Class 11 important questions for physics Friction in Soilds and Liquids

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F.Y.J.C. Science Friction in Solids and Liquids
Friction in Solids and Liquids
❖ Illustrative Examples
1) A wooden box of mass 60 kg is at rest at horizontal surface of platform. A coolie
requires a force of 78.4 N to just slide. Once the motion begins force of 39.4 N is
sufficient to keep box moving with uniform velocity. Calculate the coefficient of static
and kinetic friction. (g = 9.8 m/s2)
Data: m = 60 kg, Fs =78.4 N, Fk = 39.4 N, g = 9.8 m/s2, N = mg = 60 9.8 = 588 N
(Ans: (a) (b) )
2) Find the distance travelled by a body before coming to rest, if it is moving along the
horizontal surface with a velocity of 5 m/s.
Data:
(Ans: s = 6.378 m)
3) Find the pressure 100m below the surface of the ocean, if pressure on free surface of
liquid is one atmosphere. (Density of sea water = 1060 kg/m3)
Data:
= 1060 kg/m3,
Pressure due to the liquid =( ,g ,h),
g h =
g h = (Ans: P = )
4) Rain drop of radius 0.2 mm falling with velocity of 2m/s. If coefficient of viscosity of
air is 1.8 x 10-4 Ns/m2, determine viscous force acting on the drop.
Data :
(Ans: )
5) Find the terminal velocity of a steel ball of diameter 0.2 cm when it falls through a
tube filled with glycerine. (g = 9.8 m/s2, density of steel = 8000 kg/m3, density of
glycerine = 1.33 g/cm3, = 8.33 poise = 0.833 Ns/m2).

Data: d = 0.2 cm = 0.002 m, r = 0.001m, g = 9.8m/s2 density of steel = 8000 kg/m3,
density of glycerine = 1330 kg/m3,
(Ans: )
6) In a hydraulic press, a piston of cross-sectional area 100 cm2 is used to exert a force
of 150 N on water. What cross-sectional area of the other piston which will support a
car of mass 1000 kg? (g = 9.8 m/s2)
Data: (Ans: A2 = 0.6533m2)
7) A glass slab of length 5 cm and breadth 2 cm is kept on a thin layer of glycerine of
thickness 1 mm. A horizontal force of 0.04N is just sufficient to move the slab over
glycerine with velocity of 2 cm/s, calculate .
Data:
(Ans: )
8) Diameter of a cylindrical tube is 2m and water is flowing through the tube. If
coefficient of viscosity is 200 Ns/m2 and Reynold’s number is 1000 then find critical
velocity of water. (Given: density of water = 1 g/cm3)
Data:
(Ans: )
9) Find pressure energy per kg, K.E. per kg, P.E. per kg and total energy per kg of
water which flows through tube, where pressure is 2 x 105 N/m2 at a point where
the velocity is 10 cm/s and height is 10cm above the ground level.
Data:
(Ans: (a) Pressure energy per kg = 200 J/kg
(b) K.E. per kg = 0.005 J/kg
(c) P.E. per kg = 0.98 J/kg

(d) T.E. per kg = pressure energy per kg + KE per kg + P.E. per kg = 200.985
J/kg)
10) A body starting from rest slides over a plane inclined at 600
to the horizontal. After displacement 40cm, body has
velocity 2m/s,. What is the coefficient of friction? (g = 9.8
m/s2)
Data: = 600, s = 40cm, v = 2m/s, g = 9.8m/s2, = ?
(Ans: = 0.712)
11) A uniform ladder 6m has mass of 25kg leans with its upper
end against a smooth vertical wail and its lower end is on a
rough ground. The bottom of the ladder is 4m from the
wall. Calculate the frictional force between the ladder and
the ground. (g = 10m/s2)
Data: = length of ladder= 6m
Distance between bottom of ladder and wall = x = 4m
(Ans: F = 111.8N)
12) The range of headlights of a car is 25m. Find the limiting safety speed of the car if
the coefficient of kinetic friction between the tyre of the car and the road is
0.4 (g = 9.8 m/s2).
Data: S = 25m, = 0.4, g = 9.8 m/s2, v = ? (Ans: v = 14 m/s)
13) Flow of blood in a large artery of an anesthetised dog is diverted through a Venturi-
meter. The wider part of the Venturimeter has a cross-sectional area equal to that of
the artery (i.e. A= 7.2 mm2).The narrower part has an area a = 3.6 mm2. The
pressure drop in the artery is 24 Pa. What is the speed of the blood in the artery?
(density of blood = 1.06 103 kg/m3)
Data: p = gh = 24pa,
= (Ans: V1 = 0.125 m/s)
14) A fully loaded Boeing aircraft has mass 300 tone. Its total wing area is 400 m2. It is
in level flight with a speed of 960 km/hr. Estimate the pressure difference between

