How do you decide if an object is moving? You are probably sitting in a chair as you read this. Are you moving? Your eyes blink, and your chest moves up and down. But you would probably say you are not moving. An object is in motion if its distance from another object is changing. Because your distance from your chair is not changing, you may conclude you are not moving.

 

Reference Points

To decide if you are moving, you use your chair as a reference point. A reference point is a place or object used for comparison to determine if something is in motion. An object is in motion if it changes position relative to a reference point.

Objects that we call stationary—such as a tree, a sign, or a building—make good reference points. From the point of view of the train passenger in Figure 1, such objects are not in motion. If the passenger is moving relative to a tree, he can conclude that the train is in motion.

You probably know what happens if your reference point is moving. Have you ever been in a school bus parked next to another bus? Suddenly, you think your bus is moving backward. But, when you look out a window on the other side, you find that your bus isn’t moving at all—the other bus is moving forward! Your bus seems to move backward because you used the other bus as a reference point.