Focus On Physical Science


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1 Reading Essentials An Interactive Student Textbook Focus On Physical Science ca8.msscience.com
2 Glencoe Science To the Student In today s world, knowing science is important for thinking critically, solving problems, and making decisions. But understanding science sometimes can be a challenge. Reading Essentials takes the stress out of reading, learning, and understanding science. This book covers important concepts in science, offers ideas for how to learn the information, and helps you review what you have learned. In each chapter: Before You Read sparks your interest in what you ll learn and relates it to your world. Read to Learn describes important science concepts with words and graphics. Next to the text you can find a variety of study tips and ideas for organizing and learning information: The Study Coach offers tips for getting the main ideas out of the text. Foldables Study Organizers help you divide the information into smaller, easiertoremember concepts. Reading Checks ask questions about key concepts. The questions are placed so you know whether you understand the material. Think It Over elements help you consider the material indepth, giving you an opportunity to use your criticalthinking skills. Picture This questions specifically relate to the art and graphics used with the text. You ll find questions to get you actively involved in illustrating the concepts you read about. Applying Math reinforces the connection between math and science. Academic Vocabulary defines some important words that will help you build a strong vocabulary. The main California Science Content Standard for a lesson appears at the beginning of each lesson. This statement explains the essentials skills and knowledge that you will be building as you read the lesson. A complete listing of the Grade Eight Science Content Standards appears on pages iv to vi. See for yourself, Reading Essentials makes science enjoyable and easy to understand. Copyright by the McGrawHill Companies, Inc. All rights reserved. Except as permitted under the United States Copyright Act, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. Send all inquiries to: Glencoe/McGrawHill 8787 Orion Place Columbus, OH ISBN13: ISBN10: Printed in the United States of America
3 Table of Contents To the Student ii California Science Standards iv Chapter 1 Motion Chapter 2 Forces Chapter 3 Density and Buoyancy Chapter 4 Understanding the Atom Chapter 5 Combining Atoms and Molecules Chapter 6 States of Matter Chapter 7 The Periodic Table and Physical Properties Chapter 8 Chemical Reactions Chapter 9 Acids and Bases in Solution Chapter 10 Chemistry of Living Systems Chapter 11 Our Solar System Chapter 12 Stars and Galaxies iii
4 Grade 8 Science Content Standards 1. The velocity of an object is the rate of change of its position. As a basis for understanding this concept: a. Students know position is defined in relation to some choice of a standard reference point and a set of reference directions. b. Students know that average speed is the total distance traveled divided by the total time elapsed and that the speed of an object along the path traveled can vary. c. Students know how to solve problems involving distance, time, and average speed. d. Students know the velocity of an object must be described by specifying both the direction and the speed of the object. e. Students know changes in velocity may be due to changes in speed, direction, or both. f. Students know how to interpret graphs of position versus time and graphs of speed versus time for motion in a single direction. 2. Unbalanced forces cause changes in velocity. As a basis for understanding this concept: a. Students know a force has both direction and magnitude. b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces. c. Students know when the forces on an object are balanced, the motion of the object does not change. d. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction. e. Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction). f. Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion. g. Students know the role of gravity in forming and maintaining the shapes of planets, stars, and the solar system. 3. Each of the more than 100 elements of matter has distinct properties and a distinct atomic structure. All forms of matter are composed of one or more of the elements. As a basis for understanding this concept: a. Students know the structure of the atom and know it is composed of protons, neutrons, and electrons. b. Students know that compounds are formed by combining two or more different elements and that compounds have properties that are different from their constituent elements. c. Students know atoms and molecules form solids by building up repeating patterns, such as the crystal structure of NaCl or longchain polymers. d. Students know the states of matter (solid, liquid, gas) depend on molecular motion. iv
5 e. Students know that in solids the atoms are closely locked in position and can only vibrate; in liquids the atoms and molecules are more loosely connected and can collide with and move past one another; and in gases the atoms and molecules are free to move independently, colliding frequently. f. Students know how to use the periodic table to identify elements in simple compounds. 4. The structure and composition of the universe can be learned from studying stars and galaxies and their evolution. As a basis for understanding this concept: a. Students know galaxies are clusters of billions of stars and may have different shapes. b. Students know that the Sun is one of many stars in the Milky Way galaxy and that stars may differ in size, temperature, and color. c. Students know how to use astronomical units and light years as measures of distances between the Sun, stars, and Earth. d. Students know that stars are the source of light for all bright objects in outer space and that the Moon and planets shine by reflected sunlight, not by their own light. e. Students know the appearance, general composition, relative position and size, and motion of objects in the solar system, including planets, planetary satellites, comets, and asteroids. 5. Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept: a. Students know reactant atoms and molecules interact to form products with different chemical properties. b. Students know the idea of atoms explains the conservation of matter: In chemical reactions the number of atoms stays the same no matter how they are arranged, so their total mass stays the same. c. Students know chemical reactions usually liberate heat or absorb heat. d. Students know physical processes include freezing and boiling, in which a material changes form with no chemical reaction. e. Students know how to determine whether a solution is acidic, basic, or neutral. 6. Principles of chemistry underlie the functioning of biological systems. As a basis for understanding this concept: a. Students know that carbon, because of its ability to combine in many ways with itself and other elements, has a central role in the chemistry of living organisms. b. Students know that living organisms are made of molecules consisting largely of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. c. Students know that living organisms have many different kinds of molecules, including small ones, such as water and salt, and very large ones, such as carbohydrates, fats, proteins, and DNA. v
6 7. The organization of the periodic table is based on the properties of the elements and reflects the structure of atoms. As a basis for understanding this concept: a. Students know how to identify regions corresponding to metals, nonmetals, and inert gases. b. Students know each element has a specific number of protons in the nucleus (the atomic number) and each isotope of the element has a different but specific number of neutrons in the nucleus. c. Students know substances can be classified by their properties, including their melting temperature, density, hardness, and thermal and electrical conductivity. 8. All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept: a. Students know density is mass per unit volume. b. Students know how to calculate the density of substances (regular and irregular solids and liquids) from measurements of mass and volume. c. Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced. d. Students know how to predict whether an object will float or sink. 9. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will: a. Plan and conduct a scientific investigation to test a hypothesis. b. Evaluate the accuracy and reproducibility of data. c. Distinguish between variable and controlled parameters in a test. d. Recognize the slope of the linear graph as the constant in the relationship y kx and apply this principle in interpreting graphs constructed from data. e. Construct appropriate graphs from data and develop quantitative statements about the relationships between variables. f. Apply simple mathematic relationships to determine a missing quantity in a mathematic expression, given the two remaining terms (including speed distance/time, density mass/ volume, force pressure area, volume area height). g. Distinguish between linear and nonlinear relationships on a graph of data. vi
7 1 Motion lesson 1 Determining Position Grade Eight Science Content Standard. 1.a. Students know position is defined in relation to some choice of a standard reference point and a set of reference directions. Before You Read When you move from place to place, how do you know you have moved? Write your answer on the lines below. Then read the lesson to learn more about determining position. The position of an object depends on the choice of a reference point. What You ll Learn how to describe an object s position in two dimensions why displacement is a vector Read to Learn Position and Reference Points A new student tells you that her house is three blocks east of the grocery store. Did she give you enough information to find her house? If you know where the grocery store is, then you can walk three blocks east from there. The store is the starting place for you to find the location, or position, of her house. A reference point is a starting point used to describe the position of an object. How can you describe an object s position? The new student told you where to start, which direction to walk, and how far to walk to reach her house. You had to start at the grocery store, which was the reference point. The direction you had to walk was east. Finally, you had to walk a distance of three blocks. To describe an object s position you must include three things in your description: a reference point, a direction from the reference point, and a distance from the reference point. You describe the position of an object using units of length, such as meters. For longer distances, kilometers might be used. For shorter distances, centimeters might be used. Underline As you read, underline material you do not understand. Reread the information until you understand it. If the text is still unclear, ask your teacher for help. A Record Information Make four note cards. Label the quarter sheets as illustrated and use them to record what you learn about the position of objects, and terms and definitions introduced in the lesson. Terms Speed Formulas Definitions Main Ideas Reading Essentials Chapter 1 Motion 1
