General Science Grade 8 Student Textbook PDF

General Science GRADE 8 Student TextBook UNIT ONE BASICS OF SCIENTIFIC INVESTIGATION Learning Outcomes: At the end of this unit, You will be able to: identify the basic and derived units of measurements; explain the concept of...

General Science GRADE 8 Student TextBook UNIT ONE BASICS OF SCIENTIFIC INVESTIGATION Learning Outcomes: At the end of this unit, You will be able to: identify the basic and derived units of measurements; explain the concept of measuring physical quantities; describe the components of a scientific investigation; demonstrate ability to work effectively and respectfully with others in performing fair testing. Main contents 1.1 Scientific Measurments 1.2 Doing Scientific Investigation 1 General Science GRADE 8 Student TextBook Introduction This unit contains two sub units: scientific measurement and do- ing scientific investigation. Under scientific measurement the in- digenous and modern methods of measurement, the classification of physical quantities into fundamental and derived quantity and the difference between accuracy and precision will be discussed. Under doing scientific investigation, the importance, procedures and ethical issues of a scientific investigation will be discussed. Finally using locally available materials, a simple investigation will be conducted. 1.1 Scientific Measurements At the end of this section, you will be able to: explain the concept of measuring physical quantities; describe the various indigenous methods of measurement; distinguish between the basic and derived physical quantities; categorize the basic and derived units of measurements (length, mass, time, temperature, volume, area, density, force); identify prefixes and perform conversions among units of measurements; distinguish between accuracy and precision in measurements. Introduction Making observation is common experience in science. Similarly, it is usual asking the basic questions like how big an object is? How tall are you? To answer these questions, measurements have to be made. Measurement is the process of obtaining the magnitude of a quantity relative to an agreed standard. 2 General Science GRADE 8 Student TextBook In this section both the indigenous and modern methods of measurement will be discussed. The indigenous method of measurement refers to a measurement practiced locally while the modern method refers to a measurement applied by the scientific community. Indigenous Methods of Measurements An indigenous method of measurement refers to measurement methods that are practiced locally for a long period of time and are passed from generations to generation. In this section, we will pay attention to the measurement of length, mass, and time. A. Length Length is a measure of the distance between two points. In Ethiopia we use different indigenous units of length measurement. The commonly used ones are: 1. Hand-span: The hand-span is the measure from the tip of your little finger to the tip of your thumb when your hand is stretched out, Fig 1.1 (a). 2.Digit: A digit is the width of an adult human male fingertip, Fig 1.1 (b). 3.Cubit: A measure of distance from the tip of one’s elbow to the tip of the middle finger when your arm is extended, Fig 1.1 (c). 4.Foot: A measure of distance from the back of the heel to the tip of the big toe, Fig 1.1 (d). 5.Pace: A linear distance measure of a person’s extended walk. A pace is a unit of length consisting either of one normal walking step. The pace is the distance measured from the heel of one foot to the heel of the same foot when it next touched the ground, Fig 1.1 (e). 6. Arm span: Arm span also known as fathom is the distance from the middle fingertip of the left hand to that of the right hand when you stretch your arms out as far as they can reach, Fig 1.1 (f). 3 General Science GRADE 8 Student TextBook Figure 1.1 Indigenous Length measurements Figure 1.1 Indigenous Measurement Length Activity 1.1: Make a group containing 5 students. Using your hand Activity 1.1: Make a group containing 5 students. Using your hand span, span,cubit cubit and digit digit measure measurethe thewidth widthof of a table a table or aor a desk desk in your in your classroom.Record classroom. Recordyour yourmeasurement measurement in in the the table table below. below. No Name of the student Measurement result making measurement 1 2 3 4 Question: Did each of you obtain the same measure for that bench? Question: Did each of you obtain the same measure for that table or Justify desk? the difference Justify of students‘ the difference measurement. of students’ measurement. 