JEE Physics Mechanical Properties Of Fluids MCQs Set 09

Multiple Choice Questions (MCQs) for JEE Physics: Mechanical Properties Of Fluids

Access targeted multiple-choice questions for Mechanical Properties Of Fluids designed to align with the latest JEE academic syllabus for JEE Physics. These objective practice sets help students evaluate their conceptual understanding and improve exam readiness.

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Multiple Choice Questions

Question 1: Two bubbles of radii 0.03 cm and 0.04 cm come in contact with each other. The radius of curvature of the common surface is:
(a) 0.03 cm
(b) 0.06 cm
(c) 0.12 cm
(d) 0.24 cm
Show Answer & Explanation

Answer: (c) 0.12 cm

Explanation:
1. r = r1 r2/(r2 - r1) = 0.03 x 0.04/0.01 = 0.12 cm

Teacher's Note:
1. Apply the standard formula and check the units.

Question 2: The work done in forming a soap bubble of radius R is W. The work done in forming a soap bubble of radius 2R will be:
(a) 2W
(b) 4W
(c) W/2
(d) W/4
Show Answer & Explanation

Answer: (b) 4W

Explanation:
1. Work is proportional to the surface area, hence to R^2
2. Doubling R gives 4W

Teacher's Note:
1. Apply the standard formula and check the units.

Question 3: At which contact angle will a liquid not wet a solid?
(a) zero
(b) acute
(c) 45 degrees
(d) obtuse
Show Answer & Explanation

Answer: (d) obtuse

Explanation:
1. An obtuse contact angle means the liquid does not wet the solid

Teacher's Note:
1. Apply the standard formula and check the units.

Question 4:
Assertion (A): Gases also behave as fluids.
Reason (R): Gas molecules are bound by cohesive forces.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (c) A is true, but R is false.

Explanation:
1. Gases flow, so they are fluids and A is true
2. Gas molecules are practically free, so R is false

Teacher's Note:
1. Decide whether each statement is true, then whether R explains A.

Question 5: The internal radius of a capillary tube is 1/28 cm and the surface tension of water is 70 dyne/cm. If the angle of contact is zero, water will rise in the tube up to a height of:
(a) 4 cm
(b) 2 cm
(c) 14 cm
(d) 18 cm
Show Answer & Explanation

Answer: (a) 4 cm

Explanation:
1. h = 2T/(r rho g) = 2 x 70/((1/28) x 1 x 980) = 4 cm

Teacher's Note:
1. Apply the standard formula and check the units.

Question 6: The excess pressure inside a soap bubble A is twice that in another soap bubble B. The ratio of the volumes of A and B is:
(a) 1 : 2
(b) 1 : 4
(c) 1 : 8
(d) 1 : 16
Show Answer & Explanation

Answer: (c) 1 : 8

Explanation:
1. Excess pressure is proportional to 1/r, so rA = rB/2
2. Volume ratio = (1/2)^3 = 1 : 8

Teacher's Note:
1. Apply the standard formula and check the units.

Question 7: A soap film on a rectangular frame of wire has an area of 4 x 4 cm2. If the area of the soap film is increased to 4 x 5 cm2, the work done is (surface tension of the soap film = \( 3\times10^{-2} \) N/m):
(a) \( 12\times10^{-6} \) J
(b) \( 24\times10^{-6} \) J
(c) \( 60\times10^{-6} \) J
(d) \( 96\times10^{-6} \) J
Show Answer & Explanation

Answer: (b) \( 24\times10^{-6} \) J

Explanation:
1. Increase in area = 4 cm2 = 4e-4 m2
2. The film has two surfaces, so work = 2 x 3e-2 x 4e-4 = 24e-6 J

Teacher's Note:
1. Apply the standard formula and check the units.

Question 8: The work done in blowing a soap bubble of radius 0.2 m, given that the surface tension of the soap solution is \( 60\times10^{-3} \) N/m, is:
(a) \( 24\pi\times10^{-4} \) J
(b) \( 8\pi\times10^{-4} \) J
(c) \( 96\pi\times10^{-4} \) J
(d) \( 192\pi\times10^{-4} \) J
Show Answer & Explanation

Answer: (d) \( 192\pi\times10^{-4} \) J

Explanation:
1. Work = 2 x 4 pi r^2 x T
2. = 8 pi x 0.04 x 0.06 = 192 pi x 1e-4 J

Teacher's Note:
1. Apply the standard formula and check the units.

Question 9:
Assertion (A): Pressurised fluid contains energy.
Reason (R): Work must be done to compress a fluid.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The work done in compressing is stored as pressure energy
2. So R explains A

Teacher's Note:
1. Decide whether each statement is true, then whether R explains A.

