Grade 7 Physics: Physics and Measurement PDF

UNIT PHYSICS AND 1 MEASUREMENT Unit outcomes: After completing this unit you should be able to:  appreciate the interrelatedness of all things.  search for patterns or relationships in experimental data.  use a w...

UNIT PHYSICS AND 1 MEASUREMENT Unit outcomes: After completing this unit you should be able to:  appreciate the interrelatedness of all things.  search for patterns or relationships in experimental data.  use a wide range of possibilities for developing knowledge of the major concepts with in physics. A B 0 1 2 3 4 5 a) What is the distance between A and B? A cm b) What is the length of block A (LA = ?) B cm c) What is the length of block B (LB = ?) A B d) What is the length of blocks A and B (LA +LB) = ? 1  Grade 7 Physics 1 Physics and Measurement Introduction In the lower grades you learnt about science in general. For example, environmental science and integrated science. In this and next grades you will learn about physics, chemistry and biology separately. In this unit you will learn what physics is and about measurement. Activity 1.1 Discuss the following questions in a group. i. What is science? What does science deal with? ii. What are the major branches of science? In a simple term, science is the study of the world around us. It deals with the knowledge of the world Science is a around us. The major classifications of science are systematized knowledge given in chart 1. arising from What are the two categories of science? observation, Name the three branches of natural sciences? study and experimentation. Chart 1. Classification of science   Science    Natural Science Social Science  Physics Chemistry Biology 2 Grade 7 Physics 1 Physics and Measurement 1.1 Definition of Physics a) Meaning of physics Physics is defined in different ways. The following is one definition of physics. Physics is a way of observation of the world around us. Through observation we understand our world and how objects in the world behave (laws of nature). Physics is simply the science of observation and measurement. Activity 1.2 i. From the explanations given above, describe in your own words what physics is. ii. What do we call the person who studies physics? The word 'Physics' has its origin in the Greek word meaning ‘nature'. Hence, physics is the branch of natural science. It is the study of the nature of matter, energy and their interactions. A person who studies physics is called physicist. Isaac Newton (1643- Michael Faraday James Prescott Joule Marie Curie 1727) discovered the (1791-1867), discovered (1818-1889) , studied the (1867-1934), won the Nobel laws of motion and the generation of electr- nature of heat and Prize for the discovery of the law of gravity. icity from magnetism. He discovered the elements polonium and built the 1st dynamo. relationship between radium. mechanical energy and heat energy Fig1.1 Some known physicists and their works In order to understand, the definition of physics well, you need to have clear idea of 'matter' and 'energy'. Discuss with your friends and parents on the questions below and write a short note on: Challenging Questions i. What is matter? iii. What is energy? ii. Some properties of matter. Iv. Explain how matter and energy are interrelated. 3 Grade 7 Physics 1 Physics and Measurement Activity 1.3 Mention at least five areas of study or topics i. related to physics ii. not related to physics b) Purposes of studying physics Physics is studied as a separate subject in grades 7 and 8 and also in secondary schools. Activity 1.4 Discuss in a group with your friends why you study physics. Report your answers to the whole class. The following are some specific purposes of studying physics Physics helps you to understand the working principles of many of your daily utensils and tools. Physics helps you discover some of the unknown parts of nature and makes you familiar with the modern world. Physics helps you to understand some of the natural phenomena in other subjects like: biology, chemistry, geology, astronomy, etc. Physics enables you to understand why it is difficult to walk on a smooth plane, why the electric fan moves, how the cars, airplanes, space-rockets, refrigerators, alarm-clocks, radios, televisions, etc work. Generally studying physics helps you to: understand concepts, relationships, principles and laws of nature. do activities (experiments), to formulate and to check theories. describe some applications of physics on your daily life. solve practical problems (real life problems). understand the cause and effect of natural phenomenon. 4 Grade 7 Physics 1 Physics and Measurement c) Areas of study where physics does not address There are areas or activities which does not require direct knowledge of physics. For example, history, civics and ethical education, politics, religion, etc. are not directly related to physics. d) The main goal of the study of physics The main goal of learning physics is to gain a better understanding of the world around you and the things in it. By understanding the world around you and the things in it, you discover facts. These facts form scientific concepts, theories, laws, principles and relationships. For example all objects fall towards the earth. This is a scientific fact. Corresponding to this there is a scientific law known as law of gravity; which is derived from that fact. Similarly when light falls on the surface of a mirror, it is reflected. Hence, from this fact, the law of reflection of light is derived. Activity 1.5 Form a group with your friends and list down some examples of ‘scientific facts’ and related ‘scientific law’. Scientific facts Scientific laws/ principles e.g. All objects fall towards the earth Law of gravity e) Relationship of physics to other sciences and disciplines There is no clear boarder line between the different branches of natural sciences. Knowledge of physics overlaps with the knowledge of chemistry, biology, astronomy, etc. For example, chemistry and physics knowledge are studied as a subject called physical science/physical chemistry. The following are some areas of studies where physics is combined with other science disciplines. Biophysics: combination of biology and physics. Astrophysics: combination of astronomy and physics. Geophysics: combination of geology and physics. 5 Grade 7 Physics 1 Physics and Measurement f) Branches of Physics Physics is divided into different branches. Some of the branches of physics are given in Table 1.1 Table 1.1 Branches of physics Branches Purpose Mechanics Deals with motion of a physical body. Sound Studies production, transmission and other properties of sound. Optics Studies production, transmission and other properties of light. Electricity and Deals with charged bodies at rest and in motion and Magnetism relationship between electrical and magnetic properties of bodies. Heat Deals with temperature, heat transfer and exchange in molecular level. Nuclear physics Deals with interaction in the atomic nuclear Astrophysics Deals with celestial bodies like planets, stars, galaxies, etc. Activity 1.6 Write five practical examples from your everyday life where the branches of physics are observed. g) Relationship of physics to engineering and technology Activity 1.7 i. What is the relation between physics and technology? ii. What are the differences and similarities between technology and engineering? iii. Which comes first, physics or technology? You have already seen what physics is. Now, you will see what a technology is. Technology is the use of scientific knowledge to help human beings work easier and live better and enjoy their environment more. Things such as automobiles, TV sets, radio, airplane and home tools (appliances) are the products of technology. A person who studies technology is called a technologist. 6 Grade 7 Physics 1 Physics and Measurement Technologists apply physics and mathematical knowledge and skills to produce a very useful tool. What are the products shown in Fig 1.2? a) b) c) d) Fig1.2 Products of Technology Engineers are technologists who design, construct and assemble products. What are the criteria for a good technological product? Products of engineers can be judged on six criteria. These are: 1. Is the product functioning as it should? 2. Is the product durable? 3. Is the product cost effective? Cheap? 4. How does the product affect the individual? 