11 hand-span, digit, cubit, foot, pace Exercise 1.1: Compare the size of your and arm-span and write them in order of increasing value. B. Mass The amount of matter present in a substance is called mass. Like length, there is also an indigenous method of measuring mass. The following are some examples of the indigenous unit of mass measurement used in Ethiopia. 4 General Science GRADE 8 Student TextBook 1. Weqet- Weqet is a mass measuring unit usually used to measure the mass of powder of gold in local markets. 2. Quntal – Quntal (may be taken from the English word quintal) is a bag used to measure the mass of grains. It is equal to a hundred kilogram. 3. Feresula:- is used to measure the mass of pepper and coffee. It is equal to 17 kilogram. Figure 1.2 Indigenous mass measurements Exercise 1.2: Discuss about the reliability of the above three indigenous mass measuring methods. C. Time Time is the measure of the duration for an interval.There is also an indigenous method of measuring time. Our elders were used the shadow of a tree to measure time. As the position of the Sun changes from morning to evening the length of the shadow of a tree varies. In the morning and late in the afternoon, the length of the shadow is high. At noon when the Sun is overhead no shadow will be seen. Using this fact they could tell the approximate time of the day by just looking at the position of the shadow of a tree found at or near their home. Activity 1.2: Using a long tree found in your school, mark the time at different height of the shadow of the tree. Use this shadow clock for some time. Discuss your observation. 5 General Science GRADE 8 Student TextBook Project 1.1: In ancient time three commonly known time measuring devices were used: They are known as sundial, sand clock and water clock. Using internet and other sources explore how these devices were used to measure time and report your finding to the class. D. Volume Volume is the measure of the space occupied by an object. In the local markets of Addis Ababa the following tools are used for different size volume measurements. 1. Jog: A plastic cup used for measuring the volume of liquids. 2. Tassa: A can used to measure cereals, pulses ,liquids and solids. 3. Sini: A small ceramic cup often used for measuring coffee, pulses and spices. 4. Birchiko: A glass often for measuring pulses and liquids. 5. Kubaya: A mug, often used for measuring cereals, pulses and liquids. Figure 1.3 Some examples of Indigenous volume measurements Exercise 1.3: 1. Discuss about the problems there could be in using the above indigenous volume measuring devices. 2. Discuss in group about the pros and cons of indigenous measurements used in your locality 6 General Science GRADE 8 Student TextBook Project 1.2: With the help of your teacher go to the local market found near to your school. Gather information about the indigenous measuring devices used for different measurements in the market. You can also ask your elder family members and present a report to your classmates. Physical Quantities and Scientific Methods of Measurement In our day to day life, we measure many things such as the mass of vegetables, the volume of liquids, the speed of a car, the temperature of the day etc. Such quantities which could be measured are called physical quantities. A physical quantity is a property of an object that can be measured or calculated from other physical quantity. Examples of physical quantities are: length, mass, time, temperature, area, volume, density, force etc. Generally, physical quantities are classified into two types, namely: fundamental quantities and derived quantities 1.Fundamental Physical quantities and their units Fundamental quantities, also known as base quantities, are quantities which cannot be expressed in terms of any other quantity. They are the bases for other quantities. There are seven fundamental (basic) physical quantities: length, mass, time, temperature, electric current, luminous intensity and amount of a substance. In this section we will discuss only about the first four commonly measured fundamental quantities: length, mass, time and temperature. The names and symbols of the units of the fundamental quantities in the International System of units (SI) are shown in table 1.1.The International System of Units (SI, abbreviated from the French Système international (d’unités)) is a system of measurement based on base units. An International System of units (SI) is currently used all over the world. Measurement is the comparison of an unknown quantity with some known quantity. This known fixed quantity is called a unit. Thus, the result of a measurement is expressed in two parts. One part is a number and the other part is the unit of the measurement. For example, if a student has a mass of 32 kg: 7 General Science GRADE 8 Student TextBook is mass, the value