JEE Main & Advanced
Sample Paper
JEE Main Sample Paper-2
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, \[T'=2mg=mg+S\times 2\pi a\] \[\Rightarrow \,\,\,\,s=\left( \frac{mg}{2\pi a} \right)\]