Question

The force required to take away a flat circular plate of radius $5 \,cm$ on the surface of water, will be : (surface tension of water $ =75\times {{10}^{-3}}N/m $ )

• A. $3.5\, N$
• B. $7.5 \, \pi $ $ \times {{10}^{-3}}N $
• C. $15 \, \pi $ $ \times {{10}^{-3}}N $
• D. none of these

Answer 1

Answer: (B)

Using the relation, $ F=TL=T\times 2\pi R $
Here, $ R=5cm=5\times {{10}^{-2}}m $
$ T=75\times {{10}^{-3}}N/m $ or
$ F=75\times {{10}^{-3}}\times 2\times \pi \times 5\times {{10}^{-2}} $
So, $ F=7.5\pi \times {{10}^{-3}}N $