of the measurement is 32 and the unit of measure is kilograms (kg). the quantity being measured is mass, the value of the measurement is 32 This tells us that any measurement consists of two parts. The first is the and the unit of measure is kilograms (kg). number This which tells us that indicates the magnitude any measurement of theofquantity consists andThe two parts. the second first is theindicates number the which indicates of unit (standard) thethat magnitude quantity. of the quantity and the second indicates the unit (standard) of that quantity. Units can be classified into two groups: fundamental units and derived Units can be classified into two groups: fundamental units and derived units. units. TheThe units units usedused to tomeasure measurefundamental fundamental quantities quantities are are called called fundamental fundamental units. units.ItItdoes doesnotnotdepend depend onon any anyother otherunit. unit. Table 1. 1 Fundamental quantities and their SI units Quantity Name of Unit Symbol of the unit Length Meter m Mass kilogram kg Time Second S Temperature Kelvin K Derived Physical 2.Derived Physical Quantities Quantitiesand andtheir Units their Units Physical quantities Physical which quantities depend which on oneon depend or more one fundamental quantities or more fundamental for quantities their measurements derived are called are for their measurements called quantities. Speed,Speed, derived quantities. area, volume, density and force are examples of derived quantities. The area, volume, density and force are examples of derived quantities. The units used to measure derived quantities are called derived units. It units used depends to measure units on fundamental derived forquantities are called SI their measurement. derived units. derived It units aredepends described on by mathematically fundamental combining units for (dividing,SI their measurement. multiplying or derived units powering) the base units. Some of the derived quantities and their are described by mathematically combining (dividing, multiplying or units are given in table 1.2. powering) Tablethe1.base units. Some 2 Derived of the derived quantities quantities and their and their units SI units are given in table 1.2. No. Derived quantity Symbol Unit 1 Table Area1. 2 Derived quantities A and their mSI units xm = m2 2 Volume V m x m x m = m3 3 Speed V 16 m/s 4 Density ῤ Kg/m3 8 General Science GRADE 8 Student TextBook Example 1.1: Show how the unit of (a) area and (b) speed is derived from the fundamental units. Solution: (a) The equation for the area of rectangular surface is Area = length x width. Both length and width are length measurements. Hence they are measured in meter. Unit of area = unit of length x unit of width Unit of area = m x m = m2 (b) The equation for speed is Speed = distance/time Thus the unit of speed is the unit of distance (m) over the unit of time (s) = m/s Activity 1.3: Discuss in group about the importance of scientific measurement to the study of science. Let the representative of your group present what you have agreed to your classmate. Exercise 1.4: Show how the units of the following derived quantities are derived from the unit of base quantities. (a) volume, (b) density and (c) force. Prefixes and Conversion of Base Units Prefix In science we deal with quantities which are both very large and very small. A short hand form of writing very large and very small numbers is known as a prefix. A few of the prefixes used in the SI system of units are shown in Table 1.3. 9 In science we deal with quantities which are both very large and very small. A short hand form of writing very large and very small numbers General Science GRADE 8 Student TextBook is known as a prefix. A few of the prefixes used in the SI system of units are shown in Table 1.3. Table 1.3. SI prefixes Prefix Symbol Name Decimal representation Mega M million 1 000 000 Kilo k thousand 1 000 Centi c hundredth 0.01 milli m thousandth 0.001 Conversion micro of base µ units millionth 0.000001 It is often necessary to convert between units of measurement. For Conversion of base units example, It is oftena necessary mass measured in grams to convert may beunits between required to convert intoFor of measurement. example, kilogram. a mass measured in grams may be required to convert into kilogram. To convert from one unit to another within the SI, usually means To convert from one unit to another within the SI, usually means moving a decimal point. If you can remember what the prefixes mean, moving a decimal point. If you can remember what the prefixes mean, you can you can convert convert within withinthe theSISIsystem systemrelatively relativelyeasily by by easily simply simply multiplying multiplying