
#1
Jan413, 01:08 AM

P: 282

Why do we have xaxis perpendicular to yaxis? Why not 45° or something else?
Even if we keep any other angle other than 90° we can still build up all other vectors using them in the plane. So what is there is 90° that makes it special and simpler? 



#2
Jan413, 01:41 AM

HW Helper
P: 2,166

You can use any angle (and sometimes it is convenient to do so) and have all the same vectors. Several things make 90° special and simpler.
c^{2}=a^{2}+b2ab cos(t) This is most simple if cos(t)=cos(90°)=0 very small t are particularly trouble some as two axis are almost the same to determine the coordinates we must solve x.i=x_{i}i.i+x_{j}j.i x.j=x_{i}i.j+x_{j}j.j when t=90° this is easy x_{i}=x.i x_{j}=x.j 



#3
Jan413, 07:35 AM

Sci Advisor
P: 1,716





#4
Jan413, 08:37 AM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,898

Why do we choose to have perpendicular axis?
Also the dot product of two perpendicular vectors is 0 so if the x and y axes are perpendicular, and [itex]v_x[/itex] and [itex]v_y[/itex] are unit vectors in the direction of those vectors, then the components of vector v are just [itex]v\cdot v_x[/itex] and [itex]v\cdot v_y[/itex]. If the axes were not perpendicular, those formulas would be more complicated.
In more abstract situations, vector spaces of functions, for example, we typically do that the other way around: choose some convenient basis, then define the inner product to make the basis vectors orthogonal. 



#5
Jan413, 09:00 AM

Sci Advisor
P: 1,716





#6
Jan413, 06:10 PM

Sci Advisor
HW Helper
P: 9,428

once you pick one axis, what other natural choice is there for a second axis other than perpendicular? i.e. what is more natural and simpler than angles that are equal?




#7
Jan413, 10:38 PM

P: 282

A related question that I don't understand: Why perpendicular axis are independant of each other?




#8
Jan413, 10:41 PM

P: 4,570

http://en.wikipedia.org/wiki/Dot_product 



#9
Jan513, 09:16 AM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,898

We might also note that in "differential geometry", we work with surfaces, such as the surface of a sphere, on which we cannot have coordinate curves that are always perpendicular. That causes all sorts of problems, among them that we now have both "covariant" and "contravarient" components of vectors and tensors. If we stick to "Cartesian tensors" in which we only allow "Cartesian coordinate systems" with coordinate curves that are always perpendicular, the distinction between "covariant" and "contravariant" disappears.




#10
Jan513, 12:16 PM

P: 5,462

I'm not sure since you have posted this in linear and abstract algebra whether my response is relevent or not.
But does this mean Under what circumstances do we use perpendicular axes and under what circumstances do we use some other axes? Or do you think we only use perperdicular axes? The second is far from the truth. Many different arrangements are in use and the common theme is a blend of ease of presentation and ease of use. In mathematics you will find cylindrical polar and spherical coordinates. In cartography, navigation, surveying and fluid mechanics you will find hyperbolic, 'rhorho' and perhaps even parabolic coordinates. Look in some engineering texts you will find many graphs with exotic shaped cooordinates. In geology, soil mechanics and materials science you will find some strange triangular coodinates. These also appear in colour theory in lighting. Some coordinates have straight line axes, some do not. Look up 'curvilinear coordinates' go well 


Register to reply 
Related Discussions  
Perpendicular components of the perpendicular plane  General Physics  1  
find two perpendicular vectors which are perpendicular to another  Calculus  14  
Two points are on a disk that rotates about an axis perpendicular to the plane  Introductory Physics Homework  1  
How to find angular momentum of a body about an axis other than the axis of rotation?  Introductory Physics Homework  11  
Does skin perpendicular to sun’s rays burn faster than if not perpendicular?  General Physics  13 