Answer: Surface tension (adhesive forces between water and glass).
- A Surface tension (adhesive forces between water and glass)
- B Atmospheric pressure acting only inside the narrow tube
- C Viscous drag dragging water along the tube walls
- D Gravitational attraction pulling water upward into the tube
Correct answer: A. Surface tension (adhesive forces between water and glass)
Explanation: Water wets glass (adhesive force > cohesive force). Surface tension causes the meniscus to be concave. The pressure difference across this curved surface pushes water up.
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.