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
❤️___hEya mATE ___❤️
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)
Equation of continuity-
It states that velocity of fluid flow is inversely proportional to area of the opening.
Area × Velocity = constant
Here,
When we try to close the tap with fingers, the area of the opening decreases to just between gaps between fingers.
When we try to close the tap with fingers, the area of the opening decreases to just between gaps between fingers. As area decreases, velocity of water flow increases and it gushes out through small gaps between fingers.
c)
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)
The area through which water is flowing out of the hole in the vessel is small. According to equation of continuity ,. Area × velocity = constant
e)
The velocity of the air below the ball is decreased because these two motions are opposing each other. Thus, the pressure on the upper side of the ball is lesser and there is an upward force that works upon the ball. This makes the ball follow a curved path and not a parabolic one.
Answer:
❤️
(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.