A small point of mass \( m \) is placed at a distance \( 2R \) from the centre \( 'O' \) of a big uniform solid sphere of mass \( M \) and radius \( R \). The gravitational force on \( 'm' \) due to \( M \) is \( F_1 \). A spherical part of radius \( \frac{R}{3} \) is removed from the big sphere as shown in the figure and the gravitational force on \( m \) due to remaining part of \( M \) is found to be \( F_2 \). The value of ratio \( F_1 : F_2 \) is:
