Question

A metal disc of radius $r$ floats on the surface of water . The water layer goes down and makes an angle $\theta$ with the vertical edge of the disc. If it displaces a weight $w$ of water and the surface tension of water if $T$ ,then the weight of disc is

• A. $2\pi rTcos\theta$
• B. $2\pi rT$
• C. $2\pi rTcos\theta +w$
• D. $2\pi rTcos\theta -w$

Answer 1

The correct option is C $2\pi rTcos\theta +w$

When water is depressed downwards at point of contact,then the force due to surface tension acts upwards.
Therefore,apparent weight
$=F_r+w$
$2\pi rTcos\theta +w$