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I noticed in my physics textbook that we define certain relationships to be true. I can see how this is considerably helpful in deriving other relationships from these definitions; for instance, take position: we define these quantities to be so, and from it we can define other quantities like velocity, acceleration, etc. Moreover, most of the time these definitions are well-grounded and intuitive. However, at other times they aren't. To serve as some examples: force, torque, and electric fields. How were these things defined? What was the reasoning used to define these quantities? What are the motivations for these definitions?

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closed as not constructive by Qmechanic May 2 '13 at 22:29

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The motivation is that the ones you get taught are the ones that have survived (in the cases you list) centuries of repeated testing. It is certainly true that some of those basic relationships don't feel intuitively unique and obvious the first time you meet them, and we an assume that people tried a lot of different choices early on (caloric lasted a long time before it got the heave-ho for not working well enough). Indeed what you are taught in a intro course are only approximately true in any case, but it will be a while before you are ready for relativity. – dmckee Feb 1 '13 at 23:55