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<i”, , MECHANICAL PROPERTIES OF FLUIDS, , this reason that the cars or planes are given such, shapes (known as stream-lined shapes) so that, air friction is minimum, Rather the movement, of air layers on the upper and lower side of, streamlined shaped body provides a lift which, helps in reducing the solid friction, resulting in, increasing the speed of the car,, , 51. Two row boats moving parallel to each other, , and near by, are pulled towards each other., Explain. (CBSE (D) 2011], , Sol. According to Bernoulli’s theorem, when, , velocity of liquid flow increases, the pressure, decreases and vice versa., , When two boats move in parallel directions, close to each other, the stream of water between, the boats is set into vigorous motion. As a result,, the pressure exerted by water in between the, boats becomes less than the pressure of water, outside the boats. Due to this difference of, pressures, the boats are pulled towards each, other., , 52. According to Bernoulli’s theorem the, , pressure of water should remain uniform in, a pipe of uniform radius. But actually it goes, on decreasing, why is it so ?, , Sol. When water flows through a pipe, work is done, , against the viscous force. This work done is, taken from the pressure energy. As a result of it,, the pressure of the water falls, with onward, movement of water in pipe. It is due to this, , QUESTIONS, , w, =, , 10/83, , reason that a house located at a large distance, from a water tank receives water at lower, pressure than those houses located closer to, water tank., , 53. Ifa small ping pong ball is placed ina vertical, , jet of water or air, it will rise to a certain, height above the nozzle and will be spinning, there., , Sol. If a small ping pong ball is placed in a vertical, , jet of water the high velocity of water jet takes, the ball upwards along with it through a certain, height. Due to high velocity of jet of water, the, pressure between the ball and the jet of water, decreases, i.e., becomes less than atmospheric, pressure. As the pressure on the other side of, ball is atmospheric pressure, therefore, this, pressure difference pushes the ball against the, jet and the ball remains suspended., , Now jet of water exerts a tangential force on, the surface of ball. Due to it, the ball spins., , In an emergency, the vacuum brake is used, to stop, the high speed train. How does this, vacuum brake work ?, , Sol. When vacuum brake is applied to stop the high, , speed train, the steam at high speed enters the, cylinder of vacuum brake. According to, Bemoulli’s principle, the pressure in the cylinder, decreases which lifts up the piston and inturn, lifts up the brake. Then the brake works and, train is stopped., , Very Short Answer, Short Answer & Long Answer., , , , VERY SHORT ANSWER QUESTIONS, , , , I. Pressure and Hydraulic lift, , 1. What is a fluid ? Show that fluid exerts a, thrust., , , , Ans, Fluid is the name given to a substance which, begins to flow when external force is applied, , on it. When the fluid is kept in a container, the, molecules of a fluid in random motion due to, their thermal velocities are constantly colliding, with the walls of the container and rebounding, from them. They suffer a change in momentum., Due to it, they transfer some momentum to the, wall. This momentum transfered to the wall, , perunit time by the molecules of fluid accounts:, , for the thrust of liquid on the wall of the, container,, , Carrying 1 mark, , 2. Pressure is a scalar or vector quantity., , Ans, Pressure is a scalar quantity because at one level, , inside the liquid, the pressure is exerted equally, in all directions, which shows that a definite, direction is not associated with hydrostatic, pressure., , 3. The bags and suitcases are provided with, broad handles, Why ?, , Ans. So that the small pressure is exerted while, , carrying them,, 4, What is 1 torr ?, , Ans. One torr is the unit of atmospheric pressure, , where 1 torr = one mm of mercury column., , "5. What is 1 bar ?, Ans. 1 bar is the unit of atmospheric pressure used, , for metreological purposes where 1 bar = 10° Pa., , Scanned with CamScanner
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10/84, , 6. Wecan cut an apple easity with a sharp knife, , as compared to a blunt knife. Explain why ?, , (HP Board 2009), , Ans. We know that the area of sharp edged knife is, , much less than the area of blunt edged knife., , Since pressure = force/area, therefore for the, , given force pressure will be more for the sharp, , edge than for the blunt edge knife. That is why, , it is easy to cut an apple with a sharp knife than, with a blunt knife., , 7. It is difficult for a man to walk bare footed, , on sand. Why ? How can this difficulty be, removed ?, , Ans. When a man walks bare footed on sand, the sand, yields under the weight of the man. The man, does not get proper reaction from sand which is, necessary for his walking. This difficulty can, be overcome by spreading the wooden board, on the sand. In this situation, the thrust on the, sand due to our weight will get spread over the, whole area of the board and hence a very small, Pressure is exerted on the sand. Due to it, the, sand does not yield., 8. It is difficult to stop bleeding from a cut in, human body at high altitude. Why ?, , Ans. Athigh