8 Academic Vocabulary indicate (IN duh kayt) (verb) to point out or point to 1. Identify What sign is used to indicate a reference direction? How can you describe a reference direction? You can use a plus ( ) or minus ( ) sign to describe direction. The plus sign indicates the reference direction, and the minus sign indicates the opposite direction. For example, ( ) could mean toward the new student s house and ( ) could mean away from the student s house. So, the position of an object can be described as its distance from the reference point, together with a plus ( ) or minus ( ) sign. What is a vector? To describe the position of an object, you must specify two things. One is the distance from the reference point. The other is the direction from the reference point. The position of an object is an example of a vector. A vector (VEK tur) is a quantity that has both a size and a direction. For example, the size of a position vector is the distance of an object from the reference point. The direction of a position vector is the direction from the reference point to the object. A vector can be represented by an arrow. The length of the arrow represents the size of the vector. 2. Determine On a map, which best describes the term west? (Circle your choice.) a. part of the map scale b. a reference direction Position in Two Dimensions A runner moves in one direction only toward the finish line. To describe the runner s position, you could use the starting line as the reference point. The reference direction could be the direction from the starting line to the finish line. Because the runner moves in a straight line, you only need to use one reference direction. But a car traveling from San Diego to Sacramento doesn t move in a straight line. And it doesn t move only north. It moves west as well. To describe how it moves, you need to know how to show position with two directions. North and east are often chosen as the positive reference directions. How does a map show position with two directions? A map has two reference directions north/south and east/west. A map also has a scale to show the distances in meters. Suppose someone walks from the bus station four blocks west and one block south. If each city block is 90 m long, then the person would walk 360 m west and 90 m south. The bus station is the reference point, and 360 m west and 90 m south are distances and directions in two dimensions. 2 Chapter 1 Motion Reading Essentials
9 How can you locate a position in two dimensions? A twodimensional map is a graph used to show the location of an object with two reference directions. Twodimensional maps are similar to the graphs you ve used in math class. In a twodimensional map, east is the positive x direction. North is the positive y direction. To create a twodimensional map, you must choose a location that will be the origin of the graph. Suppose a visitor to your city uses a twodimensional map where City Hall is the origin of the map, as shown below. City Hall s position is x 0 m and y 0 m. The xaxis line goes east through City Hall. The yaxis line goes north through City Hall. Distance units are marked on the axes of the graph. The locations of buildings are points plotted on the graph. On the graph below, the bus station is 180 m east and 270 m north of City Hall. So the bus station s location is x 180 m and y 270 m. Changing Position Suppose you walk to a friend s home from your home, and then you walk back. How has your position changed? You might have walked a distance of many meters, but your final position is the same as your beginning position. So your distance traveled and your change in position are different. What is displacement? The change in your position is called the displacement. Displacement is the difference between the beginning position and final position of an object. 3. Describe Using a twodimensional map, how would you refer to a direction that is west? Picture This 4. Locate Circle the origin on the map. Draw a line from the origin to the reference point on the map. Reading Essentials Chapter 1 Motion 3
10 5. Determine If you know in which direction you moved on a trip, what do you need to know to determine your displacement on that trip? Picture This 6. Explain Why is the displacement in the third figure zero? How is displacement a vector? Displacement includes a size and a direction, just as the position does. As a result, displacement is also a vector. The direction of a displacement vector is the direction from the beginning position to the final position. The size of a displacement vector is the distance from the beginning position to the final position. What s the difference between distance and displacement? Distance depends on the length of the path traveled. Displacement depends only on the beginning position and the final position. For example, suppose you first walk a distance of 40 m to the east. The difference between your beginning position and final position is 40 m. This means your displacement is 40 m east. If you then walk 30 m north, the total distance you ve traveled from the starting point is 40 m 30 m, or 70 m. However, your final position is not 70 m from your beginning position. Instead the distance between your final and beginning position is 50 m. Your displacement is 50 m northeast. Suppose you continue walking and return to your beginning position. The total distance you travel is 140 m, but your displacement is 0 m. The figure below shows the difference between distance and displacement. What have you learned? The choice of a reference point and a reference direction determines an object s position. Displacement is a vector a quantity with both size and direction. 4 Chapter 1 Motion ca8.msscience.com
