question_answer 4)

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, the fast jest of water gust through the openings between our fingers. (c) A fluid flowing out of a small hole in a vessel, results in a backward thrust on the vessel. (d) The size of the needle of a syringe controls flow better than the thumb pressure exerted by a doctor while administering an injection. (e) A spinning cricket ball in air does not follow a parabolic trajectory.

Answer:

(a) When we blow over the paper, the velocity of air blow increases and hence pressure of air on it decreases (according to Bernoullis Theorem), where as pressure of air below the paper is atmospheric. Hence, the paper stays horizontal. (b) By doing so the area of outlet of water jet is reduced, so velocity of water increases according to equation of continuity, \[\frac{2S\cos \theta }{r\rho g}=\frac{2\times 0\cdot 465\times \cos {{140}^{o}}}{{{10}^{-3}}\times 13\cdot 6\times {{10}^{3}}\times 9\cdot 8}\] (c) When a fluid is flowing out of a small hokle in a vessel, it acquires a large velocity and hence possesses large momentum. Since no external force is acting on the system, a backward velocity must be attained by the vessel (according to low of conservation of momentum). As a result of it, an impulse (backward thrust) is experienced by the vessel is experienced by the vessel. (d) Here, size of the needle controls velocity of flow and thumb pressure controls pressure. According to Bernoullis theorem, \[\frac{2\times 0\cdot 465\times \left( -0\cdot 7660 \right)}{{{10}^{-3}}\times 13\cdot 6\times {{10}^{3}}\times 9\cdot 8}=-5\cdot 34\times {{10}^{-3}}m=-5\cdot 34mm\]constant shows that p occurs with power one and\[\cdot \] occurs with power two, hence the velocity has more influence. That is why the needle has a better control over flow. (e) See Art. 7(c). 16 part (4). [i.e. curved motion of spinning ball].