Answer: Surface tension creating a curved water surface that supports the needle.
- A Surface tension creating a curved water surface that supports the needle
- B Ordinary buoyant force alone, as with any floating dense object in most textbook accounts
- C The needle being hollow and filled with trapped air inside during normal conditions
- D A pocket of air trapped directly underneath the solid needle as generally observed
Correct answer: A. Surface tension creating a curved water surface that supports the needle
Explanation: When gently placed, the needle rests on the water surface. Surface tension creates a dent in the surface, and the vertical component of the surface tension forces supports the needle weight. If the surface tension is broken, the needle sinks.
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.