11 1 Motion lesson 2 Speed, Velocity, and Acceleration Grade Eight Science Content Standard. 1.d. Students know the velocity of an object must be described by specifying both the direction and the speed of the object. Also covers: 1.b, 1.c, 1.e. Before You Read Have you ever run in a race? What kinds of things are measured in a race? Write your answers on the lines below. Then read the lesson to learn more about speed and velocity. Speed, velocity, and acceleration describe how an object s position and motion change in time. What You ll Learn speed as a rate of change why velocity is a vector when acceleration occurs Read to Learn What is speed? When you describe motion, you often want to know how fast something is moving. The faster something is moving, the less time it takes to travel a certain distance. The slower something is moving, the more time it takes to travel a certain distance. Speed is the rate of change of distance with time. What is constant speed? An object that moves at a constant speed travels the same distance each second. For example, if a train travels 100 km in one hour, then it will travel another 100 km in the next hour. So in two hours it will travel 200 km. In five hours it will travel 500 kilometers. What is instantaneous speed? Many things do not travel at constant speeds. Instead, they speed up or slow down. For example, a car driving along a city street slows down and stops at a stop sign. Then it starts moving again. When the speed of an object isn t constant, it is helpful to know its instantaneous speed. The speed of an object at one instant in time is that object s instantaneous (ihn stuhn TAY nee us) speed. Outline Create an outline of this lesson as you read. Be sure to include main ideas, underlined terms, and other important information. B Record Information Make a Venndiagram Foldable and label the tabs as illustrated. Record what you learn about velocity and speed under the appropriate tabs. Explain how they are similar and different under the center tab. Reading Essentials Chapter 1 Motion 5 Velocity Both Speed
12 When is instantaneous speed constant? Now think about a car traveling on a highway at a constant speed of 80 km/h. What is the instantaneous speed of the car? When an object moves at a constant speed, its instantaneous speed is constant, too. So, the car s instantaneous speed is 80 km/h. 1. Determine What is a car s instantaneous speed when it is traveling at 65 km/h? What is average speed? The runners in a race line up at the starting line. When the starting gun is fired, the runners increase their speed until they cross the finish line. In a longer race, a runner might start quickly, slow down for a while to save energy, and then finish fast. During a race, a runner s instantaneous speed changes a lot. How can you describe speed when it is changing? You can find an object s average speed. The average speed is the total distance traveled divided by the total time. You can find average speed using this equation: average speed (in m/s) total distance (in m) total time (in s) v d t 2. Identify The average speed equation has what three variables? How can you find an unknown variable? The average speed equation has three variables: average speed, distance, and time. If you know any two of the variables, you can use the equation to figure out the third, unknown variable. Velocity The velocity (vuh LAH suh tee) of an object is the speed of the object and the direction of its motion. The velocity of an object describes how fast that object is going and in what direction. How is velocity a vector? Imagine an airplane flying at a speed of 300 km/h and moving east. The airplane s velocity is 300 km/h east. Recall that a quantity, such as velocity, that has both size and direction is called a vector. The size of a velocity vector is the speed. A velocity vector can be shown by an arrow that points in the direction of motion. The length of the arrow represents the speed. The length of the arrow increases as speed increases. 6 Chapter 1 Motion Reading Essentials
13 Acceleration When an object changes its motion, it is accelerating. Acceleration (ak sel uh RAY shun) is the rate at which velocity changes with time. Just like velocity, acceleration is a vector. To specify an object s acceleration, both a size and direction must be given. Upon what does the direction of acceleration depend? The velocity of an object changes when it speeds up or slows down. As a result, the object is accelerating. A runner taking off at the beginning of a race or a car slowing down at an intersection are both accelerating. The direction of the acceleration depends on whether an object is speeding up or slowing down. If an object is speeding up, the direction of its acceleration is in the same direction that the object is moving. If an object is slowing down, the acceleration is in the opposite direction that the object is moving. What happens to acceleration when the direction of motion of an object changes? The velocity of an object can change even if its speed doesn t change. For example, the horses on a carousel normally move with constant speed. However, as the carousel turns, the direction of motion of the horses is constantly changing. As a result, the velocity of each horse is changing and the horses are accelerating. What have you learned? Speed is the rate of change of position with time. You calculate average speed by dividing the distance traveled by the time taken to travel the distance. In Lesson 1 you read that a vector is a quantity with both size and direction. In this lesson, you learned about two vector quantities velocity and acceleration. Velocity is the speed and direction of an object s motion. Acceleration is the rate of change of velocity over time. Acceleration occurs when an object s speed or direction of motion changes. 3. Define What is acceleration? Academic Vocabulary motion (MOH shun) (noun) the process of changing place; movement 4. Explain How can the velocity of an object change if the object has a constant speed? ca8.msscience.com Chapter 1 Motion 7
14 1 Motion lesson 3 Graphing Motion Grade Eight Science Content Standard. 1.f. Students know how to interpret graphs of position versus time and graphs of speed versus time for motion in a single direction. Also covers: 9.d, 9.e. Graphs can show changes in an object s position and speed. What You ll Learn to construct a positiontime graph how motion with constant speed and changing speed appears on a positiontime graph Before You Read If someone asked you to show position and speed, how would you do it? Write your answer on the lines below. Then, read the lesson to learn about interpreting graphs. Identify the Main Idea When you read each paragraph, highlight the main idea. When you finish reading, make sure you understand each main point. Picture This 1. Identify What was the approximate position of the turtle at 50 s? Read to Learn PositionTime Graphs Graphs are useful tools for summarizing many kinds of information. One type of graph a positiontime graph is used to show how position changes with time. How do you graph positions from data? Imagine a turtle crawling across a sidewalk. You can measure the position of the turtle with a meterstick and its travel time with a watch. You can write down the position and time in a table, such as the one below. With the data in the table, you can graph the turtle s motion. The position of the turtle is plotted on the yaxis and the time is plotted on the xaxis. The data points are connected with a line. The line is a useful tool for estimating the position of the turtle for times you did not measure. Turtle s Position and Time Elapsed Time (in s) Position (in cm) Chapter 1 Motion Reading Essentials
