\(g' = \frac{3}{5}g\) (because mass remains constant)
\(g' = g - {\omega ^2}R{\cos ^2}\lambda \) \(⇒\) \(\frac{3}{5}g = g - {\omega ^2}R{\cos ^2}(0^\circ )\)
\(⇒\) \({\omega ^2} = \frac{{2g}}{{5R}}\) \(⇒\) \(\omega = \sqrt {\frac{{2g}}{{5R}}} = \sqrt {\frac{{2 \times 10}}{{5 \times 6400 \times {{10}^3}}}} \) = \(7.8 \times {10^{ - 4}}\frac{{rad}}{{\sec }}\)