the lower and upper surfaces of the wings. (Density of air Symbol is missing = 1.2
kg./m3).
Data: m = 300 tone= 3 x 105 kg, A = 400m2,
= 1.2 kg./m3, p = ? (Ans: )
15) A water tank has a hole at a distance of 5 m from free water surface. Find the
velocity of water through the hole. If the radius of the hole is 2 mm, what is the rate
of flow of water? (g = 9.8 m/s2)
Data: (Ans: Rate of flow of water = )
16) The diameter of a one end of a rube is 2cm and that of another end is 3cm. Velocity
and pressure of water at narrow end are 2m/s and 1.5 105 N/m2 respectively. If
the height difference between narrow and broad end is 2.5 m. Find the velocity and
pressure of water at broad end. (Density of water is 103 kg m-3)
Data: For narrow end of the tube.
dl = 2cm,
rl = lcm = 10-2m.
V1 = 2m/s,
P1 = 1.5 105 N/m2
For broad end of the tube,
d2 = 3cm,
r2 = 1.5cm =1.5 10-2 m,
v2 = ?
P2= ? (Ans: V2 = 0.89 m/s, P2 = 1.76 105 N/m2)
❖ Theory Questions
1) Define Friction. What are its causes?
2) Explain the origin of friction.
3) Define static friction, kinetic friction, rolling friction.
4) Explain concept of frictional force.
5) Explain why kinetic friction is less than static friction.
6) State laws of static friction.
7) Describe the experiment to verily that limiting force of static friction depends upon
normal reaction between two surf-aces in contact.
8) Describe the experiment to verify that limiting force to static friction is independent
of the apparent area of contact.
9) Describe the experiment to verify that limiting force of static friction depends upon
nature and material of surfaces in contact.

10) State laws of kinetic friction.
11) Explain the term
a) Coefficient of static friction
b) Coefficient of kinetic friction
12) Define pressure, state its SI. CGS unit and dimensions.
13) Mention few methods of reducing friction.
14) Why do we call friction as self-adjusting force?
15) Why are tyres made of rubber and not of iron?
16) Why do we slip on a rainy day?
17) Explain, how friction helps in walking?
18) Why is it difficult to walk on sand?
19) Why lubricants are used in machines?
20) Are large breaks of a bicycle wheel more effective than a small one? Explain.
21) Discuss the advantages and disadvantages of friction.
22) Derive an expression for pressure exerted by liquid column.
23) Explain the term hydrostatic paradox.
24) State Pascal's law?
25) State applications of Pascal's law.
26) Explain the effect of gravity on fluid pressure.
27) Explain the term (a) Streamline flow (b) Turbulent flow.
28) What do you mean by viscous drag?
29) State and explain Newton's law of viscosity.
30) Define a) Velocity gradient b) Coefficient of viscosity
31) State SI and CGS unit of coefficient of viscosity and find conversion factor between
them.
32) State dimensions of coefficient of viscosity.
33) State Stokes' law and derive it by dimensional analysis.
34) Obtain an expression for the terminal velocity of small spherical body falling under
gravity, through viscous fluid.
35) What is Reynold's number?
36) State Bernoulli's principle.
37) State limitations and applications of Bernoulli's principle.
38) Explain the working of a venturimeter.
39) Explain Torricelli's theorem.
40) Explain why the upper surfaces of wings of an aeroplane are made convex and lower
surface concave.
41) State the main properties of frictional force.