5. How does the product affect the society it works in? 6. How does the Product affect the environment? Check point 1.1 1. Explain what physics is. 2. List five branches of physics. 3. What is the purpose of learning physics? 4. Describe the relation ship of physics to: a. Biology b. Chemistry c. Astronomy 5. List some other disciplines related to physics. 6. What is the relationship of physics and technology? 7. What are the criteria for judging of good products in engineering? 7 Grade 7 Physics 1 Physics and Measurement 1.2 Standardization and Measurement Activity 1.8 a. i. Measure the length and width of this textbook. ii. Calculate the area of this textbook using the above measured values. iii. Measure the height of your friend, and let your friend measure your height also. iv. What instrument did you use? v. How did you write the measures of the textbook and your friend’s height? Measurement While you are doing Activity 1.8, you may come across units and numbers. These numbers by themselves means nothing. But when they are attached to some units of measurement like centimeter and meter they give you full information about your textbook and your friend’s height. i.e. - The length of this textbook is 24 centimeter. - Its width is 17 centimeter. - The height of your friend is 1 meter and 45 centimeters. Whenever you measure something, you simply compare two bodies. One of them Measurement consists of the comparison of an being a ‘standard’, and the other one unknown quantity with a being the body to be measured. known, fixed unit quantity. It consists of two parts: Measurement is one of the activities i. the unit. performed in physics. Physicists get ii. the number indicating how many units there quantitative information about objects are in the quantity through measurement. being measured. Standardization Activity 1.9 i. What traditional measuring units do you know that are used to describe length, time and mass? ii. Are they reliable (dependable)? 8 Grade 7 Physics 1 Physics and Measurement In ancient times, people in Ethiopia used to measure physical quantities such as time, mass, length, etc using traditional units. They say 'Nigat' or ' Mishet' as the sun rises or sets respectively. They say ' Ekule- ken' as the sun comes over head in the sky to measure time. Lengths at olden days were measured in 'cubits', 'spans', 'foot' and, 'stride'. a) Span b) Cubit Fig 1.3 Traditional length measuring units We still find these traditional units of length and time in our country. But they are not reliable. They do not give exact information. Activity 1.10 Group work a. i. Select friends who are shorter and taller than you. ii. Compare their cubits and spans. Are they the same? iii. What can you conclude? Are the traditional measuring units of length reliable? The development of science and technology Standard units are came up with the development of standard conventional units and reliable units of measurement. Scientists which are used to all over the world met together and agreed to measure physical have a standard unit that can be used through quantities scientifically. out the world. These standard units are known as System of International Units. In short, it is written as the SI units. 9 Grade 7 Physics 1 Physics and Measurement Physical quantities You measured, the length, width and height of your textbook. These quantities are called physical quantities. Time and mass are also examples of phyical quanties. Quantities that can be measured directly or indirectly are known as physical quantities. Physical quantities are The measured values of physical quantities are numbers with units written in terms of a number and unit. Physical which are used to quantities and units can also be written using describe physical symbols. phenomena. Where “” is length Note  = 24 cm, w = 17 cm “w” is width '' and 'w' are symbols of length. The numbers 24 and 17 are numerical values 'cm' is the symbol of the unit of length called centimeter. In activity 1.8, you measured directly the length and width of this textbook. But you calculated the area of it. What is the difference between the two ways of measuring things? The area of the book is calculated by combining two lengths, but not measured directly. From this practical activity, you can see that some quantities are directly measured, while others are calculated by combining two or more measurable quantities. Hence, Physical quantities are classified into two: 1. Fundamental physical quantities 2. Derived physical quantities. Fundamental physical quantities: are those quantities which can be measured directly. They are not defined in terms of other physical quantities. Length, mass and time are examples of fundamental physical quantities. Fundamental physical quantities are also called basic physical quantities. The units used to measure the fundamental quantities are called basic units. You see seven basic units in Table 1.2. Can you name them? 10 Grade 7 Physics 1 Physics and Measurement Beside length, mass and time there are other four basic physical quantities in science. These are temperature, electric current, amount of substances and luminous intensity. Table 1.2 The seven fundamental physical quantities Basic quantities Basic units Name symbol Name Symbol Length  Meter m Time t Second s Mass m kilogram kg Temperature T Kelvin K Current I Ampere A Amount of substance M Mole Mol Luminous Intensity - Candela Cd Challenging Questions 1. What fundamental quantities are combined to give area, volume, density, speed? 2. Explain how the basic units are combined to give the derived units of force, velocity, pressure and work. Derived physical quantities: are quantities that can be measured indirectly. They are calculated by combining two or more fundamental quantities. Area and volume are examples of derived physical quantities. The derived quantities use derived units. Table 1.3 Some derived physical quantities. Name Symbol Unit Symbol Area A square meter m2 Volume V cubic meter m3 Density ρ kilogram kg/m3 cubic meter Acceleration a meter m/s2 second 2 Force F kilogram-meter kg.m/s2 = Newton(N) second 2 Pressure P Kg.m/s.s kg.m/s2 = Newton = N/m2 square meter m 2 square meter 11 Grade 7 Physics 1 Physics and Measurement Scalar and vector quantities Some physical quantities are described completely by a number and a unit. A number with a unit is called a magnitude. However, other quantities have a direction attached to the magnitude. They can not be described by a number and unit only. Thus, physical quantities are grouped into two: i. Scalar quantity ii. Vector quantity A scalar quantity is a physical quantity which has only a magnitude. No direction. Time, mass, volume, density, temperature and energy are examples of a scalar quantity. A vector quantity is a physical quantity which has both magnitude and direction. Displacement, velocity and force are some examples of a vector quantity. Check point 1.2 1. What is measurement? 2. What are physical quantities? 3. Describe the difference between fundamental and derived physical quantities. 4. List seven fundamental physical quantities with their SI units. 5. List some derived physical quantities. 6. How can you distinguish between scalar and vector quantities? List examples of scalar and vector quantities in the given two columns Scalar Quantities Vector Quantities _________________ _____________ _________________ _____________ _________________ _____________ 12  Grade 7 Physics 1 Physics and Measurement 1.3 Measuring Length, Mass and Time 1.3.1 Measuring Length  Activity 1.11 b. What is the height and width of the blackboard? How far is your school from your home? What is the inside height and width of your classroom? When you tell the distance between your school and your home, or the height and width of your classroom, you measure length. Length is one of the fundamental (basic) physical quantities that describes the distance between two points. The symbol for length is "". Sometimes, we can also use other symbols such as ‘b’,’h’ and ’s’. When we measure length of an object, we are comparing it with a standard length that scientists have agreed to. The SI unit of length is METER (m). There are also other non- SI units of length. These are centimeter (cm), millimeter (mm) and kilometer (km). a) Ruler b) Tape Measure c) Carpenter Rule d) Vernier Caliper Fig 1.4 Length measuring instruments. Activity 1.12 a. Form a group with your classmates and do the following activities. Measure the length of different bodies using half meter ruler (50cm) and, write the length of the bodies using symbols. Estimate the a) Width of the blackboard b) Thickness of your physics textbook c) Width of the door of your classroom without using instrument. Now, measure the above quantities using length measuring instrument and compare with the estimated values. 