Physics - Fluid Question with Solution | TestHub

The force due to surface tension acts along the boundary of the object in contact with the liquid surface.

For a paper disc of radius $R$ with a circular hole of radius $r$ cut out, there are two boundaries in contact with the liquid:
1. The outer boundary of length $2\pi R$
2. The inner boundary of length $2\pi r$

The total length of the boundary in contact with the liquid is:
$L=2\pi R+2\pi r=2\pi (R+r)$

The total force on the disc due to surface tension is:
$F=T\times L$

$F=T\times 2\pi (R+r)=2\pi (R+r)T$

Answer: $2\pi (R+r)T$