Question

A disc of paper of radius $R$ has a hole of radius $r$. It is floating on the surface of a liquid of surface tension ${}^{\prime }{T}^{\prime }$. Then force of surface tension on the disc is:

• A. $T.2\pi R$
• B. $T.2\pi (R-r)$
• C. $T.2\pi (R+r)$
• D. $T.4\pi (R+r)$

Answer 1

Answer: (B)

$T.2\pi (R-r)$

Inward force = $-T2\pi r$ (consider outward direction of force as positive)

outward force = $T2\pi R$

Net Force $=-T\left(2\pi r\right)+T\left(2\pi R\right)$

$=2\pi T(R-r)$

Option B is correct.