A needle is $7 \mathrm{~cm}$ long. Assuming that the needle is not wetted by water, what is the weight of…
A needle is $7 \mathrm{~cm}$ long. Assuming that the needle is not wetted by water, what is the weight of the needle, so that floats on water?
$\left[\mathrm{T}=\text { surface tension of water }=70 \frac{\mathrm{dyne}}{\mathrm{cm}}\right]$
[acceleration due to gravity $=980 \mathrm{~cm} \mathrm{~s}^{-2}$]
1 g wt
5 g wt
3 g wt
7 g wt
Solution
The weight of the needle is balanced by the force due to surface tension
$\begin{aligned}
& \mathrm{W}=2 \mathrm{TL} \\
& =2 \times 70 \times 7 \\
& =980 \mathrm{dyne} \\
& =1 \mathrm{~g} \mathrm{~wt}
\end{aligned}$