UNIT 12 Unit 15—Forces and Motion . 697 Lesson 1 What Are Forces? . 699 Inquiry Flipchart p. 75—On a Roll/Make It Easier Engineering and Technology: Football Safety Equipment . .717 The Solar Inquiry Flipchart p. 76—Balloon Racer Inquiry Lesson 2 How Do Forces Affect Motion? . .719 Inquiry Flipchart p. 77—How Do Forces Affect Motion? Inquiry Lesson 3 What Are Balanced and Unbalanced Forces? . .721 System and Inquiry Flipchart p. 78—What Are Balanced and Unbalanced Forces? Lesson 4 What Are Newton’s Laws? . 723 Inquiry Flipchart p. 79—Forces of Loose Change/Blast Off the UniverseBig Idea Careers in Science: Safety Engineer . 735 Scientists answer Unit 15 Review . 737 questions by carrying out careful investigations. Interactive Glossary . R� Index. R23 Earth is part of a solar system which is made up of many different objects orbiting the Sun. I Wonder Why Why are most observatories built far from large cities? Turn the page to find out. xviii © Houghton Mifflin Harcourt Publishing Company HMH Credits 537 © Houghton Mifflin Harcourt Publishing Company Photo credits 5_CNLAESE589428_FMTOC.indd 18 12/28/10 9:09:15 PM 5_CNLAESE589428_U12O.indd 537 12/28/10 5:51:52 PM Here’s why Most observatories are built far from cities to avoid the brightening of the night sky caused by UNIT 12 artificial outdoor lighting which can make it impossible to Essential Question 1 see dim lights from stars. In this unit, you will explore this Big Idea, Essential What Objects Are Questions, and investigations on the Inquiry Flipchart. Part of the Solar rack Big Idea Earth is part of a solar system T ur Yo s which is made up of many different objects Prog re s System? orbiting a sun. Essential Questions Lesson 1 What Objects Are Part of Find the answer to the following the Solar System? . .539 question in this lesson and record it here. Inquiry Flipchart p. 58—Make a Scale Model/First Sightings Which planets have rings, and what are People in Science: Kalpana Chawla and Claudia Alexander. .557 the rings made of? Inquiry Lesson 2 How Do We Observe Objects in the Solar System? . .559 Inquiry Flipchart p. 59—How Do We Observe Objects in the Solar System? Lesson 3 What Are Stars and Galaxies? . .561 Inquiry Flipchart p. 60—A Small Slice of the Universe/Colorful Stars Engineering & Technology: Tools in Space . .571 Inquiry Flipchart p. 61—Improvise It: How High Is That Star? Unit 12 Review. .573 Active Reading Lesson Vocabulary Compare and Contrast List the terms. As you learn about each one, Many ideas in this lesson are connected make notes in the Interactive Glossary. because they explain comparisons and contrasts—how things are alike and different. Active readers stay focused on comparisons and contrasts when they ask themselves, How are these things alike? How are they different? Now I Get the Big Idea! Before you begin each lesson, be sure to write your thoughts about the Essential Questions. © Houghton Mifflin Harcourt Publishing Company (bkgd) ©NASA/JPL/Space Science Institute 538 Unit 12 © Houghton Mifflin Harcourt Publishing Company HMH Credits 539 5_CNLAESE589428_U12O.indd 538 12/28/10 5:52:03 PM 5_CNLAESE589428_U12L01O.indd 539 12/28/10 9:40:03 AM Here’s why Most observatories are built far from cities to avoid the brightening of the night sky caused by UNIT 12 artificial outdoor lighting which can make it impossible to Essential Question 1 see dim lights from stars. In this unit, you will explore this Big Idea, Essential What Objects Are Questions, and investigations on the Inquiry Flipchart. Part of the Solar rack Big Idea Earth is part of a solar system T ur Yo s which is made up of many different objects Prog re s System? orbiting a sun. Essential Questions Lesson 1 What Objects Are Part of Find the answer to the following the Solar System? . .539 question in this lesson and record it here. Inquiry Flipchart p. 58—Make a Scale Model/First Sightings Which planets have rings, and what are People in Science: Kalpana Chawla and Claudia Alexander. .557 the rings made of? Inquiry Lesson 2 How Do We Observe Objects in the Solar System? . .559 Inquiry Flipchart p. 59—How Do We Observe Objects in the Solar System? Lesson 3 What Are Stars and Galaxies? . .561 Inquiry Flipchart p. 60—A Small Slice of the Universe/Colorful Stars Engineering & Technology: Tools in Space . .571 Inquiry Flipchart p. 61—Improvise It: How High Is That Star? Unit 12 Review. .573 Active Reading Lesson Vocabulary Compare and Contrast List the terms. As you learn about each one, Many ideas in this lesson are connected make notes in the Interactive Glossary. because they explain comparisons and