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. \(2\pi r \cos\theta - W\)
• C. \(2\pi r T \cos\theta + W\)
• D. \(W - 2\pi r T \cos\theta\)

Hint:

Always analyze the geometry of the "bend" to determine the direction of the surface tension force. Since the disc bends the surface downwards, the elastic nature of the water surface tries to pull it upwards to restore flatness.

Answer 1

The Correct Option is C