A wire ring can be used to determine the surface tension of a liquid $($Fig$.2).$ Assume

that the contact angle is $=0,$ i$.$e$.,$ the air$/$water interface is oriented in the vertical

direction at the point of contact with the wire. For a wire ring with inner diameter

$D=50mm,$ determine the force needed to lift the ring through an air$/$water interface

$(=0\mathrm{.}073\frac{N}{m}).$ You can neglect the weight of the ring and the thin diameter of the

wire.

FIG. $2.$ Cross$-$section of ring.