Question

What is the angle between angular acceleration and radius vector?

Answer 1

I may not sure (never been asked this before) but I can share my thoughts.

Explanation:

I think we are discussing circular motion, so the acceleration must be centripetal. This means the angle between the two vectors must be `180^@` or `pi` rad.

i.e. they are in opposite directions assuming that radius is measured with its positive axis outwards.