Question

A ring of radius r and weight W is lying on a liquid surface. If the surface tension of the liquid is T, then the minimum force required to be applied in order to lift he ring up-

• A. W
• B. 2W
• C. $W+4\pi rT$
• D. $W+2\pi rT$

Answer 1

Answer: (C)

$W+4\pi rT$

When we lift the ring, its own weight is pointing in the downward direction.

Also, the force due to surface tension points in the direction opposite to motion, therefore in the downward direction

Hence, the minimum force required to lift the ring,

F = $W+T\times circumferenceofring$

So, F = $W+T\times 4\pi r$ (because 2 surfaces are involved)

Therefore, option D