42) Why is rolling friction less than sliding friction (kinetic friction)?
43) Explain why ball bearings used in machine to reduce friction are made of different
materials.
44) Define the following terms:
i. Limiting force of static friction
ii. Force of kinetic friction.
45) What is fluid?
46) Explain thrust at a point in fluid column. State the S.I. and C.G.S. units and
dimensions.
47) Discuss the main characteristics of fluid pressure.
48) A vertical off-shore structure is built to withstand a maximum stress of 109 Pa. Is
the structure suitable for putting upon top of an oil well in Bombay high? Take the
depth of the sea to be roughly 3 km and ignore ocean currents.
49) Give explanation for the same.
50) Write a short note on mercury barometer.
51) Write a short note on open tube manometer.
OR
Explain the construction and working of an open tube manometer.
52) Explain the applications of Pascal's law in hydraulic press.
53) Explain the applications of Pascal's law in
a. Hydraulic brake and
b. Hydraulic lift.
54) Pressure in the arteries in the various parts of the human body while standing or
lying down is different. But snakes, rats, rabbits will die when held vertical for
longer time.
55) Define viscosity.
56) Explain the cause and importance of viscosity.
57) Define the term streamline.
59) What is Streamline flow of liquid?
61) State the characteristics of streamline flow.
62) Define the term critical velocity.
63) State equation of continuity.
65) What is turbulent flow?
66) Explain with suitable example.

67) State the properties of turbulent flow.
68) Distinguish between streamline flow and turbulent flow.
69) Define the term poise.
70) Define terminal velocity.
71) Explain various applications of Bernoulli’s principle.
72) Write a short note on an atomiser.
73) Explain the flow of blood in artery by using Bernoulli’s principle.
74) Can Bernoulli’s equation be used to describe rapid flow of water in a river?
❖ Textbook Problems
1) A 20 kg metal block is placed on a horizontal surface. The block just begins to slide
when horizontal force of 100 N is applied to it. Calculate the coefficient of static
friction. If coefficient of kinetic friction is 0.4 then find minimum force to maintain
its uniform motion. (Ans: 0.5101, 78.4 N)
2) A swimmer jumped into swimming tank and was swimming at 8 m below the surface
of the tank, calculate the pressure on him.(Density of water = 1000 kg/m3, S = 9.8
m/s2, Atmospheric pressure ) (Ans: )
3) In a hydraulic lift, the input piston has surface area 20 cm2.The output piston has
surface area of 1000 cm2. If a force of 50 N is applied to the input piston, it raises
the output piston by 2m. Calculate weight of support on output piston and its work
done. (Ans: 2500N, 5000J)
4) The relative velocity between two layers of fluid, separated by 0.1 nun is 2 cm/s.
Calculate the velocity gradient. (Ans: 200/s)
5) Calculate the viscous force acting on a rain drop of diameter 2mm, falling with a
uniform velocity 2m/s through the air. The coefficient of viscosity of air is 1.8 x 10-5
Ns/m2. (Ans: )
6) A spherical oil drop falls at a steady rate of 2 cm/s in still air. Find diameter of the
drop. Take g = 980 cm/s2. The coefficient of viscosity of air is 1.8 10-4 Poise,
Density of oil = 0.8 g/cm3, Density of air = 1g/cm3. (Ans: 5.714 mm)
7) Calculate total energy per unit mass possessed by water at a point where the
pressure is 0.1 x 105 N/m2, velocity 0.02 m/s and height of water level from the
ground is 10 cm. (Density of water = 1000 kg/m3, g = 9.8 m/s2).(Ans: 10.9802J/kg)
8) A body starting from rest slides over a plane inclined at 45o to the horizontal. After
certain displacement, body has velocity 2m/s. If coefficient of friction between body

