Question

The length of a rubber cord floating on water is 5 cm. The force needed to pull the cord out of water is N
(surface tension of water is $7.2\times {10}^{-4}N{m}^{-1}$).

• A. $7.2\times {10}^{-3}$
• B. $7.2\times {10}^{-4}$
• C. $7.2\times {10}^{-5}$
• D. $7.2\times {10}^{-2}$

Answer 1

The correct option is C $7.2\times {10}^{-5}$

Surface Tension is acting on both side of cord , so

$F=2\times Tl$

$F=2\times 7.2\times {10}^{-4}\times 0.05$

$F=7.2\times {10}^{-5}N$

Hence option C is correct.