question_answer

A disc of radius a and mass m with small thickness, is suspended through a thread in air with the disc in horizontal plane. When the same set-up is placed above a liquid such that the plane of disc just makes contact with liquid surface, then the tension in thread is doubled. The surface tension of liquid is

A)  \[S=\frac{mg}{a}\]                         

B)  \[S=\frac{mg}{2\pi a}\]

C)  \[S=\frac{mg}{8\pi a}\]                

D)  None of the above

Correct Answer: B

Solution :

According to condition, \[T'=2mg=mg+S\times 2\pi a\]  \[\Rightarrow \,\,\,\,s=\left( \frac{mg}{2\pi a} \right)\]