13 Grade 7 Physics 1 Physics and Measurement Table 1.4 Relationship between meter and other non- SI units 1 meter (m) 1000 millimeters (mm) 1 meter (m) 100 centimeters (cm) 1000 meters (m) 1 kilometer (km) 1 meter (m) 0. 001kilometers (km) 1 millimeter (mm) 0. 001 m 1 centimeter (cm) 0.01 m 1 kilometer (km) 1000 m Example 1.1 The distance between two electric poles measures 100 meters. What is this distance in: a. centimeter b. kilometer N.B Use Table 1.4 Given Solution  = 100 m (distance) a) Since 1 m = 100 cm Then 100 m = ? 100cm× 100m ⇒ '' in cm = 1m  = 10,000 cm b) 1 m = 0. 001 km 100 m =? 100m × 0.001km ∴'' in km = 1m  = 0. 1km Challenging Questions Write down the suitable unit of length you need to use to measure: i. The distance between your school and your home. ii. The thickness of your physics book. iii. Your height. 14 Grade 7 Physics 1 Physics and Measurement 1.3.2 Measuring Mass So far you learnt how to measure length. Length is fundamental physical quantity in physics. The other important physical quantity you need to study is mass. Mass is a fundamental physical quantity. It is defined as the amount of matter contained in a body. There are two ways of measuring the mass of a body. i. Traditional way:- In traditional way things can be compared to each other to guess the approximate value of the mass of the bodies. Note: A traditional instrument does not tell us the exact value of the mass of a body. ii. Scientific way In scientific way a mass is measured using an instrument called a beam balance. A beam balance consists of uniform beam having two pans suspended from each of its ends. Fig 1.5 show different mass measuring instruments. Tell where these instruments are used in our daily life. Beam Weight Pan a) Locally made beam balance b) Beam balance c) Triple beam balance Fig 1.5 Mass measuring Instruments Activity 1.13 a. Have you ever tried to measure the mass of a body using a beam balance? Visit a shop in your living area. Write down the procedures the shopkeeper uses to measure the mass of a body using a beam-balance. Report your observations to your class. 15 Grade 7 Physics 1 Physics and Measurement The body to be measured is placed in one of the pans and a known Standard mass is placed in the other pan until a horizontal balance is obtained. At this moment the unknown masses of the body equals the standard masses. The SI unit of mass is the kilogram (kg). Other non- SI units can also be used to measure masses. Some examples are given in Table 1.5. Table 1.5 Relationship between units of mass 1000 kilogram 1 ton 100 kilogram 1 quintal 1 kilogram 1000 grams 1 gram 0.001 kg 1 milligram 0.001 gram Activity 1.14 i. Estimate without instruments the masses of the following bodies. a. Grade 7 Physics text book b. One stick of chalk c. One duster ii. Now measure the masses of the above estimated bodies using a beam balance. iii. Compare the estimated and measured values and calculate the differences. Give reasons for the differences. Example 1.2 1. In one of the pans of a beam balance the masses 1kg, 500g, 30g, 0.6g are placed to measure the mass of unknown body. What should be the mass of the body on the other side of a pan if they are in balance? Given Required m = 1kg, 500 g, 30 g, 0.6 g total mass = ? Solution m = the sum of the given masses = 1kg + 500g + 30g + 0.6g (change 1kg into g) = 1000g + 500g + 30g + 0.6g = 1530.6g or = 1.53 kg 16 Grade 7 Physics 1 Physics and Measurement 2. Abel and Zehara want to sit at the two ends of a SEE-SAW having equal distances from the pivot as shown in Fig.1.6. Zehara is 37 kg, and Abel is 29 kg. What additional mass (m x ) should Abel carry in order to balance the SEE-SAW. Fig 1.6 See-Saw Given Required Solution m A = 29 kg mx = ? mA + mX = mZ m Z = 37 kg 29kg + m X = 37kg m X = 37kg − 29kg = 8 kg Check point 1.3 1. What is a length? Name the measuring device of a length. 2. State the SI unit of length and some common non-SI units. Explain their relationship. 3. What is a mass? Name some measuring devices of a mass. 4. State the SI unit of mass and other commonly used non- SI units of mass. Explain their relationship. 5. What is time? Mention the measuring devices of time. 6. Write the relationship between SI unit of time and other commonly used non-SI units of time. 17 Grade 7 Physics 1 Physics and Measurement 1.3.3 Measuring Time What is time? The sun rises in the east in the morning and sets in the west in the evening. How long does the sun take to rise and set? People use the sunrise and sunset as a time measuring device. It is called sundial. Time is a fundamental physical quantity. It describes the duration between the beginning and end of an event. The SI unit of time is second (s). The symbol for time is 't'. Activity 1.15 Discuss: how the sun rise and sun set is used to measure the time of a day. Draw a diagram of sundial at different times of the day. a) Watch b) Stopwatch c) Digital watch Fig 1.7 Time measuring devices A clock and watch are the modern instruments used to measure time. Can you explain how the time measuring instruments indicated in Fig 1.7 are read? To measure very small or large intervals of time, there are other non- SI units of time. These are minute (min), hour (hr), day, etc. Activity 1.16 a. i. Measure the beat of your heart using a wrist watch (digital watch). Express it using symbols of quantity of time and unit of time. ii. Tell your friends and teacher how you did your activity. 18 Grade 7 Physics 1 Physics and Measurement Relationships between SI units and non- SI units of time: Activity 1.17 i. Have you ever noticed the relationships between hour, minute and second? What are the relationships? ii. Take a day (24 hrs) and list down activities you do through out the day. Time Activity Morning 12:00 1:00 2:00 etc. Are you using your time wisely? Compare your time with your friends time. Who are not using the day wisely? Discuss with your friends. Some wrist watches have an hour hand, a minute hand and a second hand. Can you define hour, minute and second using a wristwatch from your experiences? As the second hand completes one cycle, the minute hand moves one unit.(1 minute) As the minute hand completes one cycle the hour hand moves one unit (one hour). As the hour hand completes one cycle, we say 12 hours. Table 1.6 Relationship between units of time 1 hour 60 minutes 1 minute 60 seconds 1 day 24 hours 1 week 7 days 1 month 30 days 1 year 365 or 366 days Example 1.3 1. Express the following times in minutes: a) 3 hours b) 3/4 hours c) 1.25 hours. 19 Grade 7 Physics 1 Physics and Measurement Solution a) 1 hr = 60min 3hr × 60 min 3hr = ? ⇒t= 1hr = 180 min b) 1hr = 60 min 3 3 hr × 60min hr = ? ∴ t = 4 4 1hr = 45 min c) 1hr = 60min 1.25hr × 60 min 1.25 hr = ? t= 1hr = 75 min Challenging Questions Write down what unit of time, you need to use for measuring a. The beat of your heart. b. The duration of one period of your class. c. The time you take to travel from home to school. 1. How many hours, minutes and seconds are there in one day? 2. Mention some traditional ways of measuring time. 3. How many days are there in a year? 4. How old are you? Write your age in a. years b. months 5. Express the following times in seconds:- a. 75 minutes b. 2 hours c. 0.6 minutes 20 Grade 7 Physics 1 Physics and Measurement Summary In this unit you learnt that:  physics is a branch of natural science.  physics deals with the laws of nature. Physicist is a person who studies physics.  physics is applied to every development of science and technology.  Mechanics, Sound, Optics, Electricity and Magnetism, Heat, Nuclear physics, and Astrophysics are different branches of physics.  measurement is the comparison of an unknown quantity with a known one (standard unit).  measurement of an object consists of two parts: i. Unit of measurement, ii. The numerical values of the measured object.  standard units are conventional units which are used to measure physical quantities scientifically.  traditional units are not reliable and not exact.  