Chapter 10 : Work, Energy, and Power

Introduction: Work, Energy, and Power

In this chapter you will study the relation between force, distance moved, and time in relation to work, energy, power and the various sources of energy.
In everyday life, we see people pushing cars, lift books from the floor to a high shelf, and pull heavy tables across rooms. You notice that all these activities require us to use energy, apply force, use power and eventually we find ourselves doing work.
Sometimes we use machines like hoes, tractors to make our work easier. Therefore it is very necessary to understand and use the relationship between energy, work done, force, and power in our everyday life.
10.1: Work
In any casual talk or conversation, work means any physical or mental activity some body engages in. Activities like pushing a slashing machine, washing clothes, digging, sweeping, carrying a box of books and solving numbers would all be called work But now, what does it mean to do work in a physics context. Let us carry out
Activity 10.1 to understand work.
Activity10.1: Determining work done by a force

What you need: A wooden block having smooth surfaces With a hook on one side, spring balance, metre rule, and a smooth table or desk, notebooks, braille paper, stylus, slate, Perkins brailler

What to do
1.In groups, place the wooden block on a wide surface at the center of a table or desk
2.Hook the block on to the spring balance and mark the starting point on the table.
3.Pull the spring balance with a constant force such that the block moves along the table through a significant distance.
4.Read and record the magnitude of the pulling force from the spring balance.

5. Stop the pulling and mark the end point on the table where the block stops.
6. Use a metre rule to measure the distance through which the block moves along the table.
7. Determine the value of work done from the formula below.
   Work done = force applied x distance moved in the direction force
   The Sl unit of work is a joule. A joule is the work done when a force of one newton moves an object through a distance of one metre.

Work is a scalar quantity and it is only done when a force is applied at one point of the object so as to move it through a certain distance.
Factors that determine the amount of work done
All physical work that we do depends on a number of factors. Think Of any physical activity you always carry out. What do you think determines the amount of work in that particular activity? In Activity 10.2, you will investigate factors that determine the amount of work done.
Activity10.2: Investigating factors that determine the amount of work done
What you need: A wooden block having smooth surfaces with a hook on one side, spring balance measuring up to 2.5N, metre rule, a smooth table or desk, ten 100 g masses, paper tape or masking tape, notebooks, braille paper, stylus, slate, and Perkins brailler.
What to do

1.Use a spring balance to determine the force, F, in pulling a block placed on a table or desk through a distance, dl of 0.700 m.
2. With the same spring balance reading, repeat (1) for d2=O.500 m.
3. Determine the work done in each case.
4.Using masking tape attach eight 100 g masses and repeat (1) to (3).
5. Record your results in a suitable table.
6. Compare the values of work done in both loaded and unloaded block.
7. Identify the factors which affect the work done by a force.

Exercise 10.1

1. During painting of the class room block, a man of mass 65 kg climbs up a ladder 4 m high so as to clean a wall. What is the work done by the man?
2. A girl of mass 50 kg runs up a stair case of 20 steps, each 8.0 cm high leading her to the first flow of the school laboratory block. Find the work done by the girl.
3. Maryliftsasuitcaseofmass 12kgfromhertoestoherheadassheenterssch001 She dæs work of 144 J as she lifts the suitcase. What is her height in metres?
   10.3: Energy
   In our daily life we engage in many activities whether at school or at home. When we finish or even before finishing an activity, we may feel tired and exhausted.
   In most cases, people eat some food and then rest so as to regain energy. What does each activity require before it is done? Let us understand that in Activity 10.3.
   Activity10.3: Understanding the meaning ot Energy

What you need: notebooks, braille paper, stylus, slate, Perkins brailler.
What to do
In groups

1. Identify the activities that you carry out while at home.
2. State what is needed to do all the activities identified in (1).
3. Suggest a suitable definition of energy.
4. Identify and explain the different forms of energy you know.
5. Which forms of energy do we use at school?
   All living things need energy. Even machines that help us to do work also need energy. Anything that possess energy is capable of doing work.
   Therefore, energy is the capacity or ability to do work. Just like work, the Sl unit of energy is the joule (J).
   Energy is not visible, it does not occupy space and has neither mass nor other physical property that can describe it.
   Sources of Energy

In our daily lives, we use energy to carry out different activities such as cutting, slashing among others. Sometimes these activities may be done using machines or may be just done manually by ourselves. Whatever way one chooses, energy is always required to do work. Where do you think this energy we use comes from?
Can you identify any source of energy here in school? Let us do Activity 10.4 to identify different sources of energy.

Activity10.4: Identifying sources of energy
What you need: notebooks, braille paper, stylus, slate, Perkins brailler
What to do

1. In groups read about the various sources of energy.
2. Identify the type of energy used to cook the food?
3. Mention other materials from which the same energy can be obtained.
4. Discuss the following forms of energy and identify their individual sources.
   a) Mechanical energy
   b) Thermal energy
   c) Sound energy
   d) Light energy
   e) Electrical energy
   f) Nuclear energy
   Include illustrations where necessary and relate the work with your environment that is giving local examples where need be.