contrasts—how things are alike and different. Active readers stay focused on comparisons and contrasts when they ask themselves, How are these things alike? How are they different? Now I Get the Big Idea! Before you begin each lesson, be sure to write your thoughts about the Essential Questions. © Houghton Mifflin Harcourt Publishing Company (bkgd) ©NASA/JPL/Space Science Institute 538 Unit 12 © Houghton Mifflin Harcourt Publishing Company HMH Credits 539 5_CNLAESE589428_U12O.indd 538 12/28/10 5:52:03 PM 5_CNLAESE589428_U12L01O.indd 539 12/28/10 9:40:03 AM The sun, Earth, and its moon form a system in space. Earth revolves around the sun. That means Earth travels around the sun in a path called an orbit. The moon revolves around Earth. Read on The planets in our solar system are very to learn about other objects in space. far from each other. Active Reading As you read this page, underline two details that tell how all planets are alike. arth and its moon are part of a larger system in space Ecalled a solar system. A solar system is made up of a star The orbits of the planets in our and the planets and other space objects that revolve around solar system are not perfect it. A planet is a large, round body that revolves around a star. Earth is about 150 circles. They are oval-shaped, In our solar system, the planets and other objects revolve million kilometers or elliptical [eh•LIP•tuh•kuhl]. around a star we call the sun. from the sun! There are eight planets in our solar system. All of them Around and Around rotate, or spin, about an axis. This is an imaginary line that Draw an orbit for the planet. Then draw a goes through the center of a planet. Earth rotates on its axis moon and its orbit. once every 24 hours. This is the length of one day on Earth. Unlike planets, some objects don’t revolve directly around sun Some planets have many the sun. Moons are small natural objects that revolve around planet other objects. Many planets have moons. Earth has only one. It moons. Earth has only one. Venus and Mercury revolves once around Earth about every 27 days. have none! Diagrams not to scale. © Houghton Mifflin Harcourt Publishing Company © Houghton Mifflin Harcourt Publishing Company (bl) ©NASA/Corbis 540 © Houghton Mifflin Harcourt Publishing Company © Houghton Mifflin Harcourt Publishing Company (bl) ©NASA/Corbis 541 5_CNLAESE589428_U12L01S.indd 540 12/28/10 9:42:12 AM 5_CNLAESE589428_U12L01S.indd 541 12/28/10 9:42:33 AM The sun, Earth, and its moon form a system in space. Earth revolves around the sun. That means Earth travels around the sun in a path called an orbit. The moon revolves around Earth. Read on The planets in our solar system are very to learn about other objects in space. far from each other. Active Reading As you read this page, underline two details that tell how all planets are alike. arth and its moon are part of a larger system in space Ecalled a solar system. A solar system is made up of a star The orbits of the planets in our and the planets and other space objects that revolve around solar system are not perfect it. A planet is a large, round body that revolves around a star. Earth is about 150 circles. They are oval-shaped, In our solar system, the planets and other objects revolve million kilometers or elliptical [eh•LIP•tuh•kuhl]. around a star we call the sun. from the sun! There are eight planets in our solar system. All of them Around and Around rotate, or spin, about an axis. This is an imaginary line that Draw an orbit for the planet. Then draw a goes through the center of a planet. Earth rotates on its axis moon and its orbit. once every 24 hours. This is the length of one day on Earth. Unlike planets, some objects don’t revolve directly around sun Some planets have many the sun. Moons are small natural objects that revolve around planet other objects. Many planets have moons. Earth has only one. It moons. Earth has only one. Venus and Mercury revolves once around Earth about every 27 days. have none! Diagrams not to scale. © Houghton Mifflin Harcourt Publishing Company © Houghton Mifflin Harcourt Publishing Company (bl) ©NASA/Corbis 540 © Houghton Mifflin Harcourt Publishing Company © Houghton Mifflin Harcourt Publishing Company (bl) ©NASA/Corbis 541 5_CNLAESE589428_U12L01S.indd 540 12/28/10 9:42:12 AM 5_CNLAESE589428_U12L01S.indd 541 12/28/10 9:42:33 AM sun At times, the brightest object in the night sky Earth is not the moon or a star. It is Venus, one of Earth is the third planet from the sun. It has an Earth’s closest neighbors in space. atmosphere made of mostly nitrogen, oxygen, and carbon dioxide.