physical quantities are quantities that can be measured directly or indirectly. They are expressed in terms of: i. numerical values ii. unit iii. symbol  scalar quantities have magnitude only. Direction is not associated with them.  vector quantities have both magnitude and direction.  length, time, mass, temperature, current, amount of substance and luminous intensity are fundamental quantities in science. The rest are derived physical quantities expressed by combining two or more of these fundamental quantities. 1. The SI unit of length is meter (m). 2. The SI unit of time is second (s). 3. The SI unit of mass is kilogram (kg). 21  Grade 7 Physics 1 Physics and Measurement Review Questions and Problems I. Write true if the statement is correct and false if the statement is wrong. 1. One meter is 100 kilometer. 2. There are seven fundamental quantities in physics. 3. The device used to measure a mass of a body is kilogram. 4. If kilogram is added to kilogram then we have a derived unit. 5. m/s is a derived unit. II. Multiple choice 1. Which one of the following is not a vector quantity? a) Displacement c) Force b) Density d) Velocity 2. Which one of the following is not a fundamental physical quantity? a) Time c) Force b) Mass d) Length 3. Which one of the following is a derived SI unit? a) Newton c) Kelvin b) Kilogram d) Second 4. 2 hr + 20 min + 60 sec are equal to ______ minutes. a) 120min c) 150 min b) 141min d) 161 min 5. Which one of the following quantities is measured by a balance? a) Length c) Mass b) Volume d) Density III. Short answer questions 1. Why do you think that measuring length, mass and time, using traditional methods are not reliable? 2. How many centimeters are there in 9.3 m? 3. Define the following terms. a) Physics e) Vector b) Standard units f) Length c) Measurement g) Time d) Fundamental physical quantity. h) Mass 22 Grade 7 Physics 1 Physics and Measurement 4. A large bottle contains a number of medicinal tablets each having a mass of 250 mg. The mass of all the tablets is 0.5 kg. Calculate the number of tablets in the bottle. Challenging questions 1. Meter, kilogram and second are the SI units of length, mass and time respectively. They are internationally agreed standard units. Write a descriptive note about the history, methods of determination and definition of meter, kilogram and second. 2. The following four SI units were named after famous scientists; Watt, Joule, Pascal and Kelvin. Find out: i. the area of physics to which each of these scientists made a significant contribution. ii. the physical quantity measured using each of the four units. 23  UNIT MOTION 2 Unit outcomes: After completing this unit you should be able to:  understand concepts related to motion.  develop skill of manipulating numerical problems related to motion.  appreciate the interrelatedness of all things.  use a wide range of possibility for developing knowledge of the major concepts with in physics. Introduction In this unit, you will be introduced to the basic concepts and relationships in motion. Motion is one of the key topics in physics. Everything in the universe moves. We use some basic concepts when we express motion. These concepts are distance, displacement, speed, velocity and acceleration. Based on the path of a motion, there are different types of Galileo Galilee(1564-1642.) motions. Motion in a straight line is one of the forms of motion. It is the simplest form of motion in a specific direction. Challenging Questions What are the contributions of Galileo Galilee to science and physics? 24  Grade 7 Physics 2 Motion 2.1 Definition of Motion Activity 2.1 Discuss the following questions with your friends i. What is motion? ii. When would you say an object is at rest? iii. Assume you are in a car and the car is moving at a certain speed. Are you at rest or in motion? iv. What do you understand by the term" reference frame"? Consider your daily travel from your home to your school. When you go to the school, your journey begins from your home. Your home is your original position. After sometimes you will reach your school. Your school is your final position. In this process, you are continuously changing your position. You are increasing the gap between your present position and your home. This continuous change of position is known as a motion. Notice that your change of position is, observed by considering the distance from your school to home. Your home is taken as a reference frame. Motion is a continuous change in position of an object relative to the position of a fixed object called reference frame. A body is said to be at rest in a frame of The concepts of rest and motion are reference when its position in that reference completely relative; a frame does not change with time. If the body at rest in one reference frame may be position of a body changes with time in a in motion in another frame of reference the body is said to be in reference frame. motion in that frame of reference Types of Motion Activity 2.2 i. Observe the motions indicated in Fig 2.1. ii. Have you noticed any difference between the motions in Fig 2.1 (a-d)? Describe them. iii. Group these motions, based on their path. 25 Grade 7 Physics 2 Motion a) A car moving on a straight line b) Roundabout c) Simple Pendulum d) Spring-mass system Fig. 2.1 Types of motion In Fig 2.1 (a) you observe that a car is moving on a straight road. Its path is a straight line. Fig 2.1 (b) shows that the path of the moving car is a curved line. While Fig 2.1 (c and d) show the 'to and fro' motions of an object. Based on the path followed, a motion is classified into four types. The followings are types of motion of a body. 1. Rectilinear motion is the motion of a body along a straight line. Examples Motion of a car along a straight level road, A falling ball from a certain height. 2. Curvilinear Motion is the motion of a body along a curved path. Examples Motion of a car around a circular path, The motion of a ball thrown horizontally from a certain height, The motion of the moon around the earth. 26  Grade 7 Physics 2 Motion Note: Circular motion is a special case of curvilinear motion, in which the body moves along a circular path. 3. Rotary motion is the motion of a body about an axis. Examples The motion of the second or minute hand of a wrist watch, The motion of a wheel of a car. 4. Vibratory motion is a 'to and fro' or back and forth or up and down motion of a body. This motion does not have constant velocity. Examples The motion of a pendulum, The motion of objects suspended on a spring, Water wave, etc. Note: Both rotary and vibrational motions are periodic motions. Periodic motions can have constant or non-constant velocities and they repeat themselves. Activity 2.3 Write down some examples of motion for each type from your daily experiences. Discuss them with your classmates, how they are different. Types of motion Practical Examples 1. Rectilinear. 2. Curvilinear. 3. Circular motion. 4. Vibrational motion. 5. Rotary motion.   Check point 2.1  1. State at least four types of motion, and give practical examples  for each type. 2. Define what a motion is.  27 Grade 7 Physics 2 Motion 2.2 Motion Along a Straight Line Motion along straight line path is known as a rectilinear motion. Activity 2.4 Discuss about the common features of motion along a straight line. In a rectilinear motion, a body moves over a certain distance along a straight line and takes a certain time. Athletes run different distances. They run 5,000 m, 10,000 m and so on. To cover these distances they take different times. Which distance is covered in the shortest possible Fig.2.2 In competition different athletes cover the same time? What do you call the distance covered distance in different time in a unit of time? Activity 2.5 What do you understand by the terms ' Scalars' and ' Vectors' in relation to motion?(Revision) Distance: As a car moves along a straight road we can easily observe the change of its position. What is the distance traveled by the car between the initial and the final position? Distance is a physical quantity which describes the length between two points (places). It is the total path length traveled by a body. It depends on the path followed. To describe a distance it is not important to mention its direction. A distance is a scalar quantity. Observe Fig.2.3, Two persons moved from point A to point B, in different paths: path 1 and 2. What would you say about the distance covered by the two persons? 