and surface is 0.5, find displacement of the body from a start. (Ans: 28.86m)
9) A uniform ladder which is 5m long has a mass of 30kg leans with its upper end
against a smooth vertical wall and its lower end is on a rough ground. The bottom of
the ladder is 3m from the wall. Calculate the frictional force between the ladder and
the ground. (g = 10m/s2). (Ans: 112.5N)
10) A water tank has a hole at a distance 'x' m from free water surface. If the radius of
the hole is 2 mm and velocity of efflux of water is 11m/s find 'x' (g = 9.8 m/s2).
(Ans: 6.173m)
11) Water is flowing through a horizontal pipe of varying cross-section. At a certain point
where the velocity is 0.12 m/s, the pressure of water is 0.010 m of mercury. What is
the pressure at a point where the velocity is 0.24 m/s? (Ans: 0.00838m of Hg)
❖ Practice Problems
➢ Type-I : Problems based on friction
1) Calculate the force required to move a block of mass 20 kg resting on a horizontal
surface, if = 0.3 and g = 9.8 m/s2. (Ans: 58.8 N)
2) A block of mass 70 kg is kept on a rough horizontal surface. If the coefficient of
static friction between the block and the surface is 0.3, find the minimum force that
needs to be applied to the block in a direction inclined to the horizontal at angle of
300 to set the block in sliding motion. (Ans: 202.56N)
3) A mass of 200 kg is resting on a rough inclined plane of 30o. If the coefficient of
friction is , find the least and the greatest forces acting parallel to the plane to
keep the mass in equilibrium. (Ans: 0 N, 1960N)
4) A block of mass 40 kg resting on a horizontal surface just begins to slide when a
horizontal force of 120 N is applied to it. Once the motion starts, it can be
maintained by a force of 80 N. Determine the coefficients of static friction and kinetic
friction (g = 9.8 m/s2). (Ans: 0.306, 0.204)
5) Figure given below shows a man standing stationary with respect to horizontal
conveyor belt that is accelerating with 1 ms-2. What is the net force on the man? If
the coefficient of static friction between the man's shoes and the belt is 0.2, upto
what acceleration of the belt can the man continue to be stationary relative to the
belt? (Mass of the man : 65 kg)

(Ans: 1.96ms-2)
6) Find the distance travelled by a wooden block before coming to rest, if it is moving
along the rough ground with a velocity of 15 m/s. ( = 0.25) (Ans: 45.92m)
7) A wooden box of mass 60 kg is at rest at horizontal surface of platform. A coolie
requires a force of 78.4 N to just slide. Once the motion begins, force of 39.4 N is
sufficient to keep box moving with uniform velocity. Calculate the coefficient of static
and kinetic friction. (Ans: 0.1333, 0.0670)
8) A force of 49 N is just sufficient to pull a block of wood weighing 10 kg on a rough
horizontal surface. Calculate the coefficient of friction and angle of friction.
(Ans: 0.5, 26034 )
➢ Type-II : Problems based on pressure due to fluid column
9) A tank with a square base of area 1.0 m2 is divided by a vertical partition in the
middle. The bottom of the partition has a small hinged door of area 20 cm2. The tank
is filled with water and an acid (of relative density 1.7) in the other, both to a height
of 4.0 m. Compute the force necessary to keep the door closed. (Ans: 55N)
10) Calculate the depth of water level where the pressure will be equal to 4 atmospheres,
if the atmospheric pressure is 760 mm of Hg. (Ans: 31m)
11) A U-tube contains water and methylated spirit separated by mercury. The mercury
columns in the two arms are in level with 10.0 cm of water in one arm and 12.5 cm
of spirit in the other. What is the relative density of spirit? (Ans: 0.8)
12) The areas of the piston of hydraulic press are 3 cm2 and 300 cm2. A force of 4 N is
applied to the smaller piston. Find the force exerted at the larger piston.
(Ans: 400N)
13) What is the pressure on a swimmer 10 m below the surface of water in a lake?
[Po : 1.013 105 N/m2, pw = 103 kg/m3] (Ans: )
➢ Type-III : Problems based on viscosity
14) Calculate the force required to move a flat glass plate of area 10 cm2 with a uniform
velocity of 1 cm/s over the surface of a liquid 2 mm thick if the coefficient of
viscosity of the liquid is 2 Pa.s. (Ans: 0.01N)
15) A metal plate 5 cm 5 cm rests on a layer of castor oil 1 mm thick whose coefficient
of viscosity is 1.55 Nsm-

