Question

A ring is cut from a platinum tube $8.5cm$ internal and $8.7cm$ external diameter. It is supported horizontally from the pan of a balance, so that it comes in contact with the water in a glass vessel. If an extra $3.103$ g.f. is required to pull it away from water, the surface tension of water is

• A. $72dyn/cm$
• B. $70.80dyn/cm$
• C. $63.35dyn/cm$
• D. $60dyn/cm$

Answer 1

Answer: (A)

$72dyn/cm$

$(2\pi r_1+2\pi r_2)\sigma =mg$

$(2\pi \frac{8.7}{2}+2\pi \frac{8.5}{2})\sigma =3.97\times 980$

$\sigma =72dynec{m}_1$