After the comments mantained with Anshuman Dwivedi, I am posting a new answer.
I am solving the following problem:
A circle of radius $r$ is centered at origin.
Consider an imaginary line $y=r$, tangent to the circle at A = (0,r), and a segment AB of length $l$ on this line.
What shape will this segment describe if it moves rolling around the circumference and maintaining its point A on line $y=r$.
For any position of the segment around the circumference, we have:
We advance point A and rotate the segment till it is tangent to the circumference.
Coordinates of tangential point are calculated as follow
$Alfa= 2*atan((x-coordinate-of-point-A) / r)$
$Tx = r * cos( \pi/2- Alfa)$
$Ty = r * sin( \pi/2- Alfa)$
Now, new position of point B will be
$Bx = ( x-coordinate-of-point-A ) + l * cos(Alfa)$
$By = r - l*sin(Alfa)$
Following is the curve obtained