Try this first
Answer: C. C is correct. NCERT Class 11 Physics, Chapter 9, page 187 says that beyond a limiting value, called the critical speed, steady flow loses steadiness and becomes turbulent.
A is wrong: A is wrong because terminal speed is a different idea, the constant speed of a body falling through a viscous fluid; it is not the limit of steady flow.
B is wrong: B is wrong because escape speed belongs to gravitation and has nothing to do with whether a flow is steady.
D is wrong: D is wrong because drift speed belongs to electric current in a conductor, not to fluid flow.
The trap is thinking that a faster flow is a smoother one. NCERT says the opposite: the flow stays steady only up to a limit.
When a water tap is turned on slowly the water flow is smooth, but it loses its smoothness when the speed of the outflow is increased (NCERT Class 11 Physics, Chapter 9, page 186).
NCERT states it in one sentence: steady flow is achieved at low flow speeds, and beyond a limiting value, called the critical speed, the flow loses steadiness and becomes turbulent. The example it gives is a fast flowing stream meeting rocks, where small foamy whirlpool-like regions called white water rapids form (NCERT Class 11 Physics, Chapter 9, page 187). The syllabus calls this limit the critical velocity; NCERT's own word is critical speed.
How can a flow reach the limit without the tap or the pump being changed? By narrowing the tube. For an incompressible fluid in steady flow, A v = constant, so a smaller cross-section means a higher speed (NCERT Class 11 Physics, Chapter 9, page 187). If the speed in the narrow part goes past the critical speed, the flow there stops being steady.
The 2026-27 NCERT text gives no formula for the critical speed and no value for any fluid. Questions on this topic therefore give the critical speed as data, or ask only which side of it the flow is on.
Watch-out: below the critical speed the flow is steady, above it the flow is turbulent. Compare the actual speed with the limit, not with zero.
Given
Water flows steadily through a pipe whose critical speed is 3.0 m/s. In the wide section the cross-section is 5.0 cm² and the speed is 1.2 m/s. The pipe narrows to a section of cross-section 1.5 cm².
Required
Whether the flow in the narrow section is still steady.
Concept
Narrowing the tube raises the speed, because A v is constant for an incompressible fluid in steady flow (NCERT Class 11 Physics, Chapter 9, page 187). Steady flow is lost once the speed goes beyond the critical speed (NCERT Class 11 Physics, Chapter 9, page 187).
Formula
A₁v₁ = A₂v₂, then compare v₂ with the critical speed.
Substitution
v₂ = A₁v₁ / A₂ = (5.0 cm² × 1.2 m/s) / 1.5 cm².
Calculation
v₂ = 6.0 / 1.5 = 4.0 m/s. The areas are used as a ratio, so the unit cm² cancels. The given values carry two significant figures, and the result does too. Then 4.0 m/s is greater than the critical speed 3.0 m/s.
Final answer
The speed in the narrow section is 4.0 m/s, beyond the critical speed of 3.0 m/s, so the flow there is no longer steady and becomes turbulent. The flow in the wide section, at 1.2 m/s, stays steady.
Common trap
Checking only the starting speed. The wide section at 1.2 m/s is below the limit, but the narrow section is where the speed rises, and that is where the flow can go turbulent.
Similar NEET-style question
A pipe has critical speed 2.0 m/s. Water at 0.60 m/s in a section of cross-section 8.0 cm² passes into a section of cross-section 2.0 cm². Is the flow in the narrow section steady? (Answer: A v = constant gives v₂ = 0.60 × 8.0 / 2.0 = 2.4 m/s. Since 2.4 m/s is greater than 2.0 m/s, the flow there is turbulent, not steady.)
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Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
What does NCERT call the limiting value of flow speed beyond which a steady flow loses its steadiness and becomes turbulent?
Answer: C. C is correct. NCERT Class 11 Physics, Chapter 9, page 187 says that beyond a limiting value, called the critical speed, steady flow loses steadiness and becomes turbulent.