28  Grade 7 Physics 2 Motion Path 2 A Path 1 B Fig.2.3. Variation of distance with the path followed Do you remember the units of length from the previous unit? What is the SI unit of length? Do you think the units of length and distance are the same? The symbol for distance is ''s''. The SI unit of distance is meter (m). Mostly, the distance covered by a moving car or airplane or train is measured by kilometer (km). Fig.2.4 Football field Challenging Questions What is the distance around a standard football field? Displacement ( ) Azeb walked 300 m from A to B and returned back and walked 200 m and then stopped at C. What is her change in position from A to C? 300m 200m A C B Fig.2.5 Finding change of position 29 Grade 7 Physics 2 Motion When an object moves, it changes its position. This change of position in a certain direction is known as a displacement. A displacement is described by its magnitude and direction. It is a vector quantity. As shown in Fig. 2.6. a body may move from A to B in different paths such as path 1, path 2 and path 3. The distance of the three paths is different. However, the displacement made is the same. Path 1 Path 2 B A Path 3 Fig.2.6 Displacement is independent of the path Activity 2.6 Discuss with your friends Which path is the shortest? Which one of the paths has a fixed direction throughout its motion?(Fig 2.6) As you know all the lengths of the paths are 'distances'. Path 2 is a straight line and it is the shortest distance between the initial and final positions of the body. Hence, it is the displacement of the body. This straight path having a fixed direction is said to be a displacement. Hence a displacement is the shortest distance in a specified direction. The SI unit of displacement is the same as the SI unit of distance that is meter (m). The symbol of displacement is , with an arrow on the head of s. Displacement is independent of the path followed. In Fig 2.7, you observe that displacement is the difference between the final position x f and the initial position x i. a) A displacement to the right of the origin, 'O' will be a positive displacement. That is, s > 0 since x i < x f. For example, starting with x i = 60 m and ending at x f = 150m, the 30  Grade 7 Physics 2 Motion displacement is S = x f - x i = 150 m - 60 m = 90 m, to the right. b) A displacement to the left of the origin, 0 will be a negative displacement. That is, s < 0 since x i > x f. For example, starting with x i = 150 m and ending at x f = 60m, the displacement is s = x f - x i = 60 m - 150 m = - 90 m (to the left direction) c) Positions to the right of the origin are positive. Positions to the left of the origin are negative. a) s b) c) Fig.2.7 Displacement of a car at different time along x -axis 31 Grade 7 Physics 2 Motion Comparison of distance and displacement Activity 2.7 List down the similarities and differencec between a distance and displacement for rectilinear motion. Similarities Differences Distance and Displacement Speed (v) Activity 2.8 Tirunsh Dibaba ran Bejing Olympic and covered 10,000 m distance in 28 minutes. Sileshi ran the same distance and it took him 24 minutes. What were thier average speeds? Who is the fastest? Both athlets covered the same distance in different time. From the given informaton you can compute the distance they covered in one second. i.e. Tirunesh covered an average distance of 5.95 m in one second. While Sileshi covered 6.94 m in one second. Thus, the distance covered per unit time is called speed. Speed is a quantity that describes how fast a body moves. Its symbol is ‘’v’’ The SI unit of speed is meter per second (m/s). In reality, a moving body does not have a uniform speed throughout its motion. Sometimes the body will speed up, sometimes it will go at a constant speed and at other times it may slowdown. For this reason the speed you calculate is an average speed. 32  Grade 7 Physics 2 Motion An average speed is the total distance traveled divided by the total time taken. total distance traveled Average speed = ⇒ vav = total time taken The SI unit of average speed is m/s. Rearranging the formula gives s s = νt and t = ν Using the triangular symbol you can perform the above s rearrangement of the formula v×t -1 The unit of speed m/s can also be written as m s. Using the exponential expression, we write: m 1 = m ⋅ +1 = m ⋅ s −1 s s Activity 2.9 i. State some units of speed other than m/s. ii. Express 1 km/hr in m/s. iii. What is the conversion factor between m/s and km/hr? Example 2.1 1. Tirunesh Dibaba covers a distance of 5000m in 14.5 minutes. Calculate the average speed of Tirunesh in m/s. Given Required Solution s = 5000 m v av = ? S tot 5000m v av = = = 5.75m/s t tot 870s t = 14.5 min = 870s 33 Grade 7 Physics 2 Motion 2. A bus is moving in a straight line at a speed of 25m/s. What time does the bus take to cover 5km? Given Required Solution v = 25m/s t=? v = s/t ⇒ t = s/v s = 5km = 5000m 5000m = 25m / s = 200sec 3. Convert 20 m/s to km/hr. Solution 1 m/s = 3.6 km/hr 20 m/s × 3.6 km/hr 20 m/s = x ⇒x= = 72 km/hr 1 m/s 4. Convert 60 km/hr to m/s. Solution 1 km/h = m/s 3.6 60 km/hr = x 1 60 km/hr × m/s x= 3.6 = 16.67 m/s 1 km/hr Exercises Suppose four students. Almaz, Abebe, Sofia and Gemechu are running a 100m race. Alamz takes 12s, Gemuchu takes 13s, Sofia takes 14s and Abebe takes 15s to finish the race. Calculate their speeds and record them on the chart given below. From the chart find: a) Who is the fastest runner? b) Who is the slowest runner? c) What can you conclude about the relationship between speed and time? 34  Grade 7 Physics 2 Motion Distance (m) Time (s) Speed (ms-1) Almaz Abebe Sofia Gemechu Do you notice from the above chart that the speed increases as the time decreases to cover the same distance? Velocity Velocity is a physical quantity that describes how fast a body moves as well as the direction in which it moves. Hence, velocity is a vector quantity. Its symbol  is v (v with an arrow on the head)  Velocity is the rate of change of displacement s i.e. it is the displacement covered by the body per unit time. displacement Velocity = time taken   s v= t  m The SI unit of v is s  Average velocity (v av ) is the total displacement divided by the total time taken. Total displacement Average velocity =  Total time taken sT v av = tT Example 2.2 1. A car moves at a speed of 20m/s for 120 seconds due East. What is the displacement of the car? 35  Grade 7 Physics 2 Motion Given Required Solution V = 20m/s due East S=? s v= ⇒ s = vt t = 120s t S = 20m/s, East × 120 s = 2400 m due East = 2.4 km, due East 2. A bus is moving due north for 2 hr and covered a distance of 72 km. What is the velocity of the bus? Given Required Solution s = 72 km, due North v =? s 72km v= = due North t = 2 hrs t 2hr = 36 km/hr due North Activity 2.10 List down the similarities and differences between speed and velocity for a rectilinear motion. Similarities Differences Speed and Velocity Check point 2.2 1. What do you call a speed that has direction?  2. What are the main features of a velocity in a uniform motion? 3. A car moves at a speed of 20m/s east ward. What is the car’s velocity in magnitude and direction? 4. A bus travels 43 km in the first hour, 40km in the second hour and 46 km in the third hour of its journey. Calculate its average speed. 5. The speed of an airplane is 360 km/hr, and another air plane has a speed of 120 m/s. which one of these two air planes has a greater speed? 36  Grade 7 Physics 2 Motion 2.3 Qualitative Exploration of Uniform Motion and Uniformly Accelerated Motion 2.3.1 Uniform Motion Activity 2.11 The motions of two bodies are measured and recorded in tables ‘A’ and 'B'. A) s(m) 6 12 18 24 30 36 42 48 t(s) 1 2 3 4 5 6 7 8 v(m/s) B) s(m) 4 9 15 22 30 39 49 60 t(s) 1 2 3 4 5 6 7 8 v(m/s) i. Calculate the speed of the two bodies and complete the tables. ii. What is the difference between the speeds in A and B? iii. What do you call the type of speed in A and in B From table A you observe that when a body makes equal changes of displacement within equal interval of time, its velocity is said to be a uniform velocity. i.e. its speed is constant and the direction is fixed. A motion with a s uniform velocity is called a uniform motion. For uniform motion v = and t s = v ×t Uniform motion is the motion of an object along a straight line with a constant velocity or speed in a given direction. Activity 2.12 Suppose an object is moving at a constant speed of 2m/s in straight line. At the end of the first second, it travelled 2m away from its starting point. At the end of 2 second the distance travelled is 4m. Complete the table by filling the distance travelled in 3, 4 and 5 seconds. t(s) 1 2 3 4 5 s(m) 2 4 --- ---- --- Note that for a uniform motion, as the time increases the displacement also increases. If you plot a graph of s against t using data from the above table you will get the graph shown in Fig 2.8 37  Grade 7 Physics 2 Motion s (meter) 8 6 4 2 time (second) 1 2 3 4 5 Fig.2.8 Graph of s against t for motion with constant velocity From the graph in Fig.2.8, you can find the slope of the graph. change in displacement The slope of the graph = ' change in time Δs s f −s i s = = Δt t f − t i t s But by definition: v av = t Hence, the slope of s against t graph of uniform motion equals average velocity. 