Renewable and Non-renewable Sources of Energy
As previously discussed, energy can be obtained from various sources which include; water, wind, bio gas, among others. However, these sources may either continuously supply the energy without getting exhausted or supply energy for a limited amount of time. In Activity 10.5, we shall identify and classify the different sources of energy.
Activity10.5: Identifying renewable and non- renewable sources of energy
What you need: Research source like internet or library, ICT tool to aid research especially using internet, and Materials for illustration like Manila paper, notebooks, braille paper, stylus, slate, Perkins brailler.
What to do

1. In your groups, study the pictures (a),(b),(c) and (d).

2. Identify and name the energy source responsible for the electrical energy generated in (a) to (d).
3. Classify the above sources as renewable and non renewable sources of energy.
4. State and explain the energy source you would to use in your society.
5. Differentiate between renewable and non renewable sources of energy.
   10.2.2 The sun as a source of energy
   The sun is a major energy source which constantly supplies energy and has been in existence for 4.5 billion years. Let us do activity 10.5 to demonstrate the fact that the sun is a source of energy.
   Activity 10.6: Showing that the Sun is a source of energy
   What you need: notebooks, braille paper, stylus, slate, Perkins brailler, Laboratory thermometers, The sun shining brightly, Stop watch or stop clock, Solar lamp, Two containers like cups of the same colour and type, and water.

What to do

1. In groups, fill two containers of same type and size with equal amounts of water.
2. While under the shade, measure and record the temperatures of water in both containers using the thermometer.
3. Place one container under the Sun and leave the other container in the shade, start the stop clock immediately.
4. After 10 minutes or 20 minutes, place thermometer again in the twol samples of water.
5. Which sample gives the highest temperature rise? explain.
6. Write down the main forms of energy obtained from the sun in your books.
7. Write down the uses of energy got from the sun.

Exercise 10.2

1. A block of mass 2 kg falls freely from rest through a distance of 3 m. Find the kinetic energy of the block.
2. 100 g steel ball falls from a height of 1.9 m on a plate and rebounds to a height of 1.25 m. Find the;
   a) Potential energy of the ball before the fall.
   b) Kinetic energy as it hits the plate.
   c) The velocity the plate hits the ground.
   Energy Transformation
   In life as we use energy, it is simply converted from one form to another using devices such as solar panels, flat iron, electric bells among others. It is as a result f energy transformation that cars, trains, among others change their location.

Let us do Activity 10.7 to develop an understanding of how energy is transformed from one form to another.
Activity10.7: Exploring energy transformation
What you need: Research source,and manilla/flip charts or ICT tool, notebooks, braille paper, stylus, slate, Perkins brailler.
What to do

1. In groups, research about the energy transformations. The research should enable you do the tasks below.

Copy fie following schematic diagram and fill in the energy transformation that took place during;
i) Lighting a bulb using a battery
ii) Hammering a nail

3. Discuss the energy changes that take place in hydroelectric power.
4. In your group, discuss and make a presentation on how the following devices convert energy.
   a) Filament lamp
   b) Steam engine
   c) Electrical heater
   d) Nuclear power station
   Energy is always conserved during its transformation and this is in the law of energy conservation which states that energy is neither created nor destroyed but can be changed from one form to another.
   Exercise 10.3

10.7: Power
In this section, we shall basically understand the fact that power is simply a quantity used to describe how much work is done in a given amount of time. Let us do Activity 10.8 to determine power expanded.

Activity10.8: Determining the amount of power expanded
What you need: A wooden block, spring balance, and stop clock or stop watch, notebooks, braille paper, stylus, slate, Perkins brailler

What to do
1.Place the wooden block on its wide surface at the center of a table.
2.Hook the block on to the spring balance and mark the starting point on the table.
3.Pull the spring balance such that the block moves with a constant force and immediately start the stop clock or stop watch.
4.Read and note the value on the spring balance.
5.Stop timing after the block covering a significant distance. use a ruler to measure the distance covered.
6.What is the time taken for the block to move through the above distance.
7.Determine the amount of power from the formula below:

The Sl unit of power is Watts written as W. Note that a spring balance is an instrument used to measure the amount of force exerted. A watt is defined as power consumed when the rate work done on a body is one joule.

Exercise 10.4

1. A pump lifts 200kg of water from the bottom of an underground rain water harvesting tank near the school kitchen is 10 seconds. The pump is rated 1.2kW. Find the depth D of the tank. (Acceleration due to gravity=10ms-2).
2. The motor ofa lift provides a force of 2000 N; this force is enough to raise the lift by 1.8 m in 15 s. Calculate the output power of the moton
3. A man of mass 64 kg climbs a stair case of N steps each of height40.0 cm in 4 minutes and develops a power of 16W. Calculate the number of steps N on the stair case.
4. A cyclist applies brakes and brings the cycle to rest in a distance of 10 m in 20 seconds. The frictional force between the tyres and the road is 400 N. Calculate the power developed.