Two spheres , each of radius r are touching each other . The force of attraction between them is proportional to

Options

A. `r^(6)`
B. `r^(4)`
C. `r^(2)`
D. `r^(-2)`

Answer

B

Explanation

`F = (Gm_(1) m_(2))/( d^(2)) `, where d = 2 r or `F = (G 4 pi r^(3) rho)/( 3) xx (4 pi r^(3) rho)/( 3 xx (2 r)^(2))`
or ` F = (G xx 4 pi ^(2) rho^(2) r^(4))/(9) or F prop r^(4)`