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MHT CET 2021 (22 Sep, Shift 2) · Force due to Surface Tension

Force due to Surface Tension question from MHT CET 2021 (22 Sep, Shift 2)

Q15MHT CET 2021 · 22 SEP · SHIFT 2Medium

Force due to Surface Tension

A needle is $7\text{ cm}$ long. Assuming that the needle is not wetted by water, what is the weight of the needle, so that it floats on water?

$[T =$ surface tension of water $= 70\,\frac{\text{dyne}}{\text{cm}}]$

[acceleration due to gravity $= 980\text{ cm s}^{-2}$ ]

Show answer and solution

Answer: (a)

Surface tension acts along both sides of the needle: $F = 2TL$ .
$F = 2 \times 70 \times 7 = 980\text{ dyne}$ .
Weight $= \dfrac{980}{980} = 1\text{ g wt}$ . So the answer is (a).

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