15 What are the units on positiontime graphs? The values plotted on a positiontime graph have units. Each plotted point is the position at a certain instant of time. Position always has units of length, such as centimeters, meters, or kilometers. Time has units such as seconds, minutes, or years. Position (cm) Turtles Position and Time Elapsed Time (s) Picture This 2. Identify What type of data is shown on the yaxis? What is the purpose of a positiontime graph? A graph compares the motions and the speeds of objects. The graph above shows the positions of two turtles in a 200cm race. The turtles owners measured the positions of the turtles every 20 seconds. Then, they plotted the data on the same graph. The turtle that reached 200 cm first won the race. What does the slope of a line show? Recall that average speed equals the distance traveled divided by the time needed to travel the distance. The winning turtle travels 200 cm in 100 s. So its average speed is 200 cm/100 s, which equals 2 cm/s. The losing turtle travels 100 cm in 100 s, so its average speed is 1 cm/s. Notice in the graph above that the line for the winning turtle is steeper than the line for the losing turtle. The steepness of the line is called the line s slope. The steeper line means a greater average speed. How do you calculate slope? Two points must be used to calculate the slope of a line plotted on a positiontime graph. One point can be the origin of the graph. The other point can be any other point on the plotted line. First, determine the change in units in the vertical direction, the rise, from the origin to the chosen point. Next determine the change in units in the horizontal direction, the run. To calculate slope, divide the rise by the run. Academic Vocabulary data (DAY tuh) (noun) individual pieces of information C Record Information Make four note cards. Label the quarter sheets as illustrated. Use two note cards to record what you learn about positiontime graphs and speedtime graphs. Use the other note cards to draw an example of each type of graph. PositionTime Graphs SpeedTime Graphs PositionTime Graphs SpeedTime Graphs Reading Essentials Chapter 1 Motion 9
16 4. Calculate A horse runs a 2km race in 15 minutes. On the graph of the horse s race, which is the rise and which is the run? (Circle your answer.) a. rise 2 km; run 15 min b. rise 15 min; run 2 km How can you calculate average speed from a positiontime graph? On a positiontime graph, the slope equals the rise divided by the run. The rise is the same as the distance traveled. The run equals the time needed to travel that distance. Therefore, the slope of a line on a positiontime graph equals the average speed. If the rise of a slope is equal to 20 m and the run is equal to 5 s, the average speed is 4 m/s. How can you graph changing speed? Only objects that move at a constant speed have graphs with straight lines. How can you find the average speed of an object that isn t moving at a constant speed? You use the starting and ending data points and determine the slope of the line that would connect those two points. 5. Draw Conclusions If the line is not horizontal, what can you conclude about an object s movement? SpeedTime Graphs A speedtime graph compares the instantaneous speed of an object to time. Instantaneous speed is plotted on the yaxis and time is plotted on the xaxis. When the speed of an object is constant, the graph will show a horizontal line. How are speed changes shown on a speedtime graph? Sometimes, a car travels at a constant speed. Other times, its speed changes. The line on a speedtime graph for the car is horizontal until the driver brakes. If you plot the slowing speeds on a speedtime graph, the slope of the line decreases. As the driver gives the car more gas, the car gains speed. Plotted on a speedtime graph, the slope of the line increases as the car gains speed. The line becomes horizontal again when the car returns to a constant speed. What have you learned? Graphs are often used to summarize information. The slope of a line on a positiontime graph is the speed of the object. The steeper the slope, the more distance the object travels in a certain amount of time. So a steeper slope on a positiontime graph means a greater speed. On speedtime graphs, a horizontal line means the object s speed is constant. A line that slopes upward means the object is speeding up, while a line that slopes downward means the object is slowing down. 10 Chapter 1 Motion ca8.msscience.com
