Question

11. A circular wire frame of radius R is floating on the
surface of liquid of surface tension S. The force
required to just pull out the frame from the liquid is
[NCERT Pg. 266]
(1) 2πRS
(2) 4πRS
(3) πRS
(4) 3πRS
​

Answer 1

Answer:

surface tension=force/length

Therefore,

S=F/circumference of circle

F=2πRS.

Answer 2

Given:

The radius of the circular wire frame = R

The surface tension of the liquid = S

To Find:

The force  required to just pull out the frame from the liquid is

Solution:

Surface tension is the tension due to the surface film of a liquid. It is caused due to the attraction of the particles in the surface layer.

It tends to minimize the surface area.

According to the formula,

Surface Tension = Tension Force / Length along which the force acts

or S = F$_s_t$ / L

For a circular frame, L= The circumference of the frame = 2 π r

The minimum force required to pull the frame will be equal to the net downward force due to the liquid = F$_s_t$

F$_s_t$ = S X 2 π r

Hence, the  force  required to just pull out the frame from the liquid is (1) 2πRS.