Two objects are placed at a certain distance from each other in air. If they are placed at the same distance in water,what is the ratio of the gravitational force between them?

Two identical spheres each of radius $r$ are placed in contact with each other. Show that the gravitational force between them is proportional to $r^4$.

Statement $(I)$: The force of attraction due to a hollow spherical shell of uniform density on a point mass situated inside it is always zero.
Statement $(II)$: The force of attraction between a hollow spherical shell of uniform density and a point mass situated outside is the same as if the entire mass of the shell were concentrated at the center of the shell.
Which of the following is correct?

The distance between two objects $A$ and $B$ is $r$. The gravitational force between them is inversely proportional to the square of the distance. If the gravitational force between these two objects were inversely proportional to the fourth power of the distance,find the acceleration of object $A$.

Let $G, W, E$ and $S$ be the relative strengths of gravitational, weak-nuclear, electromagnetic, and strong-nuclear forces, respectively. Which of the following statements is correct?

The distance between the centre of the moon and the earth is $D$ and the mass of the earth is $81$ times the mass of the moon. At what distance from the centre of the earth will the net gravitational force on an object be zero?