Lesson Notes By Weeks and Term v3 - Junior Secondary 2
Work, Energy and Power
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Work, Energy and Power
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Subject: Basic Science
Class: Junior Secondary 2
Term: 2nd Term
Week: 4
Theme: You And Energy
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Explain the meaningof work, energy and power Explain the meaningsof potential energyand kinetic energy Apply the for mulapower = Work done Time Identify energytransfers that occurwhen work is done
J Time taken (t) = 20 s Formula: P = W / t Calculation: P = 600 J / 20 s P = 30 J/s P = 30 Watts (W)
Answer: The power exerted by the labourer is 30 Watts.
D. Energy Transfers when Work is Done Principle: The Law of Conservation of Energy states that energy cannot be created or destroyed, but it can be transferred from one form to another or from one object to another. When work is done, energy is transferred.
Key Idea: Doing work on an object means transferring energy to it. An object doing work loses energy.
Examples relevant to Nigeria:
1. Lifting a Bag of Cement: Action: A construction worker lifts a bag of cement from the ground to a platform.
Energy Transfer: Chemical energy from the worker's food (muscles) is converted into kinetic energy to move the bag, and then stored as gravitational potential energy in the bag (due to its increased height). The worker does work on the bag.
Transfer: Chemical Energy (worker) → Kinetic Energy (bag) → Gravitational Potential Energy (bag).
2. Using a Grinding Machine: Action: A market woman uses an electric grinding machine to grind pepper.
Energy Transfer: Electrical energy from the power source (grid or generator) is converted into kinetic energy of the grinding blades, which then does work on the pepper, breaking it down. Some energy is also lost as heat and sound.
Transfer: Electrical Energy → Kinetic Energy (blades) + Heat Energy + Sound Energy.
3. Water Flowing in a Hydroelectric Dam: Action: Water stored high up in a dam (like Kainji) is released and flows downwards, turning turbines.
Energy Transfer: The gravitational potential energy of the stored water is converted into kinetic energy as it flows downwards. This kinetic energy then turns the turbines, generating electrical energy.
Transfer: Gravitational Potential Energy (water) → Kinetic Energy (water) → Mechanical Energy (turbines) → Electrical Energy.
4. Kicking a Football: Action: A player kicks a football.
Energy Transfer: Chemical energy from the player's muscles is converted into kinetic energy of the leg, which is then transferred as kinetic energy to the football, causing it to move.
Transfer: Chemical Energy (player) → Kinetic Energy (leg) → Kinetic Energy (ball).
A. Work Definition: In science, work is done when a force causes an object to move a certain distance in the direction of the force. If there is no movement, or if the movement is perpendicular to the force, no work is done.
Formula: Work (W) = Force (F) × Distance (d)
Units: The standard unit for work is the Joule (J). One Joule is the amount of work done when a force of one Newton moves an object by one meter (1 J = 1 N × 1 m).
Conditions for Work to be Done:
1. A force must be applied to an object.
2. The object must move from its initial position.
3. The movement (displacement) must be in the same direction as the applied force.
Examples relevant to Nigeria: A bricklayer lifting a block from the ground to the top of a wall. (Force is upward, distance is upward). A market woman pushing a cart of goods along a flat road. (Force is horizontal, distance is horizontal). A farmer pulling a plough through the soil. (Force is horizontal, distance is horizontal).
When NO Work is Done: Pushing a stationary wall (no movement, d=0). A student carrying a bag on their head and walking on a level road (force is upward, displacement is horizontal, so the angle between force and displacement is 90 degrees; no work done by the student on the bag against gravity, though work is done by the student's legs against friction and gravity to move their body).
Worked Example 1 (Work): A local carpenter pushes a block of wood with a force of 50 N over a distance of 3 meters across his workshop floor. How much work is done by the carpenter?
Given: Force (F) = 50 N Distance (d) = 3 m Formula: W = F × d Calculation: W = 50 N × 3 m W = 150 Nm W = 150 Joules (J)
Answer: The carpenter does 150 Joules of work.
B. Energy Definition: Energy is the capacity or ability to do work. Without energy, no work can be done.
Units: Like work, the standard unit for energy is the Joule (J).
Forms of Energy: Energy exists in various forms, including mechanical, heat, light, sound, chemical, electrical, and nuclear energy. This lesson focuses on mechanical energy, specifically Potential Energy and Kinetic Energy. B
1. Potential Energy (PE)
Definition: Potential energy is the stored energy an object possesses due to its position, state, or condition. It is "potential" because it has the capacity to do work, but is not currently doing so.
