Question

The length of needle floating on the surface of water is $2\text{cm}$. Find the force in addition to its weight required to lift needle from the water surface. (Surface tension of water $=7.5\text{N/cm}$)

• A. $28\text{N}$
• B. $22\text{N}$
• C. $15\text{N}$
• D. $30\text{N}$

Answer 1

The correct option is D $30\text{N}$


Needle will be attracted from both sides along its length by the water molecules.
Length of needle, $l=2\text{cm}$
$\therefore$ Length of needle on which surface tension is acting, $L=2\times l=2\times 2\text{cm}=4\text{cm}$
$\Rightarrow$Force experienced due to surface tension $\left(T\right)$ is expressed as,
$F=T\times \left(2l\right)$
$F=7.5\text{N/cm}\times 4\text{cm}$
$\therefore F=30\text{N}$