find the distance between two stones each of mass 2 kg so that the gravitational force between them is 1n.

Newton's Law of Gravitation states that the gravitational force between any two objects is the product of their masses divided by the distance between their centres of mass squared all multiplied by the gravitational constant.

 Or: F = (Gm1m2)/r²

 So, we know that the gravitational constant is 6.67x10^-11 and that F = 1 and m1 and m2 = 2

 So, 1 = (6.67x10^-11 x 2 x 2)/r²

 rearranging, we get r² = 6.67x10^-11 x 2 x 2

 therefore r² = 2.6692x10^-10 therefore r = 1.634x10^-5 m

Or about 16 billionths of a metre.