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Is there any case/reason of not showing the directions (from negative to positive) of axes in coordinate diagrams? Sometimes I see the direction not specified, even though the discussion using that diagram assumes certain direction, which makes me confused a lot of times.

I thought it may be better to give an example domain to consider cases/reasons. In my dynamics class, I've never seen any coordinate diagrams, either 2D or 3D, with directions indicated.

Thanks.

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    Indicating directions is very high school. There are standard conventions, so there is no point to indicating directions.2012-04-08
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    Thanks for the comment. Can I ask what is "standard conventions" you're talking about?2012-04-08
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    And this is just my partial experience, but my previous mentor who is a computer scientist (who got PhD at one of the top school in Japan) always indicated the directions.2012-04-08
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    You are of course familiar with right $+$, up $+$. Then for three dimensions there is the right-hand rule.2012-04-08
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    @André Nicolas I see. Then I might only be experiencing strange cases with my dynamics textbook, where so often times "right +, up +" and RH-rule doesn't hold in the given diagram.2012-04-08
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    Btw, inspired by the comments I got, I personally figured out this particular dynamics book (Engineering Dynamics 3rd ed. by Ginsberg) is indicating directions by drawing lines of axes only in positive regions. Don't know if it's common sense or not but anyway thx!2012-04-08

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Its a common convention , one that you are familiar since your school days. So unless and until there is a different convention being used, its assumed that all directions are according to common convention and problems are solved keeping this in mind.

However if there is a difference, as far as books i have seen , the author(s) always bring it to the notice of the reader without failure

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    Thanks, I understand basics now. As I said in the comment to the original question, I might just be seeing exceptions, but still, as you said, the author almost always specifies the direction of coordinates outside of diagram. I just feel that's not friendly enough.2012-04-08