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
Explanations:
(a) To keep a piece of paper horizontal, you should blow over, not under, it
Blowing over the paper reduces the air pressure above it, as described by Bernoulli’s principle. The higher pressure below the paper pushes it upward, counteracting gravity and keeping the paper horizontal. Blowing underneath would increase the pressure, causing the paper to lift unevenly or fall.
(b) When we try to close a water tap with our fingers, fast jets of water gush through the openings between our fingers
Reducing the outlet area increases the velocity of water according to the principle of continuity (flow rate = area × velocity). As the area decreases, the velocity must increase to maintain a constant flow rate, resulting in fast jets of water through the openings.
(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
The flow rate of a liquid through the needle is governed by Poiseuille’s law, which states that the flow rate is proportional to the fourth power of the radius of the needle. Even a small change in the needle's radius has a significant effect on the flow rate, making it a more critical factor than thumb pressure, which influences the flow rate linearly.
(d) A fluid flowing out of a small hole in a vessel results in a backward thrust on the vessel
This is a consequence of Newton's third law of motion. When the fluid is expelled forward, it exerts an equal and opposite reaction force on the vessel. This backward thrust can also be explained by the principle of conservation of momentum.
(e) A spinning cricket ball in air does not follow a parabolic trajectory
A spinning cricket ball experiences the Magnus effect, where differences in air pressure caused by the spin alter its trajectory. The side spinning in the direction of the airflow moves faster, lowering the pressure (according to Bernoulli’s principle), while the opposite side has higher pressure. This asymmetric pressure creates a force perpendicular to the direction of motion, deviating the ball from a simple parabolic path.
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