Why A is wrong: A is wrong because terminal speed is a different idea, the constant speed of a body falling through a viscous fluid; it is not the limit of steady flow.
Why B is wrong: B is wrong because escape speed belongs to gravitation and has nothing to do with whether a flow is steady.
Why D is wrong: D is wrong because drift speed belongs to electric current in a conductor, not to fluid flow.
According to NCERT, at which flow speeds is a steady flow achieved?
Answer: A. A is correct. NCERT Class 11 Physics, Chapter 9, page 187 states that steady flow is achieved at low flow speeds.
Why B is wrong: B is wrong because high speeds, beyond the critical speed, are where the flow loses steadiness and becomes turbulent (trap: a faster flow is a smoother flow).
Why C is wrong: C is wrong because NCERT ties steadiness to the speed being low; an incompressible fluid can still become turbulent past the critical speed.
Why D is wrong: D is wrong because the critical speed is the limit where steadiness is lost, not a speed at which steady flow is achieved.
In NCERT's example, what forms when a fast flowing stream encounters rocks?
Answer: D. D is correct. NCERT Class 11 Physics, Chapter 9, page 187 gives the white water rapids of a fast stream meeting rocks as the sight of turbulence.
Why A is wrong: A is wrong because parallel streamlines describe steady, laminar flow, which the fast stream has lost beyond the critical speed.
Why B is wrong: B is wrong because the stream keeps flowing; what it loses is its steadiness.
Why C is wrong: C is wrong because straight, parallel streamlines are the sign of steady flow, the opposite of whirlpool-like regions.
A student opens a tap slowly and sees a smooth stream. She then opens it further and the stream turns ragged. What does this show?
Answer: B. B is correct. NCERT Class 11 Physics, Chapter 9, page 186 describes exactly this: the flow is smooth when the tap is turned on slowly and loses its smoothness when the outflow speed is increased, and page 187 names the limit the critical speed.
Why A is wrong: A is wrong because nothing in the observation changes the water's compressibility; the change is in the flow speed.
Why C is wrong: C is wrong because opening the tap further makes the outflow faster, not slower, and a faster outflow is what loses smoothness.
Why D is wrong: D is wrong because falling below the critical speed would make the flow steadier, not more ragged (trap: flow above the critical speed stays steady).
For a certain pipe the critical speed is given as 3.0 m/s. Water flows through it at 2.0 m/s. How would the flow be described?
Answer: D. D is correct. NCERT Class 11 Physics, Chapter 9, page 187 says steady flow is achieved at low speeds and is lost only beyond the critical speed; 2.0 m/s is below 3.0 m/s.
Why A is wrong: A is wrong because NCERT says low flow speeds give steady flow; turbulence needs the speed to pass the critical speed.
Why B is wrong: B is wrong because being close to the limit is not enough; turbulence sets in only beyond the limit.
Why C is wrong: C is wrong because the flow does not stop at the critical speed; it loses steadiness and becomes turbulent beyond it.
A mountain stream moves slowly through a wide pool and then fast over a rocky stretch. Where is the flow more likely to be turbulent?
Answer: A. A is correct. NCERT Class 11 Physics, Chapter 9, page 187 says a fast flowing stream meeting rocks forms white water rapids, because beyond the critical speed the flow becomes turbulent.
Why B is wrong: B is wrong because the slow flow in the pool is the low-speed case, where NCERT page 187 says steady flow is achieved.
Why C is wrong: C is wrong because turbulence depends on the flow speed relative to the critical speed, not only on the fluid.
Why D is wrong: D is wrong because streamlines cannot cross only in steady flow; the fast, turbulent stretch has lost the steady pattern.
The critical speed for a pipe is given as 4.0 m/s. Water flows at 1.2 m/s through a section of cross-section 5.0 cm² and then enters a section of cross-section 1.2 cm². What is the speed in the narrow section, and is the flow there steady?