2.3.2 Uniformly Accelerated Motion In section 2.3.1 you learnt about uniform motion. That is, where the speed is constant and the direction is fixed in a straight line. In this section you will study another kind of motion; in which the velocity changes uniformly. Activity 2.13 i. Explain what it means by the change of velocity. ii. Describe the factors which could be affected when the velocity changes. iii. What is acceleration? How is it different from velocity? Acceleration Whenever the velocity of an object changes in magnitude, or direction or both simultaneously, it is said to be accelerated. Acceleration is a measure of how much the velocity of an object changes in a unit of time (usually in one second). 38  Grade 7 Physics 2 Motion Acceleration is the time rate of change of velocity. Acceleration = change in velocity time taken    vf − vt a= t The symbol for acceleration is a. It is a vector quantity vi is the initial velocity vf is the final velocity t is the time taken The SI unit of acceleration is meter/second/second = m/s2 If a body starts from rest, then the initial velocity is zero ( v i = 0). If the velocity of a body decreases then the final velocity is less than the initial velocity. Such motion is called decelerating. Deceleration is called a negative acceleration. (that is v f < v i ). If the body comes to rest then, the final velocity is zero ( v f = 0 ). Uniformly accelerated motion is motion of an object along a straight line with a constant increase in its velocity. Examples 2.3 1. The speed of a car increases uniformly from 8m/s to 48 m/s in 10s. Calculate the acceleration of the car. Given Required Solution a =? v f −v i a= vi = 8m/s t vf = 48m/s = (48 − 8) m/s 10 s t = 10s 40 m/s = 10 s a = 4 m/s2 39 Grade 7 Physics 2 Motion 2. A car started from rest and accelerated uniformly and reached a speed of 20m/s after 5 s. What is the acceleration of the car? Given Required Solution vi = 0m/s a =? v f −v i a= vf = 20m/s t t = 5 sec (20 − 0) m/s = 5s 20 m/s = 5s a = 4 m/s2 3. A bus initially at rest accelerated uniformly with an acceleration of 4 m/s2. What is the speed of the bus at the end of 5? Given Required Solution v f −v i a = vi =0 vf =? t 2 a = 4 m/s vf – vi = a × t t = 5 sec vf = a × t + vi vf = 4m/s2 × 5s + 0 = 20m/s Falling bodies Activity 2.13 Discuss what happens to the motion of a stone. i. When you throw a stone vertically upward in air. ii. When you drop a stone from some height above the ground. Gravity is the pulling force of the earth on a body. The first person who studied about motion of a falling body was Galileo Galilee. He showed that all bodies dropped from the same height fall to the earth with the same acceleration, which is known as the gravitational acceleration ( g ). All objects falling freely in air 40  Grade 7 Physics 2 Motion accelerates uniformly by 9.8 m/s every second. Motion of a freely falling body is the natural example of a uniformly accelerated rectilinear motion. Free fall is the motion of a body under the action of the force of gravity. 2.3.3 Representation of Uniform Motion and Accelerated Motion Qualitatively Using Table Walk one pace every 2 seconds. This is represented s against t using dot plot. Table Dot plot for a constant velocity (Fig 2.8) t s. 2s 2 dot 2s 2s 2s 2s 4s 4 dot 1 dots 6s 6 dot Velocity = = ½ dot/s 2s distance distance Motion of (dot) m object’ ‘B’ 6 60 50 4 40 Motion of 30 object ‘A’ 20 2 10 time Time 0 2 4 6 (second) 0 1 2 3 (S) Fig. 2.9 Average velocity = slope of Fig. 2.10 Graphs of two bodies A and B the s against t graph travelling at different speeds. From Fig 2.10 we calculate that: 10 20 30 v av (A) = dot/s = dot/s = dot/s = 10 dots/s 1 2s 3 20 40 60 v av (B) = dot/s = dot/s = dot/s = 20 dots/s 1 2 3 The slope with 20 dots/s has higher velocity than slope with 10 dot/s. 41 Grade 7 Physics 2 Motion This means a steep slope has higher velocity than a gradual slope. Note that when you draw graphs of motions, you must 1. Label the axes 2. Put units clearly on both axes 3. Label each slope 4. Put their names on the graph Calculate the slope of the graph from initial and final as well as for several intervals. (see Fig 2.9 and 2.10) distance (Dot) distance 5 (dot) 4 3 2 1 time (s) t (second ) 1 2 3 4 5 6 7 8 b) Back ward motion a) Forward motion Fig 2.11 Graph of dot against t for constant velocity Dot plot for a constant velocity (Fig 2.11 (a)) Jemila walked for 3 seconds. She then stopped for 3 seconds and started to move. ( see Fig 2.11a) 0 1 2 3 4 5 6 7 8 9 3 second rest Average velocity = slope of the s against t graph 3−0  dots  v av =   3−0  s  3  dot  =   3  s  = 1dot/s Graphical representation of uniformly accelerated motion Dot plot for a uniformly accelerated motion 0s 2s 4s 6s 8s 42  Grade 7 Physics 2 Motion The above dot spaces represent distances for accelerated motion every 2 seconds. Let us see the following examples that describe a uniformly accelerated motion. The tables are based on the motions of a bus and a car accelerating along a straight line. Table 1. Motion of a bus (m/s) 0 10 20 30 40 50 t (s) 0 5 10 15 20 25 (m/s2) Table 2. Motion of a car (m/s) 0 20 40 60 80 100 t(s) 0 5 10 15 20 25 (m/s2) Challenging questions 1. What are the accelerations of the bus and the car? 2. What difference do you observe from the above tables? Motions of a bus and a car are given in Tables 1 and 2. Draw the v against t graphs for both objects. Slope of the v against t graph is the acceleration of the moving object. Accelerations = slope = Change in velocity Change in time Acceleration of the Bus = 20 - 0m/s = 2m/s/s = 2m/s2 10 - 0 (s) Acceleration of the Car = 40 - 0m/s = 4m/s/s = 4m/s2 10 - 0(s) v (m/s) Motion of car   The slopes of s against t and v 50 Motion of bus against t graph show velocity and 40 acceleration respectively. But the area 30  20 under the curves of graphs of v  10 against t and a against t gives the total distance covered and change in 5 10 15 20 25 t (s) velocity respectively. Fig 2.12 v against t graph for the motion of a bus and car. 43 Grade 7 Physics 2 Motion Challenging Questions 1. Explain the difference between velocity and acceleration. 2. Describe the difference between uniform motion and uniformly accelerated motion. 3. A body accelerates uniformly from rest at 2m/s2 for 5 seconds. Calculate its average velocity in this time. Check point 2.3 distance 1. What does the slope of (m) s against t graph stand 60 for? 40 2. What happens to a velocity in a uniformly accelerated motion? 20 time 0 5 10 15 (s) Fig 2.13 s against t graph 3. From the graph of Fig 2.13, answer the following questions. a) What is the distance travel by the body in 20 second? b) What is the time taken by it to cover a distance of 30 m? c) What is the speed of the body? 44 Grade 7 Physics 2 Motion SUMMARY In this unit you learnt that:  motion is a continuous change of position relative to a reference point. There are four types of motion. They are rectilinear, curvilinear, rotary and vibrational motion.  distance is the length of a path between two points  speed is the distance travelled divided by time taken. It describes how fast an object is moving in a unit time. When an object moves with constant speed in a straight line, the motion is known as Uniform rectilinear motion.  displacement is the shortest distance in specified direction. It has both magnitude and direction. Hence it is a vector quantity.  velocity is the time rate of change of displacement. It has both magnitude and direction.  acceleration is the time rate of change in velocity  the velocity of a body may increase or decrease with time. A body whose velocity is increasing is said to have 'acceleration' and a body whose velocity is decreasing is said to have deceleration or negative acceleration. Acceleration may happen due to: - either change in speed, or change in direction or change in both speed and direction simultaneously.  