Question

If a section a soap bubble of radius r by a plane through its center is considered, the force on one half due to surface tension is then

• A. $2\pi rT$
• B. $4\pi rT$
• C. $\pi r^{2}T$
• D. $2rT$

Answer 1

The correct option is B $4\pi rT$

The $T$ force point to the left and act all along the circular edge of the hemispherical film the magnitude of the force due to each surface of the film is the product of the $T$ and the circumference $\left(2\pi R\right)$ of the circle edge, $T\left(2\pi R\right)$

Total force $=2T\left(2\pi R\right)$

$=4\pi RT$