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PhysicsNEET 2024Surface tension, capillarity, excess pressure

Mechanical Properties of Fluids

A thin flat circular disc of radius 4.5 cm is placed gently over the surface of water. If the surface tension of water is 0.07 N m−10.07\ \mathrm{N\,m^{-1}}, then the excess force required to take it away from the surface is:

  1. A.198 N
  2. B.1.98 mN
  3. C.99 N
  4. D.19.8 mN✓

Solution

A disc lifted off a liquid surface is held by surface tension along its full circumference: F=T×2πr=0.07×2π×0.045≈1.98×10−2F = T \times 2\pi r = 0.07 \times 2\pi \times 0.045 \approx 1.98 \times 10^{-2} N =19.8= 19.8 mN.

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