Sample 1Medium
A horizontal pipe is drawn narrowing from section 1, of cross-section 4 cm², to section 2, of cross-section 1 cm². Water in streamline flow enters section 1 at 2 m/s. What is the speed at section 2, and how does the pressure there compare with section 1?
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The answer is B.
Continuity gives A₁v₁ = A₂v₂, so v₂ = 4 × 2 / 1 = 8 m/s. Bernoulli's principle for a horizontal pipe gives P + ½ρv² constant, so the pressure is lower where the speed is higher.
- A.
Continuity makes the speed rise, not fall, where the pipe narrows.
- C.
Higher speed means lower pressure in a horizontal pipe, not higher.
- D.
An incompressible fluid must speed up through a smaller cross-section.
NCERT: Class 11 Physics, Chapter 9