2. Find the horizontal force required to move the plate with a speed of 2 ms-l.
(Ans: 0.0775N)
16) Find the terminal velocity of a steel ball-bearing of radius 0.25 cm when it falls
through a tube filled with glycerine.
[Psteel = 8 g/cm3, pglycerine =1.33 g/cm3 and -glycerine = 8.33 Poise] (Ans: 10.9 cm/s)
17) A spherical liquid drop of diameter 2 10-4 m is falling with a constant velocity
through air under gravity. If the density of the liquid is 500 kg/m3 and the
coefficient of viscosity of air is 2 10-5 Pa.s, determine the terminal velocity of the
drop and the viscous force acting on it. Neglect the density of air. [g = 9.8 m/s2]
(Ans: 0.544 m/s, 2.05 10-8N)
18) A metal sphere of radius 1 mm falls vertically in glycerine. Find the viscous force
exerted by the glycerine on the sphere when the speed of the sphere is 1 cms -1.
[For glycerine, : 8.0 poise.] (Ans: )
19) Calculate the minimum pressure required to force a fluid vertically through a
distance of 0.5 m. Density of the fluid is 1040 kg/m3. (Neglect the friction)
[g = 9.8 m/s2] (Ans: )
20) A cylindrical beaker of radius 6 cm is filled with Hg upto a height of 12 cm. Calculate
the force exerted on the bottom of the beaker.
[Atmospheric pressure: 105 N/m2 and density of Hg = 13.6 x 103 kg/m3,g =10m/s2]
(Ans: )
21) A metal plate of area 25 cm2 is placed on a 0.5 mm thick castor oil layer. If a
horizontal force of 22,500 dyne is needed to move the plate with a velocity of 3cms-1,
calculate the coefficient of viscosity of castor oil. (Ans: 15 poise)
22) Find the critical velocity for air flowing through a tube of diameter 2 m. [pair =1.25
kg/m3, = 1.81 x 10-5 Pa s, NR = 2000] (Ans: 1.448 10-2m/s)
23) Find the critical velocity for air flowing through a tube of 2 cm diameter. For air,
p = 1.3 10-3 g cm-3 and =181 10-6 poise, NR = 2000. (Ans: 139.2 cms-1)
24) A ball bearing of radius 1.5 mm made of iron of density 7.85 g cm-3 is allowed to fall
through a long column of glycerine of density 1.25 g cm-3. It is found to attain a
terminal velocity of 2.25 cms -1. Determine the viscosity of glycerine in centipoise.
(Ans: 1437 centipoise)
25) Find the terminal velocity of a raindrop of diameter 3 mm, when it falls through
atmospheric air. pair = 1.2 kg/m3 , and pw =1000 kg/m3.

