Question

We use sometimes the measures to know the direction of moment of inertia. It is done by right handed coordinate system. Which is right about it for the composite bodies (consider the mentioned axis to be positive)?

(a) Thumb is z-axis, fingers curled from x-axis to y-axis

(b) Thumb is x-axis, fingers curled from z-axis to y-axis

(c) Thumb is y-axis, fingers curled from x-axis to z-axis

(d) Thumb is z-axis, fingers curled from y-axis to x-axis

I have been asked this question in examination.

The origin of the question is Moment of Inertia for Composite Areas topic in section Moments of Inertia of Engineering Mechanics

Answer 1

Correct choice is (a) Thumb is z-axis, fingers curled from x-axis to y-axis

To explain I would say: As right handed coordinate system means that you are curling your fingers from positive x-axis towards y-axis and the thumb which is projected is pointed to the positive z-axis. Thus visualizing the same and knowing the basic members of axis will not create many problems.