altitude, the atmospheric pressure is low., There will be a greater pressure difference, between blood pressure and atmospheric, pressure. As a result of it, it is difficult to stop, bleeding from a cut in the body at high altitude., , 9. A beaker containing a liquid is kept inside a, big closed jar. If the air inside the jar is, continuously pumped out. What will be the, Pressure in the liquid near the bottom of the, liquid., , Ans. The pressure in a liquid near the bottom of the, liquid will decrease. It is so because the total, Pressure at a point near the bottom of liquid is, equal to the sum of the pressure due to column, of liquid and due to air inside the jar. When the, air inside the jar is pumped out, the pressure, due to air inside the jar decreases., , 10. Why is the dam of water reservoir thick at, the bottom ?, , Ans. Itis so because the pressure of water in reservoir, increases with depth., , 11. What Is the relation between millibar and, pascal., Ans, | millibar = 10-3 bar = 10-3 x 105 Pa = 100 Pa,, , 12. If a drop of water vapour is introduced in a, , mercury barometer, how will the barometric, height change ?, , Pradeef's Fundamental Physics (XI) MI, , Ans, The barometer height will decrease if a drop of, , water is introduced in a mercury barometer,, which is due to the pressure exerted by water, vapour., , , , II. Archimede’s Principle and law of floatation, , , , 13. What is buoyancy and centre of buoyancy ?, Ans. The upward thrust acting on the body immersed, , 14,, , Ans., , 15,, , Ans., , 16., , Ans., , 7, , in a liquid is called buoyant force or simply, buoyancy. The centre of buoyancy is the centre, of gravity of the displaced liquid by the body, when immersed in a liquid., , Under which condition (i) the centre of, buoyancy coincides with the center of gravity, (ii) the centre of buoyancy does not coincide, with the centre of gravity., , (i) For a solid body of uniform density,when, immersed in a liquid the centre of gravity, coincides with the centre of buoyancy. (ii) For, a solid body having different densities over, different parts, when immersed in a liquid its, centre of gravity does not coincide with the, centre of buoyancy., , An iceberg floats in water with about onetenths of its volume outside the water. What, is the fractional volume submerged for an, iceberg on a fresh water lake of a planet, whose gravity is five times that of earth ?, When an ice berg floats in water with about onetenths of its volume outside the water, the, fractional volume of ice berg submerged in, water is nine tenth of its volume. The fractional, volume submerged is independent of the value, of acceleration due to gravity, therefore, at the, new planet, the fractional volume of ice berg, submerged in water will be nine-tenths of its, volume., , Two pieces of cork 5 one small and another, big are pushed below the surface of water., Which has greater tendency to rise swifty ?, The upthrust on a piece of cork ‘is equal to the, weight of water displaced by the cork. As bigger, piece of cork will displace more weight of water, than small piece of cork, hence upthrust is, greater on the bigger piece of cork. As a result, of it, the bigger piece of cork has greater, tendency to rise swiftly,, A wooden cylinder floats in a vessel, containing water with its axis horizontal., How will the level of water in the vessel, , change if the cylinder floats in a vessel with, Its axis vertical ?, , W, , Scanned with CamScanner
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MECHANICAL PROPERTIES OF FLUIDS, , 10/85, , , , Ans. The level of water will remain unchanged in, both the cases because in each case the floating, cylinder will displace equal volume of water., , 18. A body is just floating in a liquid whose, density is equal to the density of body. What, happens to the body if it is slightly pressed, , and released ?, , Ans, When a body is just floating in a liquid, then, weight of the liquid displaced is equal to the, weight of the body. When the body is slightly, pressed, then the downward force on body due, to its body and pressure of liquid column up to, which body is pressed is greater than the upward, thrust on the body. As a result of it, the body, , will sink to the bottom of liquid., , 19. A boat carrying a large number of stones is, floating in a water tank. What will happen, to water level if the stones are unloaded into, , water ?, , Ans. When the stones are unloaded into water, the, water level falls because the volume of the, water displaced by stones in water will be less, than the volume of water displaced when stones, , are in the boat., , 20. A block of wood is floating in a lake. What is, , apparent weight of the floating block ?, , Ans. The apparent weight of the floating block is, equal to zero because the weight of the block, acting vertically downwards is balanced by the, buoyant force acting on the block upwards., , 21. Does Archimede’s principle hold in a vessel, , in free fall ?, , Ans. Archimede’s principle does not hold good in, this situation as the vessel in free fall is in a, condition of weightlessness, where the buoyant, force accounting for Archimede’s principle does, , not exist., , 22. A body floats in a liquid contained in a, beaker. The whole system shown in the Fig., 10(Q).9 falls freely under gravity. What is the, , upthrust on the body due to the