Problem

A string of length 1 m is fixed at one end with a bob of mass 100 g and the string makes

\left(\frac{2}{\pi}\right) \text{ rev s}^{-1}

around a vertical axis through a fixed point. The angle of inclination of the string with vertical is a.

\tan^{-1}\left(\frac{5}{8}\right)

\tan^{-1}\left(\frac{3}{5}\right)

\cos^{-1}\left(\frac{3}{5}\right)

\cos^{-1}\left(\frac{5}{8}\right)