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Let's use our imagination to make sure we have a clear picture of relative motion. We'll start up a fantasy called a thought experiment. These sorts of experiments come in handy for studying tough concepts, and they're kind of fun.
We are in a spaceship with the engines turned off. Nothing is in sight. Got that? Good. Look around.
How fast are we going? A pretty meaningless question, isn't it?
But wait. Off in the distance we see another spaceship, and it doesn't have its engines running either.
If we think we are standing still, let's say we measure its speed, and it's coming towards us at five thousand miles per hour. Or is the other ship stationary, and we are moving towards it at five thousand miles per hour? It doesn't matter; both views are correct. We can even think we both move. If we crash, we won't be able to prove who ran into whom. (Spaceships always have to carry no-fault insurance.) It won't make any difference to us astronauts since we'll only be many pieces all moving relative to each other.
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