# Confused by normal Nx and Ny

**URL:** <https://community.freefem.org/t/confused-by-normal-nx-and-ny/1492>\
**Category:** General Discussion\
**Created:** [February 13, 2022, 7:19am UTC](https://community.freefem.org/t/confused-by-normal-nx-and-ny/1492 "2022-02-13T07:19:05Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![zese](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/zese/32/917_2.png) [@zese](https://community.freefem.org/u/zese)\
**Post date:** [February 13, 2022, 7:19am UTC](https://community.freefem.org/t/confused-by-normal-nx-and-ny/1492/1 "2022-02-13T07:19:05Z")

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Dear all

I got confused by two definitions. I read the manual and I have a question:

when we have a normal velocity and want to find its x and y components we write u_N.x=ux and u_N.y=uy ?  
when we have x and y components ux and uy and want to find normal velocity what should we write?

BR

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**Author:** ![sumantkr](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/sumantkr/32/2681_2.png) [@sumantkr](https://community.freefem.org/u/sumantkr)\
**Post date:** [February 14, 2022, 6:24am UTC](https://community.freefem.org/t/confused-by-normal-nx-and-ny/1492/2 "2022-02-14T06:24:12Z")

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as I know, you may proceed with

for, normal component of u along x-direction = `dx(u)*N.x`

for, normal component of u along y-direction, = `dy(u)*N.y`

combinely, for a simple normal components on some boundary edge: int1d(Th, label\_of\_edge) (dx(u)\*N.x + dy(u).N.y);

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**Author:** ![david.fabre](https://avatars.discourse-cdn.com/v4/letter/d/8e8cbc/32.png) [@david.fabre](https://community.freefem.org/u/david.fabre)\
**Post date:** [February 14, 2022, 8:54am UTC](https://community.freefem.org/t/confused-by-normal-nx-and-ny/1492/3 "2022-02-14T08:54:22Z")

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Hello  
The previous anwser was about computing the derivative in the normal direction but I’m not sur this is what you asked.  
If you simply want the normal component of a field [ux,uy] then this should be simply

Un = ux \* N.x + uy \* N.y