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Picture Yourself In a Boat on a River

 When we talk about where something is, we might say that it is 
 "on the table" or "a short drive from anywhere in town".
 Usually when someone talks about where something is, 
 or how something moved, they talk about it in relation to something else.
 What that something is, is up to the person doing the describing.
 The same place might be described as above the first floor
 or below the third floor.
 As far as we can tell, there is no absolute position of anything. 
 Position can only be described as relative to something else.
 In physics, we will often be describing the position and motion, 
 objects, fields, or forces. 
 To do this, we will usually place a coordinate system over them. 
 That is, a set of graph axis.
 The thing to realize is, 
 We can place the origin of the graph anywhere we like. 
 If the object is moving in a straight line, 
 we can make that motion along any axis we like. 
 We can say the object is moving in the positive or negative direction.
 Thought Experiment:
 Say that there are only 3 things in the universe. 

 At some point, they are arranged like this:


 At some later point, they are all touching. 
 Like this:

 The questions are: 
 Can you tell how many of them moved?
 Can you tell which ones moved?

   copyright 2005 Bruce Kirkpatrick

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