1. Unit 1 Lesson 4 Gravity and Motion

2. Down to Earth Copyright © Houghton Mifflin Harcourt Publishing Company What is gravity? Gravity is a noncontact force of attraction between objects due to their mass. All objects on Earth fall at the same rate once air resistance and other factors are removed. The force of gravity between Earth and an object is equal to the mass of an object m multiplied by a factor due to gravity g. Force = mass × acceleration due to gravity (F = mg), where g is about 9.8 m/s 2. Unit 1 Lesson 4 Gravity and Motion

3. What is gravity?

4. What if you drop a heavy object and a light object at the same time? Which would hit the ground first? Some objects can experience a lot of air resistance and fall slowly to the ground. If you take away air resistance, all objects will fall with the same acceleration. When gravity is the only force affecting the fall, a light object and a heavy object will hit the ground at the same time.

5. What is gravity?

6. All matter has mass, so all matter is affected by gravity. All objects experience gravitational attraction to all other objects. Earth and other planets are round because of gravity. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 1 Lesson 4 Gravity and Motion

7. What is gravity? Some objects in space are not round because they are too small for gravity to shape them into a sphere. The paths of the planets, the sun, and our galaxy are determined by gravity. The sun’s gravity is the force that keeps the planets moving in orbit around the sun. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 1 Lesson 4 Gravity and Motion

8. What is gravity?

9. A Weighty Issue Copyright © Houghton Mifflin Harcourt Publishing Company What determines the force of gravity? In 1687, Sir Isaac Newton formulated 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 the gravitational force is related to the mass of the objects and the distance between them. Unit 1 Lesson 4 Gravity and Motion

10. What determines the force of gravity? Gravitational force between two objects increases as the distance between their centers decrease. Gravitational force between two objects increases as their masses increase. Objects with greater mass have more attraction between them than objects with smaller mass have between them. When the distance between two objects increases, the force of gravity decreases. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 1 Lesson 4 Gravity and Motion

11. What determines the force of gravity?

13. Don’t Bring Me Down Copyright © Houghton Mifflin Harcourt Publishing Company How does gravity keep objects in orbit? Free fall is when gravity is pulling an object down and no other forces are acting on it. An object is in orbit when it travels around another object in space. When an object orbits Earth, it is moving forward but it is also in free fall. Forward motion and free-fall motion combine to cause orbiting. Unit 1 Lesson 4 Gravity and Motion

14. How does gravity keep objects in orbit?

15. Spacecraft, satellites, the moon, planets, and stars all complete orbits. Any object in curved motion is constantly changing direction. As the planets orbit the sun, or the moon orbits Earth, centripetal force keeps them moving inward in a circular path instead of flying off in a straight line. Gravity is the force that pulls objects toward the center of an orbital path. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 1 Lesson 4 Gravity and Motion

16. How does gravity keep objects in orbit?