17 2 Forces lesson 1 Combining Forces Grade Eight Science Content Standard. 2.b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces. Also covers: 2.a, 2.c, 9.g. Before You Read On the lines below, describe what you would do to move a shopping cart around a grocery store. Read the lesson to learn about the forces that cause motion. When more than one force acts on an object, the combined effect is caused by the sum of all applied forces. What You ll Learn what a force is how balanced and unbalanced forces affect motion Read to Learn What is a force? A push or a pull is called a force. When you throw a ball, your hand exerts, or puts, a force on the ball. Forces are exerted by one object on another object. What are contact forces? A force that is exerted only when two objects are touching is a contact force. Some contact forces are small, such as the force you use to push a pencil across a sheet of paper. Some contact forces are large, such as the force exerted by a tow truck as it pulls a car behind it. What are noncontact forces? When you jump up in the air, you are pulled back to the ground, even though nothing seems to be touching you. A noncontact force is a force that one object exerts on another when they are not touching. Gravity, the force that pulls you back to Earth, is a noncontact force. Two objects do not have to touch to exert a gravitational pull on one another. Other noncontact forces include magnetic force and electric force. Make Flash Cards As you read, write main ideas and vocabulary terms on note cards. When you finish reading, use your flash cards to make sure you understand the main ideas and terms. 1. Identify Which list of forces are noncontact forces? (Circle your answer.) a. gravity, magnetism, and electricity b. throwing a ball and pushing a pencil Reading Essentials Chapter 2 Forces 11
18 How is force measured? Recall that a vector, such as velocity, has a size and a direction. A velocity vector is often represented by an arrow. The arrow points in the direction of motion. The length of the arrow represents the object s speed. Forces are also vectors that can be represented by an arrow. The direction of the arrow shows the direction of the push or pull. The length of the arrow represents the size, or strength, of the force. Force is measured in newtons (N). The force needed to lift a hamburger is about 1 N. The force needed to lift a 2L bottle of water is about 20 N. Academic Vocabulary task (TAHSK) (noun) an assigned job or thing to do 2. Explain What happens when forces push in the same direction? Combining Forces You would need to use a lot of force to push a heavy dresser. But if someone helped you push, the task would be much easier. More than one force would be acting on the dresser. When this happens, the forces combine. The combination of all the forces acting on an object is called the net force. Forces combine differently, depending on the direction of the forces exerted on an object. How do forces in the same direction combine? Imagine that you and a friend push on the same side of the dresser. You are both exerting force in the same direction. When forces push in the same direction, they add together to form the net force. In the case of the dresser, the net force is in the direction that you both push. You should always give a reference direction when discussing forces. For example, you could choose to the right as the positive reference direction for the dresser. Then, both forces would be positive. What happens when forces are in opposite directions? Imagine the dresser again. This time, you are pushing on one side of the dresser and a friend is pushing on the other side. The two forces are in opposite directions. If to the right is the positive reference direction, then one force is positive and the other is negative. The net force is in the direction of the stronger force. If you push on the dresser harder than your friend does, the net force is in the direction of your push. 12 Chapter 2 Forces Reading Essentials
19 What are unbalanced and balanced forces? When you pushed on the dresser with your friend, the net force on the dresser was not zero. Even when you pushed in opposite directions, one of you was pushing harder than the other. So, the net force was still not zero. When the net force on an object is not zero, the forces are called unbalanced forces. However, if you and your friend pushed on the dresser with equal forces, but in opposite directions, the net force would be zero. When you add the forces together, they cancel each other out. When the net force on an object is zero, the forces are called balanced forces. A Sketch and Describe Make a twotab Foldable. Label the tabs as illustrated. Describe and sketch examples of balanced forces and unbalanced forces on the front tabs and describe the importance of each under the tabs. Balanced Forces How do forces affect motion? Changes in motion occur when an object changes speed or changes direction. Whether the motion of an object changes depends on whether the forces acting on an object are balanced or unbalanced. What happens to the motion of an object when the forces are unbalanced? If you pushed on the dresser with more force than your friend, it would move in the direction of your push. The net force on the dresser is not zero. This means that the forces acting on the dresser are unbalanced. Only unbalanced forces cause a change in an object s motion, shown in the figure on the right, below. What happens to the motion of an object when the forces are balanced? Imagine that you and a friend push on opposite sides of a dresser. If you both push with equal force, the dresser will not move. The forces acting on it are equal, but in opposite directions. The net force on the dresser is zero. This means that the forces acting on the dresser are balanced. Balanced forces do not change the motion of an object, as shown in the figure on the left, below. Unbalanced Forces Picture This 3. Determine What do the different sized arrows suggest about the amount of force being exerted on the box in the figure on the right? Reading Essentials Chapter 2 Forces 13
