Discussion
When a drop of liquid at the tip of a capillary is balanced by surface tensional forces, its weight is equal to :
Answer: B
The weight of the drop equals 2πrγ.
The drop hangs from the tip of the capillary, held by surface tension acting along the circumference of the tip. Surface tension γ is a force per unit length, and the length of the contact line is the circumference 2πr. At the moment of detachment the upward force just balances the weight (Tate's law):
\[ mg = 2\pi r\gamma \]
This is the basis of the drop-weight (stalagmometer) method. πr2γ uses an area where a length is needed, and the other two forms are not dimensionally a force.
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