liquid-?, , , , FIGURE 10(Q).9, , , , , , , , , , Ans,, , Ans., , Ans., , "26,, , Ans., , When the system falls freely under gravity, the, apparent weight of the body in the system, becomes zero. So the upthrust on the body due, to the liquid becomes zero., , . A plece of ice Is floating in a vessel containing, , water and inside the ice is a bubble of air., What will be the effect on the level of water,, , . when the ice melts ?, Ans., , The level of water remains the same. It is so, because for a floating ice, the weight of ice is, equal and opposite to the weight of the water, displaced by the immersed part of ice in water., When the ice melts, the water formed must be, of volume which was displaced by the immersed, part of the ice., , . A piece of ice is floating in a jar containing, , water. What will be the effect on the level of, water in jar, when ice melts and the, temperature of water falls from 4°C to 1°C., , The level of water in a jar rises, when the, whole ice will melt and temperature changes, from 4°C to 1°C. It is so because, the density of, water is maximum at 4°C and is less at 1°C. Due, to it, the given mass of the water in jar will, occupy more volume at 1°C than at 4°C., , . A block of ice is floating in a liquid of specific, , gravity 1-2 contained in the beaker. What will, be the effect on the level of liquid in the, beaker when the whole ice melts ?, , The level of water in a beaker will rise. It is, so because the density of water formed by, melting of ice is less than the density of liquid, in a beaker. Due to it, the volume of the water, formed by melting the ice will be more than the, volume of the portion of the ice in liquid while, floating in liquid., , Equal masses of two substances of densities, , _P; and p, are mixed together. What is the, , density of the mixture ?, Let m be the mass of each substance,, Total mass = m+m=2m, , Total volume = 2% 4% =m | Prt P2, Pp; Po Py Pz, , Density of mixture,, , , , , , ps total mass = 21 Py, total volume “1p, +P, ) 7 P, +P,, 4 P2, , Scanned with CamScanner
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r, , , , 10/86, , rn =. _, TIT. Surface tension and Surface energy, , 27. Mercury does not cling to glass. Why ?, , 5 (HP Board 2004), , Ans, It is because the force of adhesion between, mercury and glass molecules is less than the, , force of cohesion between mercury molecules., , 28. Why does the free surface of a liquid at rest, behave like a stretched elastic membrane ?, , Ans. All the molecules in the surface film of liquid, experience downward force which increases, upwards as we move from bottom of surface, film to its top. As a result of it all the molecules, in the surface film have potential energy which, Increases from bottom of surface film to its top., To be in stable equilibrium at rest, the surface, film must have minimum potential energy., While doing so, the free surface tends to contact, to minimise area and hence behaves like a, stretched elastic membrane,, , 29. When a drop is split into large number of, drops, the change in the total potential energy, of small drops and the potential energy of, the bigger drop is zero, positive or negative., , Ans. Positive, because on splitting a big drop into, small drops, the surface area increases and hence, the work is to be done in doing so, which, appears as potential energy of small drops., , 30. A number of small drops of mercury coalesce, isothermally to form a single drop. What will, be the effect on temperature of the drop ?, , Ans. Temperature of the drop remains unchanged,, , 31. What will the effect on the temperature, if, the number of small drops of mercury, coalesce adiabatically to form asingle drop ?, , Ans. When large number of drops coalesce, the, surface area will decrease. The energy will be, liberated. Due to adiabatic conditions, its, temperature will increase., , 32. What is the work done in blowing a soap, bubble of radius r‘and surface tension S ?, , Ans. Work done = S.T. x area of soap bubble, , =Sx(4nrP)x2=8nSr7, , 33. When a shaving brush is taken out of water, its hair cling together, why ? _——, , .. Due to surface tension, the water films forme, Ans between the hairs will tend to make the surface, area minimum. As a result of it, the hairs of, , shaving brush come closer to each other. ., , 34. When a greased iron needle is placed gently, , on the surface of water at rest, it floats on, the surface of water. Why ?, , Pradecfe's Fundamental Physics (X1) tepay, , Ans. Due to surface tension, the free surface of water, at rest behaves like a stretched membrane, When, an iron needle is placed gently on the surface of, water so that the needle does not prick the water, surface the upward components of the forces, of surface tension acting on the iron needle, balance its weight on a stretched membrane of, water surface. Then the needle floats on the, surface of water at rest., , 35. Surface tension of all lubricating oils ang, paints is kept low. Why ?, , Ans. In order to have low value of surface tension so, , that it can spread over large area., , In soldering, addition of flux makes soldering, , easily. Why ?, , Ans. The addition of flux reduces the surface tension, of molten tin, hence it spreads., , 37. Why are the droplets of mercury when, brought in contact pulled together to form a, bigger drop ? Also state with