Answer: B. B is correct. A v = constant (NCERT Class 11 Physics, Chapter 9, page 187) gives v = 1.2 × 5.0 / 1.2 = 5.0 m/s. That is beyond the critical speed of 4.0 m/s, so the flow loses steadiness and becomes turbulent (page 187).
Why A is wrong: A is wrong because 0.29 m/s multiplies by the area ratio the wrong way round; the narrower section must be faster.
Why C is wrong: C is wrong because the speed is right but the conclusion is not: 5.0 m/s is beyond the critical speed of 4.0 m/s, so the flow is not steady.
Why D is wrong: D is wrong because the speed changes when the cross-section changes; only the product A v stays constant.
Water flows at 0.60 m/s in a section of cross-section 6.0 cm². The critical speed is given as 3.0 m/s. What is the smallest cross-section to which the pipe can be narrowed while the flow stays below the critical speed?
Answer: C. C is correct. With A v = constant (NCERT Class 11 Physics, Chapter 9, page 187), the speed reaches 3.0 m/s when A = 6.0 × 0.60 / 3.0 = 1.2 cm². A narrower section would push the speed past the critical speed (page 187).
Why A is wrong: A is wrong because 3.6 cm² is only 6.0 × 0.60; it was never divided by the critical speed.
Why B is wrong: B is wrong because 0.20 is the speed ratio 0.60 / 3.0; the area factor 6.0 cm² was left out.
Why D is wrong: D is wrong because 30 cm² inverts the ratio; a larger area would slow the flow, not bring it to the critical speed.
The critical speed for a pipe is given as 4.0 m/s. Water flows at 1.5 m/s through a section of cross-section 8.0 cm² and then enters a section of cross-section 2.0 cm². What is the speed in the narrow section, and is the flow there steady?
Answer: A. A is correct: A v = constant gives v = 8.0 × 1.5 ÷ 2.0 = 6.0 m/s, which is beyond the critical speed of 4.0 m/s, so the flow becomes turbulent. Based on NCERT Class 11 Physics, Chapter 9, page 187.
Why B is wrong: B is wrong because 6.0 m/s exceeds the critical speed, and beyond it the flow loses steadiness.
Why C is wrong: C is wrong because 0.38 m/s inverts the ratio of the areas; the narrow section has the higher speed.
Why D is wrong: D is wrong because 3.0 m/s multiplies 1.5 m/s by 2, which is not the area ratio of 4.
Water flows at 0.50 m/s in a section of cross-section 10 cm². The critical speed is given as 2.0 m/s. If the pipe narrows to 4.0 cm², what is the speed there, and is the flow steady?
Answer: A. A is correct: A v = constant gives v = 10 × 0.50 ÷ 4.0 = 1.25 m/s, which is below the critical speed of 2.0 m/s, so the flow stays steady. Based on NCERT Class 11 Physics, Chapter 9, page 187.
Why B is wrong: B is wrong because 1.25 m/s is below the critical speed, so the flow has not become turbulent.
Why C is wrong: C is wrong because 0.20 m/s inverts the ratio of the areas; the narrower section carries the faster flow.
Why D is wrong: D is wrong because 5.0 m/s forgets to divide by the new area of 4.0 cm².
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When a water tap is turned on slowly, the water flow is smooth initially, but loses its smoothness when the speed of the outflow is increased.
-- NCERT Class 11 Physics, Ch. 9, p. 186which is called the equation of continuity and it is a statement of conservation of mass in flow of incompressible fluids. In general Av = constant (9.11) Av gives the volume flux or flow rate and remains constant throughout the pipe of flow. Thus, at narrower portions where the streamlines are closely spaced, velocity increases and its vice versa.
-- NCERT Class 11 Physics, Ch. 9, p. 187Steady flow is achieved at low flow speeds. Beyond a limiting value, called critical speed, this flow loses steadiness and becomes turbulent. One sees this when a fast flowing stream encounters rocks, small foamy whirlpool-like regions called ‘white water rapids are formed.
-- NCERT Class 11 Physics, Ch. 9, p. 187No question in our NEET 2020–2025 set targets this topic directly.
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