The $SI$ unit of $\frac{G}{g}$ is $(g = \text{acceleration due to gravity}, G = \text{gravitational constant})$

$A$ uniform sphere of mass $M$ and radius $R$ is surrounded by a concentric spherical shell of the same mass but radius $2R$. $A$ point mass $m$ is kept at a distance $x$ $(R < x < 2R)$ in the region bounded by the spheres as shown in the figure. The net gravitational force on the particle is:

Two spheres of radius $R$ are placed in contact as shown in the figure. If the mass density of each sphere is $\rho$,find the gravitational force between them.

Two point objects of mass $2x$ and $3x$ are separated by a distance $r$. Keeping the distance fixed,how much mass should be transferred from $3x$ to $2x$,so that the gravitational force between them becomes maximum?

$A$ black hole is a:

The force of mutual attraction between any two objects by virtue of their masses is