1. Unit 2 Lesson 2 Gravity and the Solar System Copyright © Houghton Mifflin Harcourt Publishing Company

2. Gravity Copyright © Houghton Mifflin Harcourt Publishing Company What is gravity? Gravity is a force of attraction between objects that is due to their masses and the distances between them. Every object in the universe pulls on every other object. Unit 2 Lesson 2 Gravity and the Solar System

3. What is gravity? Objects with greater masses have a greater force of attraction than objects with lesser masses have. Objects that are close together have a greater force of attraction than objects that are far apart have. Gravity is the weakest force in nature, yet it is one of the most important forces in the universe. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

4. What is gravity? Gravity accounts for the formation of planets, stars, and galaxies. Gravity also keeps smaller bodies in orbit around larger bodies. An orbit is the path that a body follows as it travels around another body in space. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

5. What is the law of universal gravitation? Using Kepler’s laws, Sir Isaac Newton became the first scientist to mathematically describe how the force of gravity behaves. He reasoned that gravity is the force that accounts for both the fall of an apple from a tree and the movement of the moon around Earth. In 1687, Newton formulated the law of universal gravitation. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

6. What is the law of universal gravitation? The law of universal gravitation states that all objects in the universe attract each other through gravitational force. The strength of this force depends on the product of the masses of the objects. Gravitational force is also inversely proportional to the square of the distance between the objects. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

7. How does gravity affect planetary motion? If a ball is attached to a string and is swung around, it moves in a circular path. The inward force that causes an object to move in a circular path is called centripetal force. If the string breaks, the ball will move off in a straight line. When the string is intact, the centripetal force prevents the ball from flying off. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

8. How does gravity affect planetary motion? When planets orbit the sun, a force similar to centripetal force prevents them from moving out of their orbits and into a straight line. The sun’s gravity is the force that keeps the planets moving in orbit around the sun. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

9. Collapse Copyright © Houghton Mifflin Harcourt Publishing Company How did the solar system form? The formation of the solar system is thought to have begun 4.6 billion years ago when a cloud of gas and dust collapsed. This kind of cloud, from which solar systems form, is called a solar nebula. In a solar nebula, the inward pull of gravity is balanced by the outward push of gas pressure in the cloud. Unit 2 Lesson 2 Gravity and the Solar System

10. How did the solar system form? Scientists think that an outside force, perhaps the explosion of a nearby star, caused the nebula to compress and contract under its own gravity. The solar system formed in a single region of the nebula, perhaps several light-years across. The sun probably formed from a region that had a mass that was slightly greater than today’s mass of the sun and planets. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

11. How did the solar system form? As a region of the solar nebula collapsed, gravity pulled most of the mass toward the center of the nebula. As the nebula contracted, it began to rotate with increasing speed and flattened out into a disk. This disk, called a protostellar disk, is where the central star, our sun, formed. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

12. How did the solar system form? As the protostellar disk continued to contract, most of the matter ended up in the center of the disk. Friction from matter that fell into the disk heated its center to millions of degrees, resulting in the fusion of hydrogen atoms into helium atoms. The process of fusion released large amounts of energy, the gas and dust stopped collapsing, and the sun was born. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

13. How did the solar system form? As the sun was forming, dust grains collided and stuck together to form dust granules, which increased in size to form meter-sized bodies. Collisions between these bodies formed larger bodies, called planetesimals, from which planets formed. The protostellar disk became the protoplanetary disk in which the planets formed. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

14. How did the solar system form? The inner part of the protoplanetary disk was so hot that only rocks and metals were in solid form. The collisions of rocky planetesimals in the inner disk led to the formation of rocky, metallic planets. We call these inner planets the terrestrial planets. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

15. How did the solar system form? In the cold, outer disk, massive planets made of icy and rocky planetesimals may have formed. The gravity of these planets was so strong that they captured gas and other matter as they grew. Therefore, these outer planets have rocky or metallic cores and deep atmospheres of gas and ice, and they are called the gas giant planets. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System

16. How did the solar system form? Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Gravity and the Solar System How does this diagram illustrate the formation of the solar system?