(Ans: 249.7m/s)
26) Water flows out from a tap of diameter 1 cm at the rate of 5 10-5 m3/s. Is the flow
streamline or turbulent? : 10-3 Pa.s, p = 103 kg/m3. (Ans: turbulent)
➢ Type-IV : Problems based on Bernoulli’s principle
27) In a test experiment on a model aeroplane in a wind tunnel, air flows at speeds of 70
m/s and 60 m/s on the upper and lower surfaces of the wing respectively. Find the
difference in pressures on the lower and upper surfaces of the wing. Neglect the
thickness of wing. (pair= 1.3 kg/m3) (Ans: 845Pa)
28) Water is flowing through a horizontal pipe of varying cross-section. If the pressure of
water equals 2 cm of Hg where velocity of flow is 32 cms-1, what is the pressure at
another point where the velocity of flow is 40 cms-1? (Ans: 2.02cm of Hg)
29) The flow of blood in a large artery of an anaesthetized dog is diverted through a
venturimeter. The wider part of the meter has a cross-sectional area equal to that of
the artery i.e. 8 mm2. The narrower part has an area 4 mm2. The pressure-drop in
the artery is 24 Pa. What is the speed of blood in the artery? Given that: density of
the blood: 1.06 x 103 kg/m3. (Ans: 0.123m/s)
30) Water is filled in a circular flask having cross sectional area 2 m2 upto a height of 30
cm. If the atmospheric pressure is 1.01 x 105 Pa, find the force exerted by water on
the bottom. [g : 9.8 m/s2, density of water = 1,000 kg/m3] (Ans: )
31) A small oil drop is falling in air at a uniform speed of 1 x 10-3 m/s. Find the radius of
the drop of oil whose density is 0.9013 x 103 kg/m3.
[density of air : 1.3 kg/m3, coefficient of viscosity of air : 1.8 x 10-6 kg/ms.]
(Ans: )
32) A cylinder of height 20 m is completely filled with water. Find the velocity of efflux of
water through a small hole on the side wall of the cylinder near its bottom.
[Given, g = 10 ms-2] (Ans: 20ms-1)
➢ Type-V : Miscellaneous
33) Water is flowing in a horizontal tube. The pressure at a point is 0.01 m of mercury at
which the velocity is 0.8 m/s. Find the pressure at a point where the velocity is 0.9
m/s. (Ans: 0.00936 m of mercury)
34) A horizontal water line necks down from a pipe of 50 mm diameter to a pipe of 25
mm diameter. The speed of water in the wider pipe is 2.4 m/s and the gauge
pressure is 53.2 kPa. What is the speed of water in the narrower pipe? What is the

flow rate in these pipes? What is the gauge pressure at the constriction? [Density of
water =103 kg/m3] (Ans: )
35) In a test experiment on a model aeroplane in a wind tunnel, the flow speeds on the
upper and lower surfaces of the wing are 70 ms-1 and 63 ms-1 respectively. What is
the lift on the wings, if its area is 2.5 m2? [Take density of air to be 1.3 kgm-3.]
(Ans: 1513N)
36) Water flows along a horizontal pipe of variable cross-section. At a point where the
velocity of flow is 0.3 cm/s, the pressure is 1400 N/m2. What is the pressure at the
point where the velocity of flow is 0.6 m/s? (Ans: 1220.5N/m2)
37) An oil droplet is found to descend through air under the action of gravity with a
uniform velocity of 0.25 mm/s. Find the radius of the droplet. [poil : 800 kg / m3,
pair : 1.2 kg /m3, : 1.82 x 10-5 Ns/m2] (Ans: )
38) Calculate the minimum pressure required to force the blood from the heart to the
top of the head (vertical distance: 50 cm). Assume the density of blood to be 1.04 g
cm-3. Friction is to be neglected. (Ans: )
❖ Multiple Choice Questions
1) A spherical ball of iron of radius 2 mm is falling through a column of glycerine. If
densities of glycerine and iron are 1.3 103 kg/m3 and 8 103 kg/m3 respectively
for glycerine = 0.83 kg/m.sec, then the terminal velocity is
(A) 0.7 m/s (B) 0.07 m/s (C) 0.007 m/s (D) 0.0007 m/s.
2) The terminal velocity v of a spherical ball of lead of radius R falling through a
viscous liquid varies with R such that
(A) = constant v R (C) v = constant
(B) vR = constant (D) = constant R2
3) A spaceship entering the earth’s atmosphere is likely to catch fire. This is due to
(A) the surface tension of air.
(B) the viscosity of air.
(C) the temperature of the upper atmosphere.
(D)the greater proportion of oxygen in the atmosphere at high altitudes.
4) Rain drops fall from a great height under gravity. Check the correct statement:

