Question

The force required to drag a circular flat plate of radius $5cm$ on the surface of water is : (Surface Tension of water is $75dyne/cm$)

• A. $75dyne$
• B. $750dyne$
• C. $75\pi dyne$
• D. $750\pi dyne$

Answer 1

The correct option is D $750\pi dyne$

As surface tension is tangential force act on the circumference only because other than the circumference rest part is in regular touch with water

so force required to drag$=$force due to surface tension

$F=s\times 2\pi r$

r$=$radius of disk$=5$cm

$s=75$ dyne/cm given

$F=75\times 2\times \pi \times 5$

$F=750\pi$ dyne.