1. Chapter 15 S2 Gravity and Motion

2. Ch15 S2 Essential Questions 1.What determines the strength of the force of gravity between two objects? 2. What two factors combine to keep the moon and Earth in orbit?

3. Gravity The strength of the force of gravity between two objects depends on two factors: the masses of the objects and the distance between them.

4. A force is a push or a pull. Gravity is the force that attracts all objects toward each other. Gravity does not need contact between objects. Gravity causes a book to fall if the book is dropped. Mass is the amount of matter in an object. The more mass an object has, the greater its force of gravity.

5. Because of gravity, Earth pulls on the moon. The moon also pulls on Earth. The moon stays in orbit around Earth because of gravity. The force of gravity is stronger when two objects are close together. The force of gravity gets weaker if the two objects are farther apart.

6. Inertia and Orbital Motion Isaac Newton concluded that two factors—inertia and gravity— combine to keep Earth in orbit around the sun and the moon in orbit around Earth.

7. Objects that are moving tend to stay in motion. Objects that are not moving tend to stay still, or at rest. This tendency of an object to not change its motion is called inertia.

8. Suppose you are in a car that stops suddenly. You will keep moving forward because of inertia. Isaac Newton stated his ideas about inertia as a scientific law. Newton’s first law of motion says that an object at rest will stay at rest. The law also says that an object in motion will stay in motion.

9. Earth’s gravity keeps the moon from moving in a straight line. The moon’s inertia keeps the moon moving ahead. The two forces combine to keep the moon revolving around Earth.

10. Ch15 S2 Essential Questions 1.What determines the strength of the force of gravity between two objects? The masses of the objects and the distance between them 2. What two factors combine to keep the moon and Earth in orbit? Inertia and gravity