- #71
Simon Bridge
Science Advisor
Homework Helper
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... you are free to call the bond axis anything you like. If you don't like "x" how about "y" or "z" (z is a very popular choice because: cylindrical coordinates)... or "bob" or "kate" or anything you like. It's just a bit of shorthand.gracy said:
Earlier on - you drew a diagram with a big arrow on it labelled "x-axis", and you immediately ran against a convention in maths drawings that an arrow with an axis label written next to it is the axis in question but you intended it to indicate the axis. That's just you learning how to talk in maths, and it seems to have got you sidetracked a bit.
One of the things that can happen to people as they learn maths is they start thinking of x-y-z as somehow being fixed in space and everything else is positioned against these fixed axes. But this is not the case. You can choose any direction and call it anything you like... it's just that Cartesian axes are handy labels. As soon as we pick something else, we then have to write down a bunch of stuff to define what we mean and that's already been done for x-y-z 400-odd ago(?) by Rene Decartes. So we all use his by default.
In physics, our axes are chosen to correspond to some physical object or process ... in your case it makes sense to put one cartesian axis along the bond-axis. It does not matter where the others point. I chose x because you seemed to be used to using the x axis. But you can pick another one if you like. You don't even have to line up the cartesian axes to the bond if you don't want to - it makes only cosmetic differences to the final result while making the calculation itself a lot harder.