20 4. Explain What do Newton s three laws explain? (Circle your answer.) a. how forces cause objects to move b. how an object moves when balanced forces act upon it 5. Compare two objects that you have moved recently. Which required more net force to move? Newton s First Law of Motion Isaac Newton was a scientist who lived from 1642 to He explained how forces cause objects to move. He developed three laws of motion. Newton s first law of motion describes how an object moves when the forces acting on it are balanced. According to Newton s first law of motion, if the net force on an object is zero, an object at rest remains at rest, or, if the object is moving, it continues to move in a straight line with constant speed. Simply put, if the net force on an object is zero, the motion of the object will not change. What is inertia? According to Newton s first law of motion, objects resist changing motion. Objects only change motion when unbalanced forces act on them. The tendency of an object to resist a change in its motion is called inertia. A book sitting on a table is not moving. The book doesn t move unless an unbalanced force acts on it. A book sliding on a table is moving. The book will keep sliding with constant speed unless an unbalanced force acts on it. What is the relationship between Change in Motion and mass? It is harder to change the motion of an object that has more mass. Imagine trying to stop a basketball or a bowling ball moving at the same speed. The bowling ball can have 12 times more mass than the basketball. You have to exert more force to stop the bowling ball than to stop the basketball. What have you learned? In this lesson you read that forces acting on an object can be added together to determine the net force acting on the object. Forces are vectors, so the size and direction of the force must be considered when calculating the net force. If the forces add to a zero net force, the forces are balanced and motion of the object does not change. Newton s first law of motion states that the motion of an object will not change if the net force is zero. If the net force is not zero, the object will move in the direction of the greater force. 14 Chapter 2 Forces ca8.msscience.com
21 2 Forces lesson 2 Types of Forces Grade Eight Science Content Standard. 2.d. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction. Also covers: 2.a, 2.c, 2.e, 2.f Before You Read On the lines below, write a descriptive sentence about what you know about the force of gravity, friction, or elastic force. Read the lesson to learn more about each type of force. Different types of forces act on objects. What You ll Learn the force of gravity depends on mass and distance to analyze static and sliding friction forces about elastic forces Read to Learn What is gravity? Picture a basketball game. The basketball is at rest until a player picks it up. The player exerts an unbalanced force on it. After shooting the ball, the player no longer exerts a force on it. According to Newton s first law of motion, the ball should move in a straight line at a constant speed unless an unbalanced force acts on it. The basketball does not move in a straight line. It moves in a curved path toward the basket. So, there must be an unbalanced force acting on it. Gravity, an attractive force between all objects that have mass, is the force that causes the ball to follow the curved path. What is the law of universal gravitation? When Isaac Newton was thinking about gravity, he wondered if the motion of falling objects and the motion of the Moon around Earth are caused by the same type of force. Newton found that it was gravity that pulled objects downward and caused the Moon to orbit Earth. In 1687, Newton published the law of universal gravitation (yew nuh VER sul gra vuh TAY shun) that showed how to calculate this force. According to the law of universal gravitation, all objects are attracted to each other with a force that depends on the masses of the objects and the distance between them. Underline Main Ideas As you read, underline the main ideas under each heading. After you finish reading, review the main ideas that you have underlined. B Define and Explain Make a sixtab Foldable. Label the tabs as illustrated. Define each term under the tabs. Compression Force Elastic Force Force of Friction Force of Gravity Normal Force Tension Force Reading Essentials Chapter 2 Forces 15
22 Picture This 1. Identify What is being compared in the table? What affects the force of gravity? The size of the force of gravity depends on the mass of objects and the distance between them. The gravitational force becomes stronger as the mass of one or both of the objects increases. The force of gravity becomes weaker as objects move away from each other. The table below compares the force of gravity exerted on a 70kg person by a book, the Sun, and Earth. The force exerted by the textbook is extremely small because its mass is small. The force exerted by the Sun is also small because it is so far away. Only Earth is close enough and massive enough to exert a noticeable gravitational force on the person. Object Gravitational Forces on 70kg Person Mass of Object (kg) Distance to Object (m) Size of Force (N) Book Sun Earth Explain What does it mean that mass is not a vector? (Circle your answer.) a. Mass changes depending on location. b. Mass does not change with location. How do weight and mass differ? When you stand on a bathroom scale, you are measuring the pull of Earth s gravity a force. The weight of an object is the gravitational force exerted on an object. Recall that mass is the amount of matter in an object. Mass is not a vector, and it does not change with location. In contrast, weight is a force vector. Weight has a size and a direction. Your weight is a force that always points toward the center of Earth. The size of an object s weight at the surface of Earth is proportional to the object s mass. For example, if the mass of an object doubles, the weight of the object doubles. If the mass of an object is reduced by half, the weight of the object is reduced by half. Weight and Mass High Above Earth In addition to mass, the distance between objects also affects weight. For example, an astronaut on the surface of Earth may have a mass of 70 kg and weight of 690 N directed toward the center of Earth. While is orbit, the astronaut s mass doesn t change. However, the gravitational force on her would be smaller because she is farther from Earth. As a result, the astronaut s weight would be reduced to about 620 N. 16 Chapter 2 Forces Reading Essentials