orordividing dividing thethe number number by value by the the value of theofprefix. the prefix. Example 1.2: Convert 6.5 kilogram (kg) to gram (g). Example 1.2: Convert 6.5 kilogram (kg) to gram (g). Solution: Since killo (k) is a prefix representing 1000, so: Solution: 6.5 Since kg = 6.5 k is a prefix × (1000) representing g = 6500 g 1000, so: Example 6.5 kg = 6.51.3: Convert × (1000) g = 200 6500meters g to kilometers. We know1.3: Example thatConvert 1 km 200= 1000m. Then meters to we will ask if 1000m is 1km kilometers. then what will be 200m in km? 18 Solution: 1 km = 1000 m 200 m = 1 km × 200 m 200 km = = 0.2 km ? = 200m 1000 m 1000 Exercise 1.5 1. Convert the following: a) 0.6 km to cm b) 500 g to kg c) 30 min to hour d) 50 m to mm e) 0.25 kg to g f) 0.5 hour to second 2. Write the following quantities in units with the appropriate prefixes: a) 3500 m b) 0.0012 sec c) 0.01 g 10 General Science GRADE 8 Student TextBook Measuring Physical Quantities The measurement of a physical quantity is done by using measuring instruments. In this section we will discuss how to measure mass, length, time, and temperature using their appropriate devices. Measuring the mass of objects Instruments which are used to measure mass are known as balances. Theused to measure balance mass. Itthe compares workmass basedofonan the object principlewith that the amount mass. a known of extension (or Different compression) types of a springare of balances is proportional there, to seethe mass Fig of1.4. the object attached to it. Figure 1.4: Instruments Used to Measure Mass Note that, before taking measurement check that the balance is on a level surface, and reads zero when no load is placed on it. Note that, before taking measurement check that the balance is on a Thesurface, level SI or base andunit of Mass reads zero is kilogram when (kg).isFor no load smallon placed mass it. we use gram (g). To measure the mass of objects less than 1 gram, we can use Themilligram. SI unit of To mass measure kilogram is the (kg). mass of big For we objects small mass we use quintal anduse tone.gram (g). To measure the mass of objects less than 1 gram, we can use 1 kg = 1000 g. milligram. To measure the mass of big objects we use quintal 1 gtone. and = 1000 mg 1 quintal = 100 kg 1 tone = 1000 kg 20 11 General Science GRADE 8 Student TextBook The relationship between different units of Length. 1 kg = 1000 g. 1 g = 1000 mg 1 quintal = 100 kg 1 tone = 1000 kg Example 1.4: How much is 1200 gram in kilogram? 1 Solution: 1200 g = 1200 × kg = 1.2 kg 1000 Exercise 1.6: Convert the following measurement: (a) 2.5 kg to gram, (b) 200 gram to milligram. Measuring Length Length is a measure of how long an object is. Depending on the size of the length of the object, we are going to use different types of length measuring instrument, see Fig 1.5. Figure 1.5 Instruments used to Measure Length The SI unit of length is meter (m). When we want to measure larger lengths, we can use kilometers. If we want to measure small lengths, we can use centimeters or millimeters. 12 General Science GRADE 8 Student TextBook The relationship between different units of Length. 1km = 1000 m 1 m = 100 cm 1cm = 10mm Note that when we are measuring length using these device do not forget to place the zero mark exactly at one end of the thing you are measuring and read the scale at the other end. Example 1.5: How many millimeters are there in a meter? Solution: 1m = 100 cm = 100 x 10 mm = 1000 mm Exercise 1.7: Convert the following into the required measures: (a) 8 meters to millimeter. (b) 5500 meters to kilometer. Measuring time Time is used to quantify the duration of events. Time is measured with a stop watch or clock. Figure 1.6 Time measuring Instruments The SI unit of time is second (s). For longer intervals of time we use: day, month , year, decades, century and millennium. 13 General Science GRADE 8 Student TextBook The relationship between different units of time 1 hour = 60 minutes 1 minute = 60 seconds 1 day = 24 hours 1 week = 7 days 1 year = 365 or 366 days Example 1.6: Convert one hour into seconds. Solution: 1 hour = 60 minutes = 60 × 60 second = 3600 seconds. Exercise 1.8: How many (a) minutes, and (b) seconds are there in one day? Measuring Temperature Thermometer is the device used to measure the temperature of an object or place. The SI unit of temperature is Kelvin. Degree Celsius (°C) and degree Fahrenheit (0F) are other units of temperature Thermometers could be analogue or digital, see Figure 1.7 Figure 1.7 Temperature Measuring Devices Activity 1.4:Measuring body temperature. measure the body temperature of two students by using thermometer. Compare the two temperatures with the standard temperature of a body which is 37°C Discuss about your observations. 