NCERT Solution (Class 11) > Physics Chapter 10 Mechanical Properties Of Fluids
NCERT Solution Class 11 Physics Chapter 10 Mechanical Properties Of Fluids: National Council of Educational Research and Training Class 11 Physics Chapter 10 Solution – Mechanical Properties Of Fluids. Free PDF Download facility available at our website.
Board |
NCERT |
Class |
11 |
Subject |
Physics |
Chapter |
10 |
Chapter Name |
Mechanical Properties Of Fluids |
Topic |
Exercise Solution |
Mechanical Properties Of Fluids Chapter all Questions and Numericals Solution
Chapter10- Mechanical Properties of Fluids
10.1) Explain why
(a) The blood pressure in humans is greater at the feet than at the brain
(b) Atmospheric pressure at a height of about 6 km decreases to nearly half of its value at the sea level, though the height of the atmosphere is more than 100 km (c) Hydrostatic pressure is a scalar quantity even though pressure is force divided by area.
Answer-
(a) The pressure of a liquid is given by
P = ρgh
Where,
P = Pressure
h = Height of the liquid column
ρ = Density of the liquid
g = Acceleration due to the gravity
from above formula It can be concluded that pressure is directly proportional to height of liquid column.
For feet height of the liquid column is more as compare to the brain hence the blood pressure in humans is greater at the feet than at the brain.
(b)The pressure of a air is given by
P = ρgh
Where,
P = Pressure
h = Height
ρ = Density of the air
g = Acceleration due to the gravity.
From above equation we can say that pressure directly proportional to height and density of air. Density of air is maximum near the sea level and it decreases as height increases. At a height of 6 km, density decreases drastically to half of its value. Hence, at a height of 6 km from the surface, pressure decreases to nearly half of its value at the sea level though the height of the atmosphere is more than 100 km.
(c)When force is applied on a liquid, the pressure in the liquid is transmitted in all directions means it does not have any fixed direction hence it is considered as scalar quantity even though pressure is force divided by area.
10.2) Explain why
(a) the angle of contact of mercury with glass is obtuse, while that of water with glass is acute.
(b) Water on a clean glass surface tends to spread out while mercury on the same surface tends to form drops. (Put differently, water wets glass while mercury does not.)
(c) Surface tension of a liquid is independent of the area of the surface.
(d) Water with detergent dissolved in it should have small angles of contact.
(e) A drop of liquid under no external forces is always spherical in shape.
Answer-
(a)
We consider the three interfacial tensions at all the three interfaces, liquid-air, solid-air and solid-liquid denoted by Sla, Ssa and Ssl, respectively as given in (a) and (b). At the line of contact, the surface forces between the three media must be in equilibrium.
For mercury with glass, Ssa < Sla. Therefore cos θ will be negative and hence θ is obtuse.
For water with glass, Ssl < Ssa. Therefore cos θ will be positive and hence θ is acute.
(b)Mercury molecules have a strong cohesive force between their molecules as compare to adhesive force between mercury molecule and glass surface. Hence mercury molecules try to come closer to each other away from solid surface and form the drop. On the other hand for water glass surface, there is strong adhesive force between water molecule and solid surface as compare to cohesive force between water molecules. Hence more molecules are going towards solid away from water molecules and spreads out on solid.
(c)Surface tension is the force acting per unit length at the interface between a liquid and other surface in contact. This is intermolecular force which is independent on the area of the surface. Hence, surface tension is also independent of the area of the liquid surface.
(d) We know that The capillary rise of a liquid (h) is directly proportional to the cosine of the angle of contact using formula,
h = (2s cosθ)/a
Where,
S = surface tension.
a = radius of capillary.
θ = angle of contact.
Detergents are used in water to help the penetration of water into the cloths so that dirt can be removed. To get more penetration (h) in small capillaries of the cloth, should be large and this is possible when the q will be small as possible hence the water with detergent should have small angle of contact.
