Exercise Short Questions of Fluid Dynamics, Chapter 6 of 1st Year Physics

Q # 1. Explain what do you know about the term `Viscosity'?

Ans. The term viscosity is used for the degree of internal friction in the fluid. This internal friction is associated with the resistance between two adjacent layers of the fluid moving relative to each other. It is denoted by the Greek word \(\eta\).

 Q # 2. What is meant by drag force? What are the factors upon which the drag force acting upon a small sphere of radius r, moving down through a liquid, depends?

 Ans. An object moving through a fluid experience a retarding force called the drag force.

 The drag force on a sphere depends upon the velocity of object in a fluid, i.e., the drag force increases as the speed of the particle increases.

 Q # 3. Why fog droplets appear to be suspended in air?

 Ans. When the magnitude of the drag force on the fog droplet becomes equal to its weight, the net force acting on the droplet is zero.

 In such a case, the droplet starts falling with very small terminal speed and appears to be suspended in air.

Q # 4. Explain what the difference between laminar and turbulent flow.}

 Laminar Flow

 The flow is said to be streamline or laminar, if every particle that passes a particular point moves along exactly the same path, as followed by particles which passed through that points earlier.

 Turbulent Flow

 The irregular or unsteady flow of the fluid is called turbulent flow.

 Q # 5.State Bernoulli's relation for a liquid in motion and describe some of its applications?

 Ans.  The principle states that the sum of pressure, the kinetic and potential energy per unit volume for an ideal fluid remains constant at every point of its path.

 Mathematically, it is described as:

\[P_{} + \frac{1}{2}\rho v_{}^{2} + \rho gh_{} = constant\]

 Where P is the pressure, v is the velocity and \(\rho\) is density of the fluid.

 The Bernoulli relation is important in nozzle design and in flow measurements.

Q # 6. A person is standing near a fast moving train. Is there any danger that he will fall towards it?

 Ans. When a person is standing near a fast moving train, then the air between them is also fast. According to Bernoulli, where the speed is high, pressure will be low. So the pressure between the person and train will be low as compared to the pressure of side way. So there will be a chance of force acting on the person from high pressure region to the low pressure region and that he may fall towards the train.

Q # 7. Identify the correct answer. What do you infer from Bernoulli's theorem?

1. Where the speed of the fluid is high the pressure will be low.
2. Where the speed of the fluid is high the pressure is also high.
3. This theorem is valid only for turbulent flow of the liquid.

Ans.  The correct answer is (1) where the speed of the fluid is high the pressure will be low.

 This is in accordance with the Bernoulli's equation, which states for a horizontal pipe,

\[P_{1} + \frac{1}{2}\rho v_{1}^{2} = P_{2} + \frac{1}{2}\rho v_{2}^{2}\]

Q # 8.Two row boats moving parallel in the same direction are pulled towards each other. Explain?

Ans. When two boats are moving parallel in the same direction, then the water between them is also flowing fast. According to Bernoulli, where the speed is high, pressure will be low. So the pressure between the two boats decreases as compared to the pressure of side way. So the side way high pressure pushes the two boats towards each other.

Q # 9. Explain, how the swing is produced in a fast moving cricket ball?

Ans.  When the cricket ball is thrown in such a way that is spins as well as moves forward, the velocity of the air on one side of the ball increases due to the spins and hence the pressure decreases.

This gives an extra curvature to the ball known as swing which deceives opponent player.

Q # 10. Explain the working of a carburetor of a motor car using Bernoulli's principal.

Ans. The petrol tank is attached with Carburetor's pipe through a very small inlet. Air moves very fast through this pipe. As a result, pressure in the pipe decreases as compared to the pressure in the petrol tank which is atmospheric pressure. So the petrol moves from the tank to the air pipe (i.e., from high pressure to low pressure), and a correct mixture of petrol and air reaches the engine.

Q # 11. For what position will the maximum blood pressure in the body have the smallest value (a) Standing up right (b) Sitting (c) Lying Horizontally (d)Standing on one's head?

Ans.  The option (c) is correct. The blood pressure will have the smallest value when a person lying horizontally.

Q # 12. In orbiting space station, would the blood pressure in the major arteries in the leg ever be greater than the blood pressure in major arteries in the neck.

Ans.  In an orbiting space station, due to weightlessness, the blood pressure in the major arteries in the leg will be equal to the blood pressure in major arteries in the neck.