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

Answer 1

The correct option is A $7.2\times {10}^{-3}N$

Surface Tension is acting on both side of cord.
$\therefore F=7.2\times {10}^{-4}\times 2\times \left(.05\right)$
$=7.2\times {10}^{-5}N$