Question

A needle of length $5cm$ is floating on water of surface tension $0.07J{m}^{-2}$ and it is not wetted by water. Then the weight of the needle will be:

• A. $7\times {10}^{3}N$
• B. $7\times {10}^{-3}N$
• C. $5.2\times {10}^{-2}N$
• D. $4\times {10}^{-2}N$

Answer 1

Answer: (B)

$7\times {10}^{-3}N$

The needle will not be wetted by water, if its weight is balance by surface Tension force
$\Rightarrow T\left(2l\right)=W$
$\Rightarrow 0.07\times 5\times {10}^{-2}\times 2=W$
$\Rightarrow W=7\times {10}^{-3}N$