Ans. Viscosity: “The property of fluids by which they resist their flow due to the internal friction”. Explanation: Different layers of a flowing fluid have frictional effect, as one layer slide over another. Viscosity measures the force required due to these inter-layer frictional effect.
Q.2 What is meant by drag force? What are the factors upon which drag force acting upon a small sphere of radius r, moving down through liquid, depend?
Ans. Drag force: “The retarding force experienced, when an object move through a fluid”. Factors: According to Stoke’s Law, the drag force, F is; F = 6πη rv ⇒ F depends upon η = coefficient of viscosity, r = radius of the sphere v = speed of the sphere through the fluid. This drag force increases as the speed of the object increases.
Q.3 Why fog droplets appear to be suspended in air?
Ans. Due to drag force. As fog droplet falls, soon its weight becomes equal to the drag force. And net force becomes zero. So it appears to be suspended in air.
Q.4 Explain the difference between laminar flow and turbulent flow.
Ans. Laminar flow: “Smooth sliding of layers of fluid past each other”. Turbulent flow: “Disorderly and changing flow pattern of fluids” Difference: In laminar flow, every particle that passes a particular point, moves along exactly same path, as followed by particles, which passed that points earlier. But in turbulent flow, the exact path of the particle cannot be predicted due to irregular and unsteady flow.
Q.5 State Bernoulli’s relation to a liquid in motion and describe some of its applications.
Ans. Bernoulli’s equation: In the steady frictionless motion of a fluid acted on by external forces which posses a gravitational potential ρgh , then P + ½ ρ v2 + ρgh = constant where P and ρ are the pressure and density of the fluid, v is the velocity of the fluid along a streamline. Applications: a) Torricelli’s Theorem: from Bernoulli’s relation, P1 + ½ ρ v12 + ρgh1 = P2 + ½ ρ v22 + ρgh2 for v1 << v2 & P1 = P2 = P = atmospheric pressure P + ρgh1 = P + ½ ρ v22 + ρgh2 ⇒ v2 = √2g (h1 – h2 ) i.e. “The speed of efflux is equal to the velocity gained by the fluid in falling through the distance (h1 – h2 ) under action of gravity”.
b) Venturi Relation: We have P1 + ½ ρ v12 + ρgh1 = P2 + ½ ρ v22 + ρgh2 taking v2 >> v1 (= 0), for horizontal pipes h1 = h2 = h P1 + ρgh = P2 + ½ ρ v22 + ρgh or P1 - P2 = ½ ρ v22 the above Venturi relation used to measure speed of liquid flow. c) Relating speed & pressure: We have PA + ½ ρ vA2 + ρghA = PB + ½ ρ vB2 + ρghB For horizontal flow, hA = hB = h so PA + ½ ρ vA2 + ρgh = PB + ½ ρ vB2 + ρgh or PA + ½ ρ vA2 = PB + ½ ρ vB2 ⇒ where the speed is high, the pressure will be low, e.g. i) spinning of tennis ball, ii) swing of cricket ball iii) designing of aeroplane wing. Other application are: a) checking human blood pressure b) functioning a filter pump c) working of perfume or paint sprayer d) working of a chimney for smoke exhaust.
Q.6 A person is standing near a fast moving train. Is there any danger that he will fall towards it?
Ans. Yes. He will fall towards the train. As the relative speed of air is high, the pressure will be low. So the greater air pressure behind the person will push him towards low pressure.
Q.7 Identify the correct answer. What do you infer from Bernoulli’s theorem?
(i) Where the speed of the fluid is high the pressure will be low.
(ii) Where the speed of the fluid is high the pressure is also high.
(iii) This theorem is valid only for turbulent flow of the fluid.
Ans. The correct answer is (i) , where the speed of the fluid is high the pressure will be low. Inference: Please see, answer 6.5—relating speed and pressure.
Q.8 Two row boats moving parallel in the same direction are pulled towards each other. Explain.
Ans. Relative speed of water and air between the boats is high, the pressure will be low, so both boats pulled towards each other.
Q.9 Explain, how the swing is produced in a fast moving cricket ball.
Ans. The velocity of the air on one side of the ball increases due to spin and air speed in the same direction and so pressure decreases. This gives swing to the ball.
Q.10 Explain the working of a carburetor of a motor car using Bernoulli’s principle.
Ans. Carburetor: “An apparatus used to charge air with gas from petrol for producing light or power”. Air is drawn outward through small pipe with a piston. High velocity of air produces low pressure. So petrol-air mixture is drawn inside.
Q.11 For which position will the maximum blood pressure in the body have the smallestvalue. (a) Standing up right (b) Sitting (c) Lying horizontally (d) Standing on one’s head?
Ans. (c) lying horizontally, position will have smallest value of maximum blood pressure in the body have the smallest value. In this position all parts of the body are nearly in level with the heart.
Q.12 In an orbiting space station, would the blood pressure in major arteries in the leg
ever be greater than the blood pressure in major arteries in the neck?
Ans. No. Due to lack of force of gravity, (as we use to experience on the earth) The blood pressure in major arteries in the leg will be equal than in arteries in the neck, due to weightlessness.
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