Types: Gravitational Potential Energy: Energy stored in an object due to its height above a reference point (e.g., ground level).
Elastic Potential Energy: Energy stored in a stretched or compressed object (e.g., a stretched rubber band, a coiled spring).
Chemical Potential Energy: Energy stored in the bonds of chemical compounds (e.g., food, fuel, batteries). Formula (for Gravitational Potential Energy): PE = m × g × h Where: PE = Potential Energy (in Joules, J) m = mass of the object (in kilograms, kg) g = acceleration due to gravity (approximately 9.8 m/s2 or often approximated as 10 m/s2 for JSS calculations) h = height above the reference point (in meters, m)
Examples relevant to Nigeria: Water stored behind the dam wall at Kainji or Shiroro Hydroelectric Power Stations (high height). A bag of cement lifted to the top floor of a building under construction. A child at the top of a slide. A stone held above the ground. A stretched catapult before releasing the stone. Fuel (petrol, diesel, kerosene, cooking gas) in a tank (chemical potential energy).
Worked Example 2 (Potential Energy): A market woman lifts a 5 kg basket of tomatoes from the ground to a table 1.2 meters high. Calculate the potential energy gained by the basket of tomatoes. (Take g = 10 m/s2).
Given: * Mass (m) cement lifted to the top floor of a building under construction. A child at the top of a slide. A stone held above the ground. A stretched catapult before releasing the stone. Fuel (petrol, diesel, kerosene, cooking gas) in a tank (chemical potential energy).
Worked Example 2 (Potential Energy): A market woman lifts a 5 kg basket of tomatoes from the ground to a table 1.2 meters high. Calculate the potential energy gained by the basket of tomatoes. (Take g = 10 m/s2).
Given: Mass (m) = 5 kg Height (h) = 1.2 m Acceleration due to gravity (g) = 10 m/s2 Formula: PE = mgh Calculation: PE = 5 kg × 10 m/s2 × 1.2 m PE = 50 N × 1.2 m PE = 60 Joules (J)
Answer: The basket of tomatoes gains 60 Joules of potential energy. B
2. Kinetic Energy (KE)
Definition: Kinetic energy is the energy an object possesses due to its motion. Any object that is moving has kinetic energy.
Formula: KE = 1⁄2 × m × v2 Where: KE = Kinetic Energy (in Joules, J) m = mass of the object (in kilograms, kg) v = velocity (speed) of the object (in meters per second, m/s)
Examples relevant to Nigeria: A moving "Okada" (motorcycle) or "Keke NAPEP" (tricycle). Flowing water from a burst pipe or a river. A child running during a playground game. A stone thrown by a child. Wind blowing across farmlands.
Worked Example 3 (Kinetic Energy): A 0.5 kg stone is thrown by a child with a velocity of 8 m/s. Calculate the kinetic energy of the stone.
Given: Mass (m) = 0.5 kg Velocity (v) = 8 m/s Formula: KE = 1⁄2 mv2 Calculation: KE = 1⁄2 × 0.5 kg × (8 m/s)2 KE = 1⁄2 × 0.5 kg × 64 m2/s2 KE = 0.25 kg × 64 m2/s2 KE = 16 Joules (J)
Answer: The kinetic energy of the stone is 16 Joules.
C. Power Definition: Power is the rate at which work is done or the rate at which energy is transferred. It tells us how quickly work is performed.
Formula: Power (P) = Work done (W) / Time taken (t) Power (P) = Energy transferred (E) / Time taken (t)
Units: The standard unit for power is the Watt (W). One Watt is the rate of doing one Joule of work per second (1 W = 1 J/s). Larger units include kilowatt (kW) = 1000 W and megawatt (MW) = 1,000,000
W. Analogy: Two individuals may do the same amount of work (e.g., lifting a bag of rice to the same height), but the one who does it faster is more powerful.
Examples relevant to Nigeria: Comparing a small generator (e.g., "I better pass my neighbour") to a large industrial generator: the industrial generator has more power because it can generate more electrical energy per second. A commercial grinding machine vs. a traditional grinding stone: the machine is more powerful as it grinds the same amount of pepper faster. Vehicles with higher engine power (in horsepower or kilowatts) can accelerate faster or carry heavier loads more easily.
Worked Example 4 (Power): A labourer pushes a wheelbarrow of sand, doing 600 Joules of work in 20 seconds. Calculate the power exerted by the labourer.