(A) Their velocity continuously increases till they hit the earth with the same final
velocity.
(B) They fall with a terminal velocity which is different for drops of different sizes.
(C) They fall with a terminal velocity which is the same for every drop.
(D)Their velocity goes on increasing continuously till they hit the earth, and the final
velocity of each drop is different.
5) The coefficient of viscosity for hot air is
(A) greater than the coefficient of viscosity for cold air.
(B) smaller than the coefficient of viscosity for cold air.
(C) same as the coefficient of viscosity for cold air.
(D)increases or decreases depending on the external pressure.
6) A liquid disturbed by stirring comes to rest after sometime due to_________.
(A) surface tension (C) viscosity
(B) stability (D) attraction forces between molecules
7) The velocity of falling rain drops attains limited value because of___________.
(A) surface tension (C) viscous force exerted by air
(B) up thrust due to air (D) air current.
8) A rain drop of radius 0.3 mm has a terminal velocity in air = 1 m/s. The viscosity of
air is 18 x 10-5 poise. The viscous force on it is
(A) 101.73 10-4 dyne (C) 16.95 10-4 dyne
(B) 101.73 10-5 dyne (D) 16.95 10-5 dyne
9) A fluid of density d and viscosity is flowing with an average velocity v measured
over the cross-section of a pipe of radius r. The Reynold’s number R is then given by
(A) (B) (C) (D)
10) The viscous drag on a spherical body moving with a speed V is proportional to
(A) (B) V (C) (D) V2
11) A sphere is dropped gently into a medium of infinite extent. As the sphere falls, the
net force acting downwards on it
(A) remains constant throughout.
(B) increases for some time becomes constant and then becomes constant.
(C) decreases for some time and then becomes zero.
(D)increases for some time and then decreases.
12) Which of the following statements is NOT true?
(A) For gases, in general, viscosity increases with increase in temperature.

(B) For liquids, viscosity varies directly with pressure.
(C) For gases, viscosity is independent of pressure.
(D)For liquids, viscosity varies inversely with pressure.
13) The reading of a manometer fitted to a closed tap is 3.5 105 N/m2. If the valve is
opened, the reading of the manometer falls to 3 105 N/m2. The velocity of water is
(A) 1 m/s (B) 10 m/s (C) 100 m/s (D) 0.1 m/s.
14) Bernoulli’s equation is important in the field of
(A) electrical circuits. (C) flow of liquids.
(B) magnetism. (D) photoelectric effect.
15) In a streamline (laminar flow), the velocity of flow at any point in the liquid
(A) does not vary with time.
(B) may vary in direction but not in magnitude.
(C) may vary in magnitude but not in direction.
(D)may vary both in magnitude and direction.
16) In Bernoulli’s theorem, which of the following is conserved?
(A) Mass (C) Energy
(B) Linear momentum (D) Angular momentum
17) The flow of liquid is laminar or streamline is determined by
(A) Reynold’s Number (C) Stake’s law.
(B) density of fluid (D) coefficient of viscosity of liquid.
18) Water is flowing through a horizontal pipe in streamline flow. At the narrowest part
of the pipe,
(A) velocity is maximum and pressure is minimum.
(B) pressure is maximum and velocity is minimum.
(C) both the pressure and velocity are maximum.
(D)both the velocity and pressure are minimum.
19) A body of weight 50 N is placed on a smooth surface. If the force required to move
the body on the surface is 30 N, the coefficient of friction is
(A) 0.6 (B) 0.4 (C) 0.3 (D) 0.8
20) A block of weight 1 kg sliding on a surface with a velocity of 2 m/sec is stopped by
friction in 10 seconds. The force of friction will be
(A) -2 N (B) 3 N (C) 4 N (D) 0.02 N
21) A block of mass 0.1 kg is pressed against a wall with a horizontal force of 5 N. If the
coefficient of friction between the wall and the block is 0.5, then the frictional force
acting on the block will be
(A) 9.8 N (B) 4.9 N (C) 0.98 N (D) 0.49 N

