Question

The length of a needle floating on water is 2.5 cm. How much minimum force, in addition to the weight of the needle will be needed to lift the needle above the surface of water? Surface tension of water is $7.2\times {10}^{-4}N/cm$ .

• A. $7.2\times {10}^{-4}N$
• B. $18.0\times {10}^{-4}N$
• C. $36.0\times {10}^{-4}N$
• D. none of the above

Answer 1

Answer: (C)

: Surface tension $=\frac{Force}{2(length)}$ $\therefore$ $7.2\times {10}^{-4}\frac{N}{cm}=\frac{Force}{2(2.5)cm}$ $\therefore$ Force $=7.2\times {10}^{-4}\times 2.5\times 2N$ $=36.0\times {10}^{-4}N$