14 General Science GRADE 8 Student TextBook In using thermometer, hold the thermometer at the top, do not hold the bulb of a thermometer and do not let the bulb touch the glass. Activity 1.5: Measuring the temperature of water. Using a laboratory thermometer, measure the temperature of a warm water. Record your observations Safety!! Never put a laboratory thermometer into your mouth. Accuracy and Precision in Measurement Accuracy refers to how close a measurement is to its accepted or known value. Example 1.7: If in a laboratory you obtain a mass measurement of 8.2 kg for a given substance, but the actual or known mass is 10 kg, is your measurement accurate? Answer: This measurement is not accurate, because your measurement (8.2 kg) is not close to the known value (10kg). Precision refers to how close two or more measurements are to each other, regardless of whether those measurements are accurate or not. Example 1.8: In the above example 1.4, if you measure the mass of the given substance five times, and get 3.2 kg, 3.1 kg, 3.25 kg, 3.3 kg and 3.2 kg. Is your measurement precise? Answer: This measurement is precise, because the values are close to each other but not accurate because it is far from the known value (10 kg). This shows that precision is independent of accuracy. You can be very precise but inaccurate. You can also be accurate but not precise. Exercise 1.9: The figure below shows 3 results of a student playing a dart game. In the space provided below each figure, write whether the result is (a) accurate but not precise (c) precise but not accurate (b) accurate and precise (d) neither precise noraccurate 15 General Science GRADE 8 Student TextBook Figure 1.8 Dart game Exercise 1.10: 1. Define the following terms: physical quantity, fundamental quantity, derived quantity. 2. State the various indigenous methods of measurement used in Addis Ababa. 3. What are prefixes? 4. What is the difference between accuracy and precision in measurements? 1.2 Doing Scientific Investigation At the end of this section, you will be able to: describe the components of a scientific investigation; demonstrate ability to work effectively and respectfully with others in performing fair testing; practice scientific investigation procedures using appropriate contents to their age levels. Introduction to Scientific Investigation Science is a process of learning about the world through observation, inquiry, formulating and testing hypotheses, gathering and analyzing data, and reporting and evaluating findings.This process is referred as the scientific investigation or scientific method. 16 General Science GRADE 8 Student TextBook 1.2 Scientific Method Activity 1.6 What are the applications of scientific method? All sciences, including the social sciences, employ variations of what is called the scientific method. Scientific method is the process by which scientists approach their work. The Steps of the Scientific Method Based on the type of question being asked, the type of science being applied and the laws that apply to that particular branch of science, you may need to modify the method and alter or remove one or several of the steps. 1. Ask Questions A scientific investigation typically begins with observations. Observations often lead to questions. This question will include one of the key starters, which are, how, what, when, why, where, who or which. The question you ask should also be measurable and answerable through experimentation. It is often something that can be measured with a numerical result, although behavioral results are part of the scientific method as well. 2. Perform Background Research With your question formulated, conduct preliminary background research to prepare yourself for the experiment. You can find information through online searches or in your local library, depending on the question you are asking and the nature of the background data. You may also find previous studies and experiments that can help with your process and conclusions. 3. Establish your Hypothesis Based on the data that were gathered, the researcher formulated a hypothesis. A hypothesis is a tentative explanation for a set of observations. Your hypothes should also include your predictions that you can measure through experimentation and research.A hypothesis must be based on scientific knowledge, and it must be logical. 17 General Science GRADE 8 Student TextBook 4. Test your Hypothesis Next, test your hypothesis by conducting an experiment. Your experiment is a way to quantifiably test your predictions and should be able to be repeated by another scientist. Assess your scientific process and make sure that the conditions remain the same throughout all testing measures. If you change any factors in your experiment, keep all others the same to maintain fairness. After you complete the experiment, repeat it a few more times to make sure the results are accurate. 