Question

A thin metal disc of radius $r$ floats on water surface and bends the surface downwards along the perimeter making an angle $\theta$ with vertical edge of the disc. If the disc displaces a weight of water $W$ and surface tension of water is $T$, then the weight of metal disc is

• A. $2 \pi r T+W$
• B. $2 \pi r T \cos \theta-W$
• C. $2 \pi r T \cos \theta+W$
• D. $W-2 \pi r T \cos \theta$

Answer 1

Answer: (C)

Weight of metal disc $=$ total upward force upthrust force + force due to surface tension
= weight of displaced water $+T \cos \theta(2 \pi r)$
$=W+2 \pi r T \cos \theta$