A thin flat circular disc of radius 4.5 cm is placed gently over the surface of water. If surface tension of water is 0.07 N m-1, then the excess force required to take it away from the surface is
- 19.8 mN
- 198 N
- 1.98 mN
- 99 N
Answer
(1) 19.8 mN
Solution
The water surface pulls the disc down along its circumference, so the excess force is
F = T 2 r
= 0.07 2 3.14 0.045
= 1.98 10-2 N = 19.8 mN
Answer: (1)