Given: Work done (W) = 600 J Time taken (t) = 20 s Formula: P = W / t Calculation: P = 600 J / 20 s P = 30 J/s P = 30 Watts (W)
Answer: The power exerted by the labourer is 30 Watts.
D. Energy Transfers when Work is Done Principle: The Law of Conservation of Energy states that energy cannot be created or destroyed, but it can be transferred from one form to another or from one object to another. When work is done, energy is Teacher Activities: Introduction (Engage): Begin by asking learners about everyday activities that involve effort, such as fetching water, pushing a heavy table, or climbing stairs. Ask them what makes some tasks "harder" or "take more effort." Introduce the terms "work," "energy," and "power" as scientific ways to describe these efforts. Explanation and Definition (Explore & Explain): Define Work, explaining its conditions (force and displacement in the same direction).
Use simple demonstrations: Demonstration 1:* Push a desk across the classroom.
Ask: "Is work being done?" "Why?" (Yes, force applied, desk moved).
Demonstration 2:* Push against a classroom wall.
Ask: "Is work being done?" "Why?" (No, force applied, but wall did not move).
Demonstration 3: Lift a book vertically.
Ask: "Is work being done?" (Yes). Carry the book horizontally across the room.
Ask: "Is work being done on the book against gravity?" (No, force is up, displacement is horizontal). Clarify the direction aspect. Introduce the formula W = F × d and the unit (Joule). Define Energy as the capacity to do work. Explain that work and energy share the same unit. Explain Potential Energy (PE) using examples like holding a book high, a stretched rubber band. Introduce PE = mgh. Explain Kinetic Energy (KE) using examples like a moving toy car, a running student. Introduce KE = 1⁄2 mv
2. Define Power as the rate of doing work. Introduce P = W/t and the unit (Watt). Provide analogies like comparing two students lifting the same weight, one faster than the other.
Illustrative Examples (Elaborate): Work through the provided worked examples for Work, PE, KE, and Power step-by-step on the board, encouraging student participation in identifying variables and suggesting calculations. Emphasize units.
Energy Transfer Discussion: Discuss the principle of energy conservation and provide practical Nigerian examples of energy transfers when work is done (e.g., hydroelectric dam, grinding machine, lifting loads). Draw simple diagrams or ask students to imagine the process.
Activity Guidance: Guide students through practical activities and group discussions.
Questioning and Feedback: Ask probing questions to check understanding throughout the lesson. Provide constructive feedback.
Student Activities: Brainstorming & Discussion: Participate in initial discussions about daily tasks and effort.
Observation & Participation: Observe teacher demonstrations and respond to questions about whether work is done.
Note-taking: Copy definitions, formulas, units, and examples into their notebooks.
Problem Solving: Work individually or in small groups to solve guided practice problems.
Scenario Analysis: Analyze real-life scenarios presented by the teacher and identify types of energy, work done, and energy transfers.
Questioning: Ask questions for clarification.
Agriculture and Rural Development: Application: Farmers use hoes to till soil (doing work). The effort involved (force over distance) determines the work done. Understanding power helps compare different farming equipment; a tractor performs work much faster (more powerful) than manual labour, increasing productivity. Lifting harvested produce (e.g., yam, cassava) involves gaining potential energy.
Integration: Discuss how simple machines (levers, pulleys) can reduce the force needed to do work, making tasks easier for farmers. Relate to how the government's push for agricultural mechanization aims to increase power and efficiency.
Energy Generation and Consumption: Application: Hydroelectric power stations (e.g., Kainji Dam, Shiroro Dam) convert the gravitational potential energy of water stored at height into kinetic energy as it falls, which then turns turbines to generate electrical energy. This electrical energy is transferred to homes and industries to do work (e.g., power appliances, run machinery).
Integration: Discuss the importance of energy conservation in homes and industries. Relate to the common use of generators ("I-pass-my-neighbour") in Nigerian homes, explaining that these generators convert chemical energy (fuel) into electrical energy, though often with low efficiency and high cost.
Transportation and Construction: Application: Vehicles (cars, buses, trucks, motorcycles) use chemical energy from fuel, converted to kinetic energy, to move people and goods (doing work against friction and air resistance). In construction, cranes lift heavy materials (e.g., steel beams, bags of cement) to great heights, storing potential energy. The power of the crane determines how quickly it can lift these materials.
Integration: Discuss how the design of roads and vehicles considers work and energy principles. For example, building bridges and flyovers involves doing significant work against gravity, requiring powerful machinery.