# Finding the twist angle of a Vector around axis Z

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Commented: Jan on 30 Jul 2018
Hi all
I have the positions of a vector F as a function of time, and I need to find the twist around axis Z, so I need to project it on XY plane and then find the angle with one of X or Y axis.
I know how to do the 2nd part , but I have problem with the first part. I am not sure the quiver() will give me the right result.
Also, as an Idea, could I use the gradients of the vector F along X and Y and divide them on eachother to find the angle with X or Y ?

Jan on 30 Jul 2018
Do not bump your question by posting meaningless comments. Trying to push the contributors is not liked in this forum and bumping is considered as noise.
Please provide what you have as inputs. "the positions of a vector F as a function of time" is not clear. The projection in the XY plane is trivial by setting the Z component to zero. Do you use atan2 to determine the angle? acos and asin is not stable.
You see: If you do not get an answer, adding more details by editing the question is more useful than a bumping.
Dear Jan
Sorry , My concern was about a problem that the question is not viewed in question feed.
I can't provide the inputs now , but I used this line, looping over the rows of the vector matrix :
Tors(n,1) = atan2(norm(cross(u2d(n,:),X2D)), dot(u2d(n,:),X2D))*180/pi;
I got
??? Error using ==> cross at 37
A and B must have at least one dimension of length 3.
Jan on 30 Jul 2018
The message is clear. Check the dimensions of the inputs of the cross command. Again:
Please provide what you have as inputs.
Posting the complete code is a good idea also.