Two light balls are suspended as shown in figure. When a stream of air passes through the space between them, the distance between the balls will

The excess of pressure, due to surface tension, on a spherical liquid drop of radius ' $R$ ' is proportional to

Two small drops of mercury each of radius ' $R$ ' coalesce to form a large single drop. The ratio of the total surface energies before and after the change is

In air, a charged soap bubble of radius ' $R$ ' breaks into 27 small soap bubbles of equal radius ' $r$ '. Then the ratio of mechanical force acting per unit area of big soap bubble to that of a small soap bubble is