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

Asked by Pragya Singh | 1 year ago |  73

##### Solution :-

(a) If we blow over a piece of paper, velocity of air above the paper becomes more than that below it. As K.E. of air above the paper increases, so in accordance with Bernoulli’s theorem its pressure energy and hence its pressure decreases.

Due to greater value of pressure below the piece of paper = atmospheric  pressure, it remains horizontal and does not fall.

(b) As per the equation of continuity area × velocity = constant. When we try to close a water tap with our fingers, the area of cross-section of the outlet of water jet is reduced considerably as the openings between our fingers provide constriction  (regions of smaller area)
Thus, velocity of water increases greatly and fast jets of water come through the openings between our fingers.

(c) The size of the needle controls the velocity of flow and the thumb pressure controls pressure.

According to the Bernoulli’s theorem  $$P + \dfrac{ 1 }{ 2 } pv^{ 2 }$$ = Constant

In this equation, the pressure P occurs with a single power whereas the velocity occurs with a square power. Therefore, the velocity has more effect compared to the pressure. It is for this reason that needle of the syringe controls flow rate better than the thumb pressure exerted by the doctor.

(d) This is because of principle of conservation of momentum. While the flowing fluid carries forward momentum, the vessel gets a backward momentum.

(e) A spinning cricket ball would have followed a parabolic trajectory has there been no air. But because of air the Magnus effect takes place. Due to the Magnus effect the spinning cricket ball deviates from its parabolic trajectory.

Answered by Abhisek | 1 year ago

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