Explain why
(a). To keep a piece of paper horizontal, you should blow over, not under, it
(b). When w 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.

Explanation

(a) When a piece of paper is blown over it, according to Bernoulli.s principle the velocity of air over the paper increases and the pressure over it decreases whereas the presgure below the paper remains same as the atmospheric pressure and therefore, the paper remains horizontal.
(b) By closing a water tap with our fingers the area of opening decreases and according to equation of continuity, the velocity of water flow will increase.
(c) According to the equation of continuity `A_(1) v_1)=A_(2)v_(2)`. The area of cross section where the pressure is applied by the doctor is much more than that of needle. The velocity of the fluid through the needle is much more than that inside the syringe. Now, according to Bernoulli.s theorem `P + pgh +1/2 pv^(2)` constant. As in this equation, P occurs with single power while velocity occurs with square power. Thus, the velocity being high and also raised to the power 2 has more influence as compared to the pressure. Thus, the size of needle has a better control over flow rate then the thumb pressure exerted by the doctor.
(d) When a liquid is flowing through a small hole of a vessel, it acquirea very large velocity and also momentum. In the absence of external force, the vessel should attain a velocity in the direction opposite to the flow to conserve the linear momentum. Hence, a backward thrust Gimpulse) is experienced by the vessel.
(e) A spinning cricket ball would follow a parabolic trajectory in the absence of air. But as the ball is spinning in the air, therefore, due to magnus effect of air, the ball deviates from the parabolic trajectory.