What is the ratio of the gravitational force between two masses kept at a certain distance in air to the force between them when kept in any other medium?

State whether the following statement is True or False:
All objects attract each other along the line joining their centre of mass.

Why is the value of $g$ more at the poles than at the equator?

Three students $A, B$ and $C$ were performing an experiment to establish the relation between the weight of a wooden block and the minimum force required to just move it on a horizontal surface. The experimental setup used by each is as shown in the figures given below.
The result obtained by them will be:

$A$ ball is dropped from a height of $20\, m$. $A$ second ball is thrown downwards from the same height after one second with initial velocity $u$. If both the balls reach the ground at the same time, calculate the initial velocity of the second ball. (Take $g = 10\, m s^{-2}$).

$A$ stone is released from the top of a tower of height $19.6 \, m$. Calculate its velocity just before touching the ground.