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s8pe-10301-ca 12/8/05 6:04 PM MAZER Page 71 KEY CONCEPT Gravity is a force exerted by masses. CALIFORNIA Content Standards 8.2.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. 8.2.g Students know the role of gravity in forming and maintaining the shapes of the planets, stars, and the solar system. BEFORE, you learned NOW, you will learn • Every action force has an equal and opposite reaction force • Newton’s laws are used to describe the motions of objects • Mass is the amount of matter an object contains • How mass and distance affect gravity • What keeps objects in orbit EXPLORE Downward Acceleration (8.2.d) How do the accelerations of two falling objects compare? MATERIALS PROCEDURE 1 Make a prediction: Which ball will fall faster? • golf ball • Ping-Pong ball 2 Drop both balls from the same height at VOCABULARY gravity p. 71 weight p. 73 orbit p. 74 the same time. 3 Observe the balls as they hit the ground. WHAT DO YOU THINK? • Were the results what you had expected? • How did the times it took the two balls to hit the ground compare? Masses attract each other. VOCABULARY Create a four square diagram for gravity in your notebook. When you drop any object—such as a pen, a book, or a football—it falls to the ground. As the object falls, it moves faster and faster. The fact that the object accelerates means there must be a force acting on it. The downward pull on the object is due to gravity. Gravity is the force that objects exert on each other because of their masses. You are familiar with the force of gravity between Earth and objects on Earth. Gravity is present not only between objects and Earth, however. Gravity is considered a universal force because it acts between any two masses anywhere in the universe. For example, there is a gravitational pull between the Sun and the Moon. Even small masses attract one another. The force of gravity between dust and gas particles in space helped form the solar system. check your reading Why is gravity considered a universal force? Chapter 3: Gravity, Elastic Forces, and Friction 71 PDF