Problem 1.47

A solid body starts rotating about a stationary axis with an angular acceleration $\beta=a t,$ where $a=2.0 \cdot 10^{-2} \mathrm{rad} / \mathrm{s}^{3} .$ How soon after the beginning of rotation will the total acceleration vector of an arbitrary point of the body form an angle $\alpha=60^{\circ}$ with its velocity vector?