Lesson Notes By Weeks and Term v4 - SHS 2
WAVES
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WAVES
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Subject: Physics
Class: SHS 2
Term: 1st Term
Week: 16
Grade code: 2.2.2.LI.4
Strand code: 2
Sub-strand code: 2
Content standard code: 2.2.2.CS.1
Indicator code: 2.2.2.LI.4
Theme: ENERGY
Subtheme: WAVES
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This lesson explores two practical methods for measuring the speed of sound in air. We often hear things before we see their source move (like a carpenter hammering from a distance), or we see lightning before we hear thunder. This is because sound travels at a finite, measurable speed. Understanding how to measure this speed is fundamental to many technologies, from medical ultrasound used in hospitals like Korle Bu to sonar used by fishing boats in Tema. Today, we will learn the scientific principles behind echoes (reflection) and resonance to determine this important physical constant.
This lesson focuses on two distinct methods. We will treat each one in detail. Method 1: Determining the Speed of Sound using Reflection (Echo Method)
A. Core Concepts Sound Wave: A longitudinal wave that travels through a medium (like air, water, or solids) by causing vibrations in the particles of the medium. Reflection of Sound: When a sound wave hits a hard, flat surface (like a wall, a cliff, or a large building), it bounces back. Echo: An echo is a reflected sound wave that is heard shortly after the original sound. For a distinct echo to be heard, the reflecting surface must be far enough away (at least 17 metres) so that the time delay between the original sound and the echo is at least 0.1 seconds. This is due to the persistence of hearing in the human ear.
B. The Principle and Formula
The echo method is a direct measurement technique. We measure the time it takes for a sound to travel to a distant object and return. Let `d` be the distance between the source of the sound and the reflecting surface (e.g., a wall). The sound travels to the wall (distance `d`) and then travels back to the source (another distance `d`). Therefore, the total distance travelled by the sound is `d + d = 2d`. Let `t` be the time taken for the echo to be heard after the sound is produced. We know the basic formula for speed is: `Speed = Total Distance / Total Time`. Substituting our values, we get the formula for the speed of sound (`v`):