5. Analyze the Results and Draw a Conclusion You can now take your experiment findings and analyze them to determine if they support your hypothesis or not. Drawing a conclusion means determining whether what you believed would happen actually happened. If it did not happen, you can create a new hypothesis and return to step three, then conduct a new experiment to prove your new theory. If what you hypothesized happened during the experimentation phase, the final step is putting together your findings and presenting them to others. 6. Communicating Results The last step in a scientific investigation is communicating what you have learned with others. This is a very important step because it allows others to verify your methods and results. If other researchers get the same results as yours, the hypothesis becomes stronger. However, if they get different results, they may not support the hypothesis. When scientists share their results, they should describe their methods and point out any possible problems with the investigation. Finally, communicating results can be done in a variety of ways including scientific papers, blogs, news, articles, conferences, etc. 18 General Science GRADE 8 Student TextBook Figure 1.9 Steps in Scientific Method Example1.9: Simple experiment with candle that shows the necessary of air for burning. Consider how the scientific method applies in this simple experiment with air necessary for burning under two different conditions. 1. Ask Question: Is air necessary for burning? 2. Do back ground Research: From different literatures ‘‘air is necessary for burning.’’ 3. Formulate Hypothesis: The null hypothesis is that there wil be no air needs for burning. The alternative hypothesis is that there will be air needs for burning. 4. Test Hypothesis by Experiment and Collect Data:Take a candle and fix it on a table. Light the candle. The candle will continue to burn due to continuously available fresh air providing the required oxygen for combustion.Now cover the burning candle by putting an inverted gas jar over it. After a short time, the candle stops burning and gets extinguished. 5. Analyze the Results and Draw Conclusion: When the burning candle is covered with gas jar, then the candle takes away the oxygen necessary for burning from 19 General Science GRADE 8 Student TextBook the air enclosed in the gas jar. After some time, when all the oxygen of air inside the gas jar is used up, then the burning candle gets extinguished. This proves that air is necessary for combustion or burning of substances. Figure 1.10 a) Burning of candle b) Candle stops burning 6. Communicate Results: Report your findings in the form of a written report as an oral presentation. Air is necessary for burning. Activity 1.7 Form groups and conduct investigations on activities listed below. After investigation present your findings to the class. a. What is the effect of sunlight on the growth of bean plant? b. Does a coiled nail act like a magnet? c. How do plants store their food in their leaf? Exercise 1.13 Describe the components of a scientific investigation. Project 1.3 Conduct some investigations (for example, making injera) using local materials and methods (procedures) in groups by reading different reference books or asking a person who is knowledgeable and experienced in the area. 20 General Science GRADE 8 Student TextBook Figure 1.11Injera Figure 1.12 Injera being cooked on a griddle Key Terms: -Fundamental quantity, -Derived quantity, -Fundamental unit, -Derived unit, -Prefix, Accuracy -Precision, and -Scientific method. 21 General Science GRADE 8 Student TextBook Summary Measurement is the process of obtaining the magnitude of a quantity relative to an agreed standard. Indigenous units of measurement for length: cubit, span, digit, foot and pace, for mass weqet and quntal, for time length of a shadow are used. Fundamental quantities are a set of physical quantities which cannot be expressed in terms of any other quantities. Their corresponding units are called “Fundamental units”. The physical quantities which can be obtained by mathematically combining (i.e., multiplying and dividing) the fundamental quantities are known as “Derived quantities”. Their corresponding units are called “Derived units”. Prefixes are a short hand form of writing very large or very small numbers. Accuracy refers to how close a measurement is to the accepted value while precision refers to how close measurements are to each other. Scientific method is the process by which scientists approach their work. 22