The outer shell will have no contribution in the gravitational field at point P
`:. E_(P) = (Gm_(1))/(r^(2))`
Thus, force on mass `m_(2)` placed at P is, ltbr? `F = m_(3)E_(P)`
or `F=(Gm_(1)m_(3))/(r^(2))`
The field `E_(P)` and the force F both are towards centre O.