how can we understand torsion of curve is in the direction of normal unit vector en?
Are you familiar with the Frenet-Serret formulas?
I have first seen them today I will try to study them.
It is a mathematical system which helps with space curves; it starts with the idea that the tangent to a curve can be found by taking the derivative of the curve's functional definition wrt the distance along the curve.
Then two more orthogonal vectors are created, providing a co-moving orthogonal reference system.
This is usually first encountered in a vector calculus course, though it is useful in many areas of engineering.
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