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.
Answers
(a) When we blow over the paper, the velocity of air blow increases and hence pressure of air on it decreases (according to Beroulli's Theorem), where as pressure of air blow the paper is atmospheric. Hence, the paper stays horizontal.
(b) By ddoing so the area of outlet of water jet is reduced, so velocity of water increases according to equation of continuity, Area × Velocity = Constant.
(c) The small opening of a syringe needle controls the velocity of the blood flowing out. This is because of the equation of continuity. At the constriction point of the syringe system, the flow rate suddenly increases to a high value for a constant thumb pressure applied.
(d) When a fluid flows out from a small hole in a vessel, the vessel receives a backward thrust. A fluid flowing out from a small hole has a large velocity according to the equation of continuity:
Area × Velocity = Constant
According to the law of conservation of momentum, the vessel attains a backward velocity because there are no external forces acting on the system.
(e) A spinning cricket ball has two simultaneous motions – rotatory and linear. These two types of motion oppose the effect of each other. This decreases the velocity of air flowing below the ball. Hence, the pressure on the upper side of the ball becomes lesser than that on the lower side. An upward force acts upon the ball. Therefore, the ball takes a curved path. It does not follow a parabolic path
Answer:
Explanation:
(a) according to Bernoulli theorem , for horizontal flow of fluid ,
P + 1/2dv² = constant
Therefore , when velocity of a fluid increases , it's pressure decreases and vice-versa.
When we blow over a piece of paper , the velocity of air above the paper increases . so, in accordance with Bernoulli theorem , the pressure of air decreases above the paper . due to the pressure difference of air between below and above the paper a lifting force act on paper and hence it remian horizontal.
(b) According to equation of continuity for steady flow of liquid the product of area of cross-section of the tube and velocity of liquid remains constant at each point . i.e.,
A1v1 = A2v2
Now, when we try to close a water tap with our fingers , the area of CSA of the outlet of water jet is reduced due to the restriction provided by the fingera and therefore , the velocity of water increases and fast jets of water gush through the openings between our fingers .
(c) according to Bernoulli's theorem,
P + 1/2 dv² = constant .so, the size of the needle controls the velocity of flow and the thumb pressure controls pressure. Therefore , size of the needle of a syringe controls flow rate better than thumb pressure.
(d) fluid flowing out of a small hole in a vessel have a large velocity and therefore a large momentum.As no external force is acting therefore according to law of conservation of momentum equal momentum in opposite direction and hence, a backward velocity is attained by the vessel. Therefore a backward thrust act on the vessel.
(e) a spinning cricket ball in air does not follow a parabolic trajectory due to magnus effect.