A uniform thin rod of mass m and length R is placed normally on surface of earth as shown. The mass of earth is M and its radius is R. If the magnitude of gravitational force exerted by earth on the rod is `(etaGMm)/(12R^(2))` then `eta` is

Explanation

`F=int_(R)^(2R)(GM((pm)/(R))dx)/(x^(2))=(GMm)/(2R^(2))`