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How can you tell if something is moving? Look at the baseball players in the pictures. How can you tell which player has moved? You can tell the player has moved because his position has changed. Position is the location of an object. The player’s final position is at first base. All objects have a position. You know something has moved when its position has changed.
Words such as above and below, left and right, ahead and behind, and north and south give clues about position. You could say the girl is behind the tree, or Perryville is south of St. Louis. When we describe the position of something, we compare it with nearby objects that seem not to be moving. The objects used for comparing are called a frame of reference.
What if you’re in a bus riding to school? What do you see when you look out the window? Houses, trees, signs, stores? These objects, which don’t move, are your frame of reference.
Earth is usually our frame of reference. A car, boat and bicycle all appear to move because they change their position compared with Earth’s surface. We assume Earth is not moving.
Your frame of reference is Earth, so the Sun appears to move in the sky. Sunrises and sunsets are actually caused by the movement of Earth on its axis.
However, when viewed from space, Earth is moving. It’s traveling around the Sun. If outer space is our frame of reference, then Earth and all the objects on it are moving.
Compare and Contrast • What is the difference between a person’s frame of reference and Earth’s frame of reference for motion? • Compare and contrast the position of the two baseball players in the photographs at the beginning of this lesson.
How do you know that something has moved? • Answer the question… • With a partner • In your Learning Notebook