uniformly accelerated motion of an object is a motion with constant acceleration in a given duration.  freely falling body is a practical example of a uniformly accelerated motion on the earth. 45 Grade 7 Physics 2 Motion Review Questions and Problems Solve the following 1. A bicyclist travels with an average velocity of 15 km/h North, for 20 minutes. What is his displacement in km? 2. A car accelerates from rest to 90km/h in 10 seconds. What is its acceleration in m/s2? 3. An aircraft landing on an aircraft carrier is brought to a complete stop from an initial velocity of 215 km/h in 2.7 seconds. What is its acceleration in m/s2? 4. A certain car has an acceleration of 2.4m/s2. Assume that its acceleration remains constant. Starting from rest, how long does the car require to reach a velocity of 90 km/h? How far does it travel while reaching that velocity? 5. From the graph of Fig 2.14  a) Calculate the velocity of the motion s (m) b) What is the slope of the 30 graph equals to? 25 c) What is the distance traveled 20 when t = 6 seconds? 15 10 5 6. From the graph of Fig 2.15 a) Calculate the acceleration of the 0 1 2 3 4 5 t (s) motion Fig. 2.14 Graph of s against t b) What is the slope of the graph equals to? c) What is the velocity when the v time taken is 8 seconds? m/s 40 30 20 10 0 1 2 3 4 5 t (s) Fig. 2.15 Graph of v against t 46  Grade 7 Physics 2 Motion 7. Data for a freely falling body is recorded as in table below : Using the given data: t(s) v(m/s) a) draw the graph of velocity versus time. 0 0 b) calculate the acceleration due to gravity at the 1 9. 8 place where the data are taken. c) Is the acceleration changing or constant? 2 19.6 3 29.4 4 39.2 8. What is the relationship between velocity and acceleration? 9. How does the velocity of a freely falling body change with time? How does the distance it has fallen change? How about the acceleration? 10. a) A car travels at a speed of 25 m/s. How far does it travel in 5 s? b) Draw a graph showing the distance versus time for the above car. c) What is the slope of the graph? 11. A train initially at rest, has a constant acceleration of 0.5 m/s2 a) What is its speed after 15s? b) What would be the total time it would take to reach a speed of 25 m/s? c) Draw the graph of speed against time for the train. 47  UNIT FORCE AND NEWTON'S 3 LAWS OF MOTION Unit outcomes: After completing this unit you should be able to:  develop a qualitative understanding of Newton’s laws of motion and force in static situations.  develop introductory skill of manipulating numerical problems related to Newton’s law of motion and force.  appreciate the interrelatedness of all things.  use a wide range of possibilities for developing knowledge of the major concepts with in physics. Introduction In unit two of this book you learned important concepts that describe motion. The concepts such as speed, velocity and acceleration are used for describing various types of motion around you. Activity 3.1 Discuss with your friends or parents. The concepts: speed, velocity and acceleration. The difference between uniform and accelerated motions. Do you think that force and motion have a relation? Explain. 48 Grade 7 Physics 3 Force and Newton’s Law of Motion The motion of bodies (relative to a chosen reference system) is either uniform or accelerated or slowed down, or change in directions. In the last three cases, the velocities of moving bodies are changing. That is, acceleration is produced. Clearly it is very important to be able to study acceleration. However, to understand acceleration in its full sense you must know how it emerges or is produced. 3.1. Force Activity 3.2 Discuss the followings with your friends. i. What is a force? ii. Mention some examples of forces from your daily activities. iii. explain the following phrases - Social force, - Political force, iv. Explain the following actions. - A push you exert on a wall, - A pull exerted to drag a box on a table. v. Do you think that forces in iii) and iv) are the same? Explain The term force is used in different situations in the English language. Force is a technical term in physics. The term force in physics is different from the term force in political and social. Force is a very important physical quantity. It is used to describe interactions between two bodies in nature. For example, when you kick a ball, tear a piece of paper, hold your exercise book and walk on the floor you apply forces. Hence, in physics the term 'force' is used to describe a 'push' or a 'pull' exerted on a body. A force is a push or a pull exerted on a body by another body. 49  Grade 7 Physics 3 Force and Newton’s Law of Motion Types of forces Activity 3.3 i. Throw a ball vertically upward and observe its motion. What will happen to the ball? Will it continue to move upward forever? Why? ii. Take a magnet and pieces of iron fillings. Move the magnet over the iron filings without a physical contact between the magnet and the iron fillings. Describe your observation for your teacher. iii. Move your desk from its current position. Can you do it without a physical contact? Why? iv. Explain the types of forces that exist in the above 3 activities. You know that a force is a push or a pull. But do all bodies push or pull other bodies by making a physical contact only? From your Activity 3.3, you might have noticed that bodies could be in contact to each other or they could be without contact or at a distance from each other. Therefore, forces are classified into two broad categories known as: (i) Contact forces and (ii) Non-contact forces i. Contact forces are forces exerted when two objects are in touch or contact. For example; - A force exerted by a stretched or compressed spring. - An upward force exerted by a table on a box resting on it. a) Pushing a table b) Stretching a spring c) A box resting on a floor Fig.3.1. Examples of contact forces ii. Non-contact forces are forces exerted without body contact. They are forces acting at a distance. Gravitational force, magnetic force and electrical forces are examples of non – contact forces. 50 Grade 7 Physics 3 Force and Newton’s Law of Motion A falling ball Attraction or repulsion of two magnets Electrostatic forces Fig. 3.2 Different types of non contact forces Challenging Questions 1. Discuss examples of contact forces in Fig 3.1 2. Discuss the differences between gravitational force, magnetic force and electric forces. (Fig 3.2) Effects of a force When a force is exerted on a body, the body may change its shape or size. Activity 3.4 Observe the activities shown in Fig 3.3 a. Explain the effects of forces in each activity. b. Can you summarize the effects of force? b) When the engine applies a force the car moves. a) When the person kicks the ball the ball moves in the direction of the force c) When a spring is stretched the size and the shape change. d) When a ball roles over the table and falls off the table forces are exerted. Fig.3.3 Different effects of forces 51 Grade 7 Physics 3 Force and Newton’s Law of Motion It is not simple to describe a force as you can The change in shape describe some material objects such as a chalk, or size of a body is known as pen, orange etc. You can only say what a force deformation. There can do. For example when a body at rest is are two types of deformation; acted upon by a force it will begin to move. If a I. Permanent body is already moving a force may change its II. Temporary. velocity. That is, a force produces motion or changes motion of a body. Force produces an acceleration of a body. A force has the following main effects, when it is exerted on an object: i. It changes the shape and size of the objects. ii. It changes the magnitude or direction of motion of the objects. i.e. when a force is exerted on a body: a. a stationary body starts to move b. a moving body increases its speed, c. a moving body decreases its speed and gradually stops moving d. a moving body changes it direction. Measuring a Force Activity 3.5 Do the following tasks with your friends. i. Describe methods of measuring a force. ii. What is the instrument used to measure a force? iii. Mention the SI unit of force. A force is measured using an instrument called a spring balance (Fig 3.4 a). As you can observe from Fig 3.4 there is a stretch (increase in length) of the spring when it is pulled. We can use this increase in length of a spring to measure the magnitude of the force stretching the spring. 