A needle of 12 cm long is on the surface of water. The extra force required, over its weight, to take it out from the surface of water is (surface tension = 72 dyne cm^-1)

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0.0173N

0.864N

7.2N

72N

answer is C.

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Detailed Solution

$\begin{array}{l} surface tensional force acts on the needle circumference i . e, \\ F = (2 ℓ) (T) \\ = 2 (12) (10^{- 2}) (72 \times \frac{10^{- 5}}{10^{- 2}}) = 0 .0173 N \end{array}$

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