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Q9.4: NCERT Physics - Part 2 MECHANICAL PROPERTIES OF FLUIDS 9.4 Solution: Explain why (a) To keep a piece of paper horizontal, you should …
⚠️ Common Exam Traps & Conceptual Pitfalls
Students frequently assume that blowing on any side of a surface will push it away, overlooking that according to Bernoulli’s principle the side with higher airflow speed has lower pressure; thus blowing over the paper reduces the pressure above, not below, keeping it horizontal. Always determine which region has the greater velocity before assigning the pressure change.
Explain why (a) To keep a piece of paper horizontal, you should blow over, not under, it (b) When we try to close a water tap with our fingers, fast jets of water gush through the openings between our fingers (c) The size of the needle of a syringe controls flow rate better than the thumb pressure exerted by a doctor while administering an injection (d) A fluid flowing out of a small hole in a vessel results in a backward thrust on the vessel (e) A spinning cricket ball in air does not follow a parabolic trajectory
Step-by-Step Verified Solution
**Answer:** (a) Bernoulli's principle. (b) Equation of continuity. (c) Velocity term has higher power in Bernoulli's equation. (d) Conservation of momentum. (e) Magnus effect.
**Step-by-Step Explanation:**
(a) Blowing over the paper reduces the air pressure above it, while the pressure below remains atmospheric. The higher pressure below keeps the paper horizontal [citation:31].
(b) When the tap is partially closed with fingers, the area of the opening decreases. By the equation of continuity (A_v_ = A_v_), the velocity of water increases, causing fast jets to gush out [citation:31].
(c) In Bernoulli's equation, pressure P occurs to the first power, while velocity v occurs to the second power. Hence, a small change in the needle size (which controls velocity) has a much larger effect on flow rate than thumb pressure [citation:31].
(d) The fluid flowing out of the hole carries forward momentum. By conservation of momentum, the vessel experiences an equal and opposite backward momentum (recoil) [citation:31].
(e) A spinning ball experiences a sideways force due to the Magnus effect (difference in air pressure on opposite sides caused by the spin). This makes its trajectory curved rather than parabolic [citation:31].
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