reasons whether, the temperature of this bigger drop will be, the ‘same, or more, or less than the, temperature of the smaller drop., , Ans. It is due to large cohesive forces acting between, the molecules of mercury that the droplets of, mercury when brought in contact pulled together, to form a bigger drop in order to make potential, energy minimum. The temperature of this bigger, drop increases since the total surface area, decreases., When bits of comphor are dropped on, water, they move helter-skelter. Why ?, Ans. When bits of camphor are dropped on water,, they lower the surface tension where they are, dipping. Since the shape of camphor bits is, uregular, unequal forces of surface tension act, on them. That is why they move helter-skelter, or erratically on the surface of water., , Why does oil spread over the surface of, , water ?, , Ans. Surface tension of oj] is less than that of water., When oil is dropped on the surface of water,, the force of surface tension of water being, larger than that of oil, stretches the oil drops, on all sides. Hence, the oil spreads over the, surface of water., , 40. It becomes easier to spray the water in which, some soap is dissolved, Why ?, , Ans. When some Soap is dissolved in water the, , surface tension of: water decreases. Now less, , energy is needed for Spraying the water. That is, , 36, , 38., , 39, , Scanned with CamScanner, , - ~
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MECHANICAL PROPERTIES OF FLUIDS, , why, it becomes easier to Spray the water in, which some soap is dissolved., , =, IV. Excess of pressure and Ascent formula, —_—— EEE EEE Ee, , 41., , Ans,, , 42., , Ans,, , 43,, , Ans., , 44,, , Ans., , 45., , Why excess pressure in a soap bubble is twice, the excess pressure of a liquid drop of the, same radius., , A soap bubble has two free surfaces ; One inner, surface and second outer surface which are, incontact with air, whereas the liquid drop has, only one free surface in contact with air., , The soap bubble formed at the end of the tube, is blown very slowly. Draw the graph, between excess of pressure inside the bubble, with time., , As excess of pressure inside the soap bubble of, , . | 4S 1, radiusris P=—, ie, px, ar r, ie., as r increases with time, p decreases., So, the variation of p with time (1) will be a, , parabolic curve AB as shown in Fig. 10(Q).10., , P FIGURE 10(Q).10, A, , , , , , , , t, , What will be the effect on the angle of contact, of a liquid if the temperature increases ?, , With the increase in temperature, the surface, tension of liquid decreases. Due to it, the liquid, surface on the solid surface becomes more flat,, , , , consequently the angle of contact of a liquid ., , increases with the increase in temperature., What is the effect of impurities on the surface, tension of liquid ?, , The presence of impurities either on the liquid, surface or dissolved in it, considerably affect, the force of surface tension, depending upon, the degree of contamination. A highly soluble, substance like sodium chloride when dissolved, in water, increases the surface tension of water., But the sparingly soluble substance like phenol, when dissolved in water, reduces the surface, tension of water., , When wax is rubbed on cloth, the cloth, becomes water proof ; Why ?, , Ans., , 46., , Ans,, , 47., , Ans,, , 48., , >, &, , 49., , Ans., , 50., , Ans,, , TEE ’=CS ooo, , 10/87, , The capillaries formed in threads disappear, when wax is rubbed on cloth., , What is the effect of solute on the surface, tension of liquid ?, , In case the solute is very easily soluble, then, the surface tension of liquid increases. For, example, when salt is dissolved in water, the, surface tension increases. If the solute is less, soluble, then the surface tension of liquid, decreases. For example, by adding soap or, phenol in water, its surface tension decreases., Water can rise up to a height of 10 cm ina, capillary tube. If a capillary of same diameter, but of length 6 cm is held vertically in water,, will the water come out in the form of a, fountain ? Explain., No. when the size of the capillary is less than, the height to which the water can rise, the radius, of curvature of water meniscus adjusts itself so, that there is no over flowing of water but the, product of height of liquid and radius of, curvature of the meniscus remains a finite, constant., A liquid of density p and surface tension S, rises to a height A in a capillary tube of, diameter D. What is the weight of the liquid, in the capillary tube ? Angle of contact is 0°., Weight of liquid in the capillary tube is, W=mg=nrhpg., , 2Scos8_ 25, TP& TPs, , We know that, h =, , , , & W=nr2pgx2e =mx2rxS, rps, , =nDS, , Put a piece of chalk into water. The chalk, will emit bubbles in all directions. Explain, this phenomenon., , A chalk has pores in all directions which act, as narrow capillaries. When a piece of chalk, is immersed in water, the water enters into, these capillaries and forces the air out in the, form of bubbles in water., , How is the rise of liquid affected if top of, the capillary is closed ?, , There will be a small rise in the capillary tube, if top of capillary tube is closed, because the, rise of liquid in capillary tube due to surface, tension will be opposed by the downward, force exerted by the compressed air above the, , Scanned with CamScanner
