Question

The direct cosines l, m, n are of local _____

(a) z’- axes

(b) x’- axes

(c) y’- axes

(d) None of the above

The question was posed to me in an interview for job.

This intriguing question originated from Three Dimensional Trusses topic in section Trusses of Finite Element Method

Answer 1

Correct answer is (b) x’- axes

Easy explanation: Direction cosine refers to the cosine of the angle between any two vectors and three co-ordinate axes. Direction cosines are analogous extension of the usual notion of the slope to higher dimensions. In a truss element the direction cosines l, m, n are of local co-ordinate of x’- axes.