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# Motion Every day, you see objects and people in motion. Cars, buses, and trucks move along the streets; people walk, run, and move in different directions. How would you describe motion? One way is to say it is fast or slow. However, sometimes you need to know the speed and the direction of th...

# Motion Every day, you see objects and people in motion. Cars, buses, and trucks move along the streets; people walk, run, and move in different directions. How would you describe motion? One way is to say it is fast or slow. However, sometimes you need to know the speed and the direction of the movement. It's also important to know how the motion is changing. Motion is a change in position. You don't always need to see something move to know motion has occurred. For example, consider a mail truck stopped next to a mailbox. A little while later, the same truck is further along the street. You know it moved because its position relative to the mailbox changed. Motion is a change in position of an object over time. ## Position depends on a reference frame To measure the position of an object, a *reference frame* must be chosen. A reference frame is a group of objects that are not moving relative to each other. One point in the reference frame is chosen as the reference point. Then the position of an object is the distance and the direction of the object from the reference point. ## Distance and displacement Imagine a runner jogging to the 50-meter mark, then turning around and running back to the 30-meter mark. * **Distance:** The distance the runner traveled is 50 meters + 30 meters = 80 meters. Distance is the length of the path traveled. * **Displacement:** The runner's displacement is 20 meters north. Displacement is the distance and direction from the initial position to the final position. Displacement has both a magnitude (size) and a direction. So displacement of 20 meters north is different than 20 meters south.