23 Friction Imagine pushing a book away from you across a table. As the book slides, it slows down and then stops. The force causing the book to slow down is a type of friction. Friction (FRIHK shun) is a force that opposes the movement between two surfaces in contact. The size of the friction force depends on the types of surfaces in contact. Smooth surfaces usually have less friction force than rough surfaces. What is static friction? What if you give a book on a table a tiny push? The book does not move. Why? The push is balanced by a force acting on the book in the opposite direction. This force is called static friction. Static friction occurs between two objects that are touching. It keeps the objects from sliding when a force is applied. The static friction force is exerted on the bottom of the book where it touches the table. Static friction increases when force increases. However, a strong enough force can overcome static friction. A hard push on the book causes it to slide on the table. What is sliding friction? Static friction keeps an object at rest. Sliding friction slows down an object that slides. It acts on an object in the opposite direction of its motion. Unlike static friction, sliding friction does not change when forces change. Sliding friction stays the same whether the forces are small or large. If friction did not exist, the sliding baseball player pictured below would continue moving at a constant speed. Academic Vocabulary occur (oh KUR) (verb) to happen Picture This 3. Predict What would happen to the sliding baseball player if the force of friction did not exist? Reading Essentials Chapter 2 Forces 17
24 What causes motion? People once thought that forces caused motion. In other words, an object would move only if unbalanced forces were acting on that object. Suppose you stop pushing a skateboard. The skateboard slows down and stops. You might think that the skateboard stops because there are no forces acting on it. However, the skateboard stops because friction acts on it. On Earth, friction is present whenever something moves. Without friction, the skateboard would continue to move in a straight line with constant speed. Instead of causing motion, unbalanced forces cause changes in motion. When friction is greatly reduced, objects move with a nearly constant velocity. 4. Identify According to the first law of motion, what do unbalanced forces cause? (Circle your answer.) a. motion b. changes in motion 5. Explain Which force is acting on a sweater when you pull it over your head? Explain. Elastic Forces Imagine a diver standing on the end of a diving board. She is not accelerating. So, the forces acting on her are balanced. The downward pull of Earth s gravity is one of the forces acting on her. An upward force must be acting on her to balance the downward force of gravity. This force is exerted on the diver by the diving board and is called an elastic (ih LAS tik) force. An elastic force is the force exerted by a material when the material is stretched or compressed. When the diving board is bent downward, it exerts an elastic force upward on the diver. What is tension? When you stretch a rubber band, you can feel the rubber band pulling back as it is stretched. The force the rubber band exerts is an elastic force. The force you exert on the rubber band is a tension (TEN shun) force. A tension force is a pulling force exerted on an object that can make it stretch. The elastic force exerted by the object when it is stretched is the same size as the tension force that is stretching the object. What is compression? When you squeeze a rubber ball, the ball changes shape. You can feel the ball push back on your hand as you squeeze. The force the ball exerts on your hand is an elastic force. The force you exert on the ball is a compression force. A compression force is a pushing or squeezing force applied to an object that can make the object shrink. The elastic force exerted by an object when it is compressed is the same size as the compression force that is squeezing the object. 18 Chapter 2 Forces Reading Essentials
25 What are normal forces? An elastic force balances the downward force of gravity. The force pushes upward on a diver, perpendicular to the surface of a diving platform. This force is a normal force, which is a force exerted by an object that is perpendicular to the surface of the object. The table below summarizes the forces discussed in this lesson. Gravity Types of Forces Force Properties Direction Static friction Sliding friction Tension force Compression force noncontact force strength increases as masses get closer together strength increases if one or both masses increase contact force force prevents the surfaces from sliding past each other contact force force exists when surfaces are sliding past each other contact force that causes an object to be stretched contact force that causes an object to be squeezed force on one mass is toward the other mass opposite to motion of object opposite to motion of object direction of stretching direction of squeezing Identifying Forces on an Object More than one force can act on an object at the same time. The forces can act in the same direction or in different directions. The forces acting in the vertical direction can cause an object s vertical motion. Horizontal forces can change an object s horizontal motion. How do forces balance horizontally? Suppose you push a book at a constant speed across a flat table. The book is moving in a horizontal direction with a constant velocity as you push it. According to the first law of motion, the forces acting on the book are balanced. For the forces to be balanced horizontally, an equal force must be acting on the object in the opposite direction. That force is sliding friction. Picture This 6. Determine Highlight the force that is a noncontact force. Circle the force related to stretching. 7. Identify What is the force that works against a horizontal push? Reading Essentials Chapter 2 Forces 19
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