physics questions and answer

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Short Questions

Chapter No.6

Fluid Dynamics

F.Sc 1st partBy Muzammal Safdar:-

Q.No. 6.1:- Explain what do you understand by the term viscosity?

Ans:- From the microscopic study of flowing fluid we know that fluid flows whenthe layers of fluid slid over on another. But there exists another opposing forcebetween the layers of fluid which wants to stop this sliding. This force is called theviscosity.

In short we can say that viscosity is the opposing force (frictional effects betweenthe relative motion of layers of fluid) between the layers of fluid which will slow down

the flow of fluid or stop it if it is very large.

Q.No. 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?

Ans:- When an object moving through a fluid then it will experience aretarding(opposing) force called drag force. This force increase as the speed of theobject increases in the fluid. The drag force F is given by Stockes Law

6F rv=

The factors upon which drag force depends are:(i) Speed of the sphere(ii) Radius of the sphere

(iii) Coefficient of viscosity

Q.No. 6.3:- Why fog droplets appear to the suspended in air?

Ans:- Let suppose that the fog droplet is just like a small sphere. So when it flowsthrough the air (which is fluid) then droplet will experience a retarding force of themagnitude

6F rv=

As the droplet is very small so its radius is small and as a whole the drag force will besmall. The drag force will be equal to its weight which is also very small. So the droplet will soon attains a terminal velocity which is

22

9t

grv

=

As terminal velocity also depends upon radius so the droplet will fall very slowly andit will appear to be suspended in air.

Q.No. 6.4:- Explain the difference between laminar flow and turbulent

flow.

Ans:-


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Laminar Flow:- If every particle of the fluid that passes a point moves along thesame path, as followed by particles, which passed that point earlier is called laminarflow.

Turbulent Flow:-The irregular or unsteady flow of the fluid is called turbulent flow.In this case the exact path of the particles of the fluid cannot by predicted as thevelocity of the fluid may change abruptly.

Q.No.6.5:- State Bernoullis relation for a liquid in motion and describe

some of its applications.

Ans:-Bernoullis equation is the fundamental equation in fluid dynamics whichstates that the sum of pressure and kinetic and potential energies per unit volume isa steady flow of an incompressible and non viscous liquid has the same value.

Thus;21

2P v gh = + = Constant

Where is the density of the fluid and g is the acceleration due to gravity.Some of the applications of Bernoullis equation are

(i) Lift in an airplane.(ii) Swing of a cricket ball

(iii) Working of a filter pump

(iv) Measurement of speed of liquid or gas flow through a pipe.

Q.No. 6.6:- A person is standing near a fast moving train. Is there any

danger that he will fall towards it?

Ans:-From the application of Bernoullis equation we have come to know that theplace where the speed of fluid is high there the pressure will be low and vice versa.So, a person standing near a fast moving train will obviously in danger. Becausewhen a train moving fast then speed of air near the train will be high and away fromthe person will be low. So this pressure difference will pull the man toward the fastmoving train.

Q.No. 6.7:- Identify the correct answer. What do you infer from

Bernoullis 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 liquid.

Ans:-The correct statement is (i) where the speed of the fluid is high the pressurewill be low.

Q.No. 6.8:- Two rowboats moving parallel in the same direction are

pulled towards each other. Explain.

Ans:-When two boats moving parallel in the same direction, the water betweenthem is also dragged along with them. When water dragged then the speed of waterwill be fast, than the water on the other sides of the boats. Hence the pressure will below wherethe speed is high and vice versa.

So, the high pressure at the other sides will pull the boats to the side where the

pressure is low. Hence as a result the boats will pull toward each other.

Q.No. 6.9:- Explain how the swing is produced in a fast moving cricket

ball?


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Ans:- When a fast moving cricket ball moves in such a way that it spins as well asmoves forward, then the air deflects due to the spin and unsmooth surface of ball thestream lines of air becomes closer to each other and results the increase of air speedand low pressure. On one side of ball there exists low pressure while on other sidehigh. This change in pressure will cause the ball to lift. This will give an extracurvature to the ball known as swing, which deceives the batsman.

Extra Elucidation

Often Bernoulli's principle is used to explain the topspin effect, as the difference in speed between ballsurface and air is greateron the top of the ball. For example, if the air flowing past the bottom of the ball ismoving faster than the air flowing past the top then Bernoulli's principle implies that the pressure on thesurfaces of the ball will be lower below than above. In other words, since there is more air friction occurring

on the top surface of the ball compared to the bottom, this differential causes a greater pressure to beapplied on the top of the ball, resulting in the ball being pushed down.In ball sports, topspin is a property of a ball that rotates as if rolling in the same direction as it is moving.Topspin on a shot imparts a downward force that causes the ball to drop, due to its interaction with the air. Itcan be generated by hitting the ball with an up-and-forward swing, with the racquet facing below thedirection it is moving. A topspin shot is the opposite of the slice; topspin itself is the opposite of backspin

Q.No. 6.10:- Explain the working of carburetors of a motorcar using

Bernoullis equation.

Ans:- The carburetor of a car engine uses a Venturi duct to feed the correct mix ofair and petrol to the cylinders. Air is drawn through the duct and along a pipe to thecylinders. A tiny inlet at the side of the duct is fed with petrol. The air through theduct moves very fast, creating low pressure in the duct, which draws petrol vapors

into the air stream.

Extra Elucidation

A venturi is a tube with a convex taper, (one end wider than the other) . As air enters wider end it ' s squeezed into the narrower section of the tube, lowering the air ' s pressurearea of lowest pressure is just past the narrowest point and is called the depression. Thalways seemedcounterintui t ive to me, but Bernoull i ' s Principle outlines the flu id dynamicsinvolved in this effec t. This l owered pressure, or comparative vacuum is separate f rom theengine vacuum. A variab le venturi varies the venturi diameter at the depressionby raising or lowering an obstruction. This obstruction is called a slide. On a CV the slideis called a piston or diaphragm valve.


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Q.No.6.11:- For which 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 ones head?

Ans:- When a man lying horizontally then in this case the heart will work quietsmall. Consider the lying position, in this case all the parts are in level with the heart.

When the blood circulates then every part of blood vessels make an angle of 90degree with the earth surface. And the cos90 is zero(0) so the work done by heartagainst gravity becomes zero( or approximately negligible).

Q.No.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?

Ans:-Pressure is just like a force so to overcome a force we should exert a forcewhich is equal but opposite. In case of orbiting space station where is the conditionof weightlessness, so the blood pressure will be equal in all parts of body.

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