Question

The length of a needle floating on water is $2.5cm$. The minimum force in addition to ties weight needed to lift the needle above the surface of water will be (surface tension of water is $0.072N/m$)

• A. $3.6\times {10}^{-3}N$
• B. ${10}^{-2}N$
• C. $9\times {10}^{-4}N$
• D. $6\times {10}^{-4}N$

Answer 1

The correct option is A $3.6\times {10}^{-3}N$

One will have to overcome the downward surface tension force in addition to its weight.
$F_T=2Tl\because \text{There are two contact surfaces}\Rightarrow F_T=2\times 0.072\times 2.5\times {10}^{-2}\Rightarrow F_T=3.6\times {10}^{-3}N$
Note: Arrow in the diagram shows surface tension force.