Imagine that you are on a train, traveling at a steady speed of 50 miles per hour (mph). Your physics textbook on the table in front of you.
Now, you and the textbook (and the train) are all moving at the same speed. To an outside observer standing next to the train tracks, you and the book are each rushing by at 50mph. But, from your point of view, the book isn't moving at all. That is, it's not getting closer to or farther from you.
- You are moving at 50mph relative to the observer next to the tracks.
- You are not moving, relative to the textbook.
- The book, the train, and yourself are not moving at all, relative to each other.
- Relative to the train, the ground is moving by at 50mph.