52 Grade 7 Physics 3 Force and Newton’s Law of Motion W=0 1 2 F1 W1 W2 a) b) Fig 3.4 Spring balance Each time an extra weight is added you find that there is the same extension because each object is identical. They are attracted to the earth with the same force, so what we have found is that equal force produced equal extensions of the spring. Newton meter is the scientific instrument used to measure a force. The SI unit of force is newton symbolized by N. The unit newton is named, after the great scientist Sir Isaac Newton. Fig 3.5 (a) illustrate the structure of a Newton meter. It is made up of a spring attached to a hook and a scale leveled in newton. Fig 3.5 (b) measures the weight of stone in gram. When the spring balance is held by the hand it shows a certain weight for the piece of stone. Here the weight is 500 grams. To know the weight of the stone in newton you have to multiply by 10 m/s2. Describing a Force: A force is a vector quantity. (A vector quantity is a quantity which has both magnitude and direction.) To fully describe the force acting upon an object, you must describe both its magnitude and direction. For example, “10 N of force” is not a complete description of the force acting on an object. ‘10 N downwards’ is a complete description of the force acting upon an object. 53 Grade 7 Physics 3 Force and Newton’s Law of Motion Support Spring balance held by hand Pointer 500 g Scale (Newton) Spring Stone weighs 500 g Hook b) Spring balance a) Newton meter Fig 3.5 Force measuring instruments Check points 3.1 1. What is a force in physics? 2. Name two types of force and give example for each type. 3. Describe some effects of a force. 4. Write the unit of force both in word and symbol. 5. Read the weight of a body from the spring balance. 3.2. Newton's Laws of Motion In unit two of this book you learned about the motion of bodies in a straight line. In the first section of this unit, you learned the major effects of a force. Combining these two backgrounds, it is now necessary to study the relationship between force and motion. Galileo Galilee (1564-1642 AD) and Sir Isaac Newton (1642-1727 AD) tried to explain the causes of motion of bodies in a certain direction or why bodies stop their motion. These ideas were put together by Sir Isaac Newton in the form of laws of motion called Newton's laws of motion. Newton's First Law of Motion Activity 3.6 Discuss the following questions with your friends. i. Place any object (text book, or pen, or eraser) on a floor. ii. What happens to the state of motion of the object, when you don't exert a force on it? iii. Exert a force (push or pull) on the objects. What happens to their states of motion? 54 Grade 7 Physics 3 Force and Newton’s Law of Motion Based on the discussion of activity 3.6 you can generalize that unless you or someone else exerts a force on the object, an object at rest will remain at rest. But when a force is applied it starts to move. Similarly, a body moving with a constant velocity along a straight line will not increase or decrease its speed unless an external force is applied on it. These conditions led Newton to state the important law called Newton’s first law of motion. Newton’s first law of motion states that: "an object continues in its state of rest or of uniform motion in a straight line unless it is forced to change that state by the application of an external force." This means, in the absence of an external force, a body at rest will remain at rest and a body in motion will continue its motion in a straight line with uniform velocity. This law is also called the Law of Inertia. This law points out that force is something that changes the state of a body. In other words we can say that if the state of a body changes, a force is acting on it. A force may be defined as a push or a pull which produces or tends to produce motion, stop or tend to stop motion. Activity 3-7 i. Discuss with your friends and report to your teacher. ( Fig 3.6) a. pull both cans with the same force. Which can is easy to move? Why? b. If both cans are moving towards you, which is easier to stop its motion? c. What do you call the property of a body to resist change in its motion? ii. What is the use of seatbelt/safety belt in a car? Ask a driver or a traffic a) Empty can b) Can full of Sand police and discuss your findings with Fig 3.6 Bodies having different masses your group members. iii. When you are standing in a moving bus, you fall or tend to fall forward when it suddenly stops. How can you explain this effect? What are the forces acting on you? iv. Explain the term 'inertia' using practical examples. 55 Grade 7 Physics 3 Force and Newton’s Law of Motion From your Activity 3.7 you noticed that an object at rest would insist to be at rest. A moving object would like to continue its uniform motion in a straight line. This is the property of all objects and it is known as inertia. Inertia is the property of a body to retain its state of rest or state of uniform motion in a straight line in the absence of an external force. Activity 3.8 i. Place your pen on the surface of a floor. Push the pen and observe its motion. Similarly apply the same amount of force on a table; standing on a floor. What effects do you notice in both activities? ii. Is the speed of the table the same as that of the pen? iii. Do you think that for the same applied force the change in velocity is the same? Explain your answer. From the above two activities you noticed that mass and inertia are the same. To move a large mass, a large force is required for motion to begin; and if the mass is small, a small force is required. We use the term mass instead of inertia in this book. Generally large masses have greater inertia and smaller masses have less inertia. Activity 3-9 Do the following activity to understand the effects of inertia. (Fig 3.7) i. Pile of four or five smooth wooden blocks on the top of a table. ii. Give a sharp kick with a hammer on the bottom block. iii. What did you observe? iv. Why do the blocks of wood drop vertically down when the bottom block is kicked with the hammer? Fig.3. 7 A pile of wooden blocks 56 Grade 7 Physics 3 Force and Newton’s Law of Motion When the bottom block is given a sharp kick it causes out of the stack while the top three blocks drop vertically down as shown in Fig. 3.7. Do the rest in the same manner as the first. You will observe that when the kicked block moves away the remaining will be dropped vertically down. Activity 3.10 i. Apply a force to move a heavy box placed on a smooth floor, it will resist to stay at rest or don't move. This means you didn't bring a change in motion even though you applied a force. ii. Why do you think the body does not move when you apply a force? The above activities show that mass and inertia are the same. To move a large mass, a large force is required, and to move a small mass, a small force is required. In this book we use the term mass instead of inertia. Newton's second law of motion Newton's first law describes the qualitative property of a force. It describes how force changes the state of rest or uniform motion of a mass of body. In other words, it states that every change in the magnitude or direction of a body's velocity is caused by applying an external force. In Newton's second law you will learn how to measure the magnitude of a force required to bring a given body to rest or set in motion. Activity 3-11 Consider two boxes 'A' and 'B' as in Fig 3.8. Let the mass of 'A’ is 20 kg and that of 'B’ is 40 kg. Both are at rest. Suppose you push separately the two boxes with the same force of 10 N. which box change its motion easily? Explain it. F=10 N F=10 N A B Fig.3.8 Different masses acted by the same force Activity 3.11 helps you to know that, when the same force is applied on two bodies of different masses, the smaller mass accelerates more than the larger mass. 57 Grade 7 Physics 3 Force and Newton’s Law of Motion Mathematically you can state as follows. Acceleration is inversely proportional to the mass of a body for a given applied force. i.e. a α where 'm' is the mass of the body and 'a' is the acceleration. α is proportionality symbol. Activity 3-12 a. i. Consider two bodies of equal masses and different forces are applied to make them move. (Fig 3.9) Which one of the masses do you think will accelerate more? Wh