heat: The objects get warmer. Sound: You hear the crash. Deformation: The objects get bent, dented, or broken. A Special Case: Perfectly Inelastic Collisions This is where the maximum possible kinetic energy is lost, and the colliding objects stick together after the collision, moving with a single, common final velocity. Real-world Examples: Most collisions in our daily lives are inelastic. A car crash (cars get dented, and there's a loud sound). A ball of kenkey dough dropped on the floor (it doesn't bounce back). A football player tackling another (they move together for a moment). An arrow sticking into a target. Summary Table

| Feature | Elastic Collision | Inelastic Collision | | :--- | :--- | :--- | | Momentum | Conserved | Conserved | | Kinetic Energy | Conserved | NOT Conserved (is lost) | | Objects after collision | Bounce apart | Can bounce apart, but often deform or stick together | | Energy Conversion | No conversion of KE | KE is converted to heat, sound, deformation | | Example | Billiard balls colliding | A car crash |

Evaluation guide

• Examine the differences between elastic and inelastic collisions of moving objects.
• Inquiry -based Teaching:
•  Let learners explore the differences between elastic and inelastic collisions and collision of
• moving objects.