22) The coefficient of sliding friction for steel on ice is 0.05. What is the force required to
keep a man with steel skates moving at a constant speed along the ice, if the mass of
the man is 80 kg? (Take g = 10 m/s2)
(A) 20 N (B) 40 N (C)80 N (D) 50 N
23) A body lies on a table. Its weight is balanced by the_________.
(A) frictional force (C) force causing motion on the body
(B) normal force (D) surface of the table
24) The friction that exists between the surface of two bodies in contact when one body
is sliding over the other, is called _________.
(A) rolling friction (C) kinetic friction
(B) friction (D) static friction.
25) The pressure due to liquid column does NOT depends upon
(A) height of the liquid column. (C) direction of force.
(B) density of the liquid. (D) acceleration due to gravity.
26) The pressure exerted by column of liquid on the base depends only on the
(A) height of the liquid. (C) material of the vessel.
(B) shape of the vessel. (D) viscosity of the liquid.
27) Bernoulli’s principle is true under which of the following assumptions?
(A) The fluid is viscous and streamline.
(B) The fluid is non-viscous and streamline.
(C) The fluid is non-viscous and turbulent.
(D)The fluid is viscous and turbulent.
28) If vc is the critical value of velocity when a liquid flows inside a tube with velocity v
then flow of liquid inside a tube remains streamline if
(A) v < vc (B) v = vc (C) v > vc (D) v = 2vc
29) The S.I. unit and dimensions of Reynold’s number respectively are
(A) Poise, [L°M0T°] (C) No unit, [L°M°T°]
(B) NS/m2, [L1M1 T1] (D) No unit, [L1M1T1]
30) Bunsen’s burner works on the principle of
(A) terminal velocity. (C) Newton’s law.
(B) Stake’s law. (D) Bernoulli’s principle.
31) In case of coefficient of static friction ( ), kinetic friction ( ) and rolling friction
( ), which of the following relation is true

(A) > > (C) > >
(B) > > (D) > >
32) Limiting force of static friction does NOT depend on
(A) actual area of contact.
(B) geometrical area of contact.
(C) interlocking between surfaces in contact.
(D)intermolecular forces of attraction between molecules of the two surfaces.
33) Pascal’s law is applied in which of the following case?
(A) Hydraulic brakes (C) Venturi-meter
(B) Air purifier (D) Blood pressure gauge
34) Choose the correct statement:
(A) The viscosity of liquid decreases with temperature while that of gas increases
with temperature.
(B) The viscosity of liquids and gases decrease with temperature.
(C) The viscosity of liquids and gases increase with temperature.
(D)The viscosity of liquid increases with temperature while that of gas decreases
with temperature.
35) A wooden block of 50 kg is at rest on the table. A force of 70 N is required to just
slide the block. The coefficient of static friction is
(A) (B) (C) (D)
36) Reynold’s number of a liquid is 1500, the nature of flow of liquid is
(A) Turbulent (C) unstable
(B) laminar (D) changing the streamline to turbulent
37) A body begins to slide over the surface of another when pulled with a force of 20 N. If
we pull with a force of 15 N, the force of friction that will come into play is
(A) zero (B) 0.15 N (C) 15 N (D) 300 N
38) A horizontal force of 10 N is necessary to just hold a block stationary against wall.
The coefficient of friction between the block and the wall is 0.3. The weight of the
block is
(A) 33 N (B) 30 N (C)13N (D) 3N
39) A vehicle is in motion with velocity v along a straight horizontal road. If the
coefficient of friction between the tyres and road is , what would be the shortest
distance in which the vehicle can stop?

(A) (B) (C) (D)
❖ Answers to Multiple Choice Questions
1. (B) 2. (D) 3. (B) 4. (B) 5. (A) 6. (C) 7. (C)
8. (A) 9. (B) 10. (B) 11. (C) 12. (D) 13. (B) 14. (C)
15. (A) 16. (C) 17. (A) 18. (A) 19. (A) 20. (D) 21. (C)
22. (B) 23. (B) 24. (C) 25. (C) 26. (A) 27. (B) 28. (A)
29. (C) 30. (D) 31. (A) 32. (B) 33. (A) 34. (A) 35. (D)
36. (B) 37. (C) 38. (D) 39. (B)

Class 11 important questions for physics Friction in Soilds and Liquids

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Class 11 important questions for physics Friction in Soilds and Liquids