## FSc Notes: Physics XI: Chapter 06 Fluid Dynamics Exercise Short Question:

**Question 6.1 Explain what do you understand by the term viscosity?**

Answer 6.1

Answer 6.1

**Viscosity**means how much force is required to slide one layer over another in a fluid. Substances that do flow easily like honey or tar have large co-efficient of viscosity. On the other hand fluid those flows easily like water have low value of co-efficient of viscosities.

**Question 6.2 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 a liquid, depend?**

Answer 6.2An object moving through a fluid experience a retarding force which opposes its motion is known as

Answer 6.2

**drag force**.

**By Stoke’ Law its eq is: F = 6πηrv**

**Velocity:**Drag force is directly proportional to the velocity of the body in the fluid.

**Radius:**Drag force also depends upon the radius of the body.

**Nature of Fluid:**It also depends on the co-efficient of viscosity of the fluid. i.e. nature of fluid.

**Question 6.3 Why fog droplets appear to be suspended in air?**

Answer 6.3As we know that:

Answer 6.3

**vt=mg/6πηr.**

As mass of the fog droplet is very small so its terminal velocity will be very small. For this reason fog droplet appears to be suspended in air.

**Question 6.4 Explain the difference between laminar flow and turbulent flow?**

Answer 6.4The flow is said to be laminar if every particle that passes a particular point moves along exactly the same path as followed by particles which passed that points earlier. i.e. the traffic on motorway. The irregular or unsteady flow of the fluid is called turbulent flow. i.e. as a driver changes line randomly on a road.

Answer 6.4

**Question 6.5 State Bernoulli’s relation for a liquid in motion and describe some of its applications?**

Answer 6.5

Answer 6.5

**Bernoulli’s relation P + ½ ρ v2 + ρgh = constant**

**Torricelli’s Theorem:**The speed of efflux is equal to the velocity gained by the fluid in falling through the distance (h1-h2) under the action of gravity.

**v2 = √2g (h1 – h2)**

**Venturi Relation:**The effect of the decrease in pressure with the increase in speed of the fluids in a horizontal pipe is known a venturi relation.

**P1 - P2 = ½ ρ v2(2).**

**Question 6.6 A person is standing near a fast moving train. Is there any danger that he will fall towards it?**

Answer 6.6From Bernoulli's Theorem. Where speed is faster streamlines are forced together and pressure will be low there. Between train and a person streamlines are close due to high speed but pressure exerted by fluid will be low. On the other hand streamlines are further and pressure will be greater. Therefore there will be a danger that a person will fall towards a fast moving train.

Answer 6.6

**Question 6.7 Identify the correct answer. What do you infer from Bernoulli's theorem?**

- Where the speed of the fluid is high the pressure will be low.
- Where the speed of the fluid is high the pressure is also high.
- The theorem is valid only for turbulent flow of the liquids.

**Answer 6.7**Correct answer is

**(i) Where the speed of the fluid is high the pressure will be low**.

**Question 6.8 Two row boats moving parallel in the same direction are pulled towards each other. Explain?**

Answer 6.8According to Torricelli Theorem Where the speed is high, the pressure will be low. Between row boats velocity is high and pressure is low outside row boats velocity is low but pressure is high so boats are pulled towards each other.

Answer 6.8

**Question 6.9 Explain, how the swing is produced in a fast moving cricket ball?**

Answer 6.9When a bowler bowls a cricket ball it spins and moves forward. The velocity of air on the shiny Surface is greater than on the dull surface and pressure is low on the shiny surface and greater on the dull surface. This gives an extra curvature to the ball which is known as swing which may deceive the batsman.

Answer 6.9

**Question 6.10 Explain the working of a carburetor of a motorcar using by Bernoulli's principle.**

Answer 6.10The carburetor of the car uses venturi duct to feed the correct mix of air and petrol to the cylinders. Air is drawn through the duct and along a pipe to the cylinders. A tiny inlet at the side of the duct is fed with petrol. The air through the duct moves very fast, creating low pressure in the duct, which draws petrol vapors into the air stream.

Answer 6.10

**Question 6.11 For which position will the maximum blood pressure in the body have the smallest values?**

- Standing up right.
- Sitting.
- Lying horizontally.
- Standing on one’s head.

**Answer 6.11**The maximum blood pressure in the body has the smallest value when body is lying horizontally

**Question 6.12 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?**

Answer 6.12 No. Due to lack of force of gravity, the blood pressure in major arteries in the leg will be equal to that in arteries of the neck, due to weightlessness.

Answer 6.12 No

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