Answer: 2T/r.
- A 2T/r
- B 4T/r
- C T/r
- D T/2r
Correct answer: A. 2T/r
Explanation: A liquid droplet has one surface (unlike a soap bubble with two). Excess pressure inside = 2T/r, where T is surface tension and r is the radius of the droplet.
Since the same volume of fluid must pass every cross-section per second (continuity), the fluid speeds up where the pipe narrows; Bernoulli's equation then says this faster-moving fluid has lower pressure - the principle behind a venturi meter and an aircraft wing's lift.
Concept context
Pressure, buoyancy, Bernoulli equation, viscosity, and surface tension.