Answer: 2TA (factor 2 for two surfaces).
- A 2TA (factor 2 for two surfaces)
- B TA, counting only a single surface of the soap film
- C 4TA, double-counting both surfaces of the film twice over
- D T/A, dividing the surface tension by the area instead of multiplying
Correct answer: A. 2TA (factor 2 for two surfaces)
Explanation: A soap film has two surfaces (front and back). Surface energy = T × total area = T × 2A = 2TA.
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.