Source: Sun and Earth, Oracle ThinkQuest

What do you think of when you hear the word motion? Are you moving right now? You may not think so, but you are. Remember, the Earth is moving around the Sun, and the Sun is moving around the Milky Way Galaxy. Everything in the universe moves. From the atoms that make up all matter to the largest of objects, all things are moving. Motion is defined as a change in an object's position over a given period of time.

Source: Cars, Atlas Signs and Plaques

Whether or not you perceive an object to be moving depends on your point of view, or point of reference. An object is in motion if it changes position relative to a reference point. A reference point is a place or object used for comparison to determine if something is in motion. Have you ever been sitting in a car stopped at a red light and seen the car next to you moving and thought that the car you were in was moving backward? You were using the car next to you as a reference point and assumed that it was sitting still. It is important to choose your reference point carefully. Buildings and light posts are good reference points because they are stationary.

The study of motion is a large part of physics. Scientist use words like speed, velocity and acceleration to describe motion. In this lesson, you will learn the differences in these motion concepts.