(e)A liquid always try to acquire the minimum surface area due to surface tension. Out of all the figures surface area of a sphere is the minimum for a given equal volume. Hence under no external forces, liquid drops always take spherical shape.
10.3) Fill in the blanks using the word(s) from the list appended with each statement:
(a) Surface tension of liquids generally … with temperatures (increases / decreases)
(b) Viscosity of gases … with temperature, whereas viscosity of liquids … with temperature (increases / decreases)
(c) For solids with elastic modulus of rigidity, the shearing force is proportional to … , while for fluids it is proportional to … (shear strain / rate of shear strain)
(d) For a fluid in a steady flow, the increase in flow speed at a constriction follows (conservation of mass / Bernoulli’s principle)
(e) For the model of a plane in a wind tunnel, turbulence occurs at a … speed for turbulence for an actual plane (greater / smaller)
Answer-
(a) The surface tension of a liquid is the intermolecular force per unit length. As temperature increases, kinetic energy of all molecules increases which decreases the cohesive force between them which directly impact the intermolecular force and hence surface tension. Hence we can say surface tension is inversely proportional to temperature.
(b) Viscosity of gases increases with temperature, while viscosity of liquids decreases with temperature.
(c) we know that modulus of rigidity for solids is the ratio of shearing force to shear strain. Hence for solids shearing force is proportional to the shear strain. For fluids, the shearing force is proportional to the rate of shear strain.
(d)Conservation of mass/Bernoulli’s principle. For a steady-flowing fluid, an increase in its flow speed at a constriction follows both conservation of mass and Bernoulli’s principle.
(e)For the model of a plane in a wind tunnel, turbulence occurs at a greater speed than it does for an actual plane this is because the turbulence depends upon Reynolds number. And Reynolds number for model is more than the actual plane.
10.4) Explain why
(a) To keep a piece of paper horizontal, you should blow over, not under, it
(b) When we try to close a water tap with our fingers, fast jets of water gush through the openings between our fingers
(c) The size of the needle of a syringe controls flow rate better than the thumb pressure exerted by a doctor while administering an injection
(d) A fluid flowing out of a small hole in a vessel results in a backward thrust on the vessel .
Answer-
(a) For any horizontal surface pressure on upper side is equal to pressure on lower side. Change in pressure results into fall or rise in surface. When air is blown under a paper, the velocity of air under the surface increases. According to Bernoulli’s principle, atmospheric pressure reduces under the paper. As pressure is more on upper side so paper falls. When we blow over the paper velocity of air increases above the paper which results in reduction in atmospheric pressure above the surface of paper hence To keep a piece of paper horizontal, you should blow over, not under, it.
(b)When we try to close a tap of water with our fingers we are actually reducing area of water flow. But According to the equation of continuity: Area × Velocity = Constant .as area decreases it results into increase in velocity to make their product constant hence fast jets of water gush through the openings between our fingers.
(c) Answer of this question lies in bernaullis principle it states that p + 1/2 x p x v2 + pgh = constant. Effect of velocity is more in this equation because it is present with square term. small opening of a syringe controls the high velocity of bloos flowing out as area is small according to equation of continuity. Hence The size of the needle of a syringe controls flow rate better than the thumb pressure exerted by a doctor while administering an injection.
(d) According to the equation of continuity: Area × Velocity = Constant . When a fluid flows out from a small hole in a vessel it has more velocity. As there is no external force acting on system to satisfy law of conservation of momentum vessel attains velocity in backward direction and we can say there is backward thrust on the vessel.
10.5)
10.9)
10.11) Can Bernoulli’s equation be used to describe the flow of water through a rapid in a river? Explain.
Answer-Bernoulli’s equation is applicable to ideal laminar steady flow of fluid .but for rapid flow of water through river there is turbulence and condition for laminar steady flow gets violated hence Bernoulli’s equation cannot be used to describe the flow of water through a rapid in a river.
10.12) Does it matter if one uses gauge instead of absolute pressures in applying Bernoulli’s equation? Explain.
Answer-
10.13)
10.14)
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