Question 10: Adding detergents to water helps in removing greasy stains because: (a) it increases the oil-water surface tension, (b) it decreases the oil-water surface tension, (c) it increases the viscosity of the solution, (d) dirt is held suspended, surrounded by detergent molecules.
(a) (b) and (d)
(b) (a) only
(c) (c) and (d)
(d) (d) only
Show Answer & Explanation

Answer: (a) (b) and (d)

Explanation:
1. Detergent lowers the oil-water interfacial tension and forms micelles around dirt, keeping it suspended

Teacher's Note:
1. Apply the standard formula and check the units.

Question 11: If the radii of two soap bubbles are R1 and R2, and they are combined in vacuum under isothermal conditions to form a single soap bubble, the radius of the combined bubble is:
(a) \( R_1R_2/(R_1+R_2) \)
(b) \( \sqrt{R_1^2+R_2^2} \)
(c) \( (R_1+R_2)/2 \)
(d) \( R_1R_2/(R_1-R_2) \)
Show Answer & Explanation

Answer: (b) \( \sqrt{R_1^2+R_2^2} \)

Explanation:
1. In vacuum, pressure times volume and surface energy balance gives R^2 = R1^2 + R2^2

Teacher's Note:
1. Apply the standard formula and check the units.

Question 12: Water flows with velocity 4 m/s in a cylinder of diameter 8 cm. It is connected to a pipe whose end tip has diameter 2 cm. The velocity of water at this free end is:
(a) 4 m/s
(b) 8 m/s
(c) 32 m/s
(d) 64 m/s
Show Answer & Explanation

Answer: (d) 64 m/s

Explanation:
1. v is proportional to 1/d^2
2. v = 4 x (8/2)^2 = 64 m/s

Teacher's Note:
1. Apply the standard formula and check the units.

Question 13: The pressure head in Bernoulli's equation is:
(a) \( P\rho/g \)
(b) \( P/\rho g \)
(c) \( \rho g \)
(d) \( P\rho g \)
Show Answer & Explanation

Answer: (b) \( P/\rho g \)

Explanation:
1. Dividing the pressure term P by rho g gives a length, the pressure head

Teacher's Note:
1. Apply the standard formula and check the units.

Question 14:
Assertion (A): Soil particles inside water are freely placed, but they stick together when taken out of water.
Reason (R): Thin films formed create a pressure difference.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Water films between grains have curved surfaces, giving a pressure difference that pulls the grains together

Teacher's Note:
1. Decide whether each statement is true, then whether R explains A.

Question 15: A spherical body of diameter D is falling in a viscous medium. Its terminal velocity vt is proportional to:
(a) \( D^{1/2} \)
(b) \( D^{3/2} \)
(c) \( D^2 \)
(d) \( D^{5/2} \)
Show Answer & Explanation

Answer: (c) \( D^2 \)

Explanation:
1. Terminal velocity = 2 r^2 (rho - sigma) g/(9 eta), which is proportional to D^2

Teacher's Note:
1. Apply the standard formula and check the units.

Question 16: Four wires are made of the same material. Which will have the largest extension when the same tension is applied?
(a) length 300 cm, diameter 3 mm
(b) length 50 cm, diameter 0.5 mm
(c) length 100 cm, diameter 1 mm
(d) length 200 cm, diameter 2 mm
Show Answer & Explanation

Answer: (b) length 50 cm, diameter 0.5 mm

Explanation:
1. Extension is proportional to L/d^2
2. The values are 33.3, 200, 100 and 50 respectively, so the second wire stretches most

Teacher's Note:
1. Apply the standard formula and check the units.

Question 17: The wettability of a surface by a liquid depends primarily on:
(a) angle of contact between the surface and the liquid
(b) viscosity
(c) surface tension
(d) density
Show Answer & Explanation

Answer: (a) angle of contact between the surface and the liquid

Explanation:
1. A small (acute) angle of contact means good wetting

Teacher's Note:
1. Apply the standard formula and check the units.

Question 18: A non-viscous fluid of constant density 1000 kg/m3 flows in streamline motion along a tube of variable cross-section. The areas of cross-section at two points P and Q at heights 5 m and 3 m are 40 cm2 and 20 cm2 respectively. If the velocity of the fluid at P is 3 m/s, the velocity at Q is:
(a) 3 m/s
(b) 4 m/s
(c) 5 m/s
(d) 6 m/s
Show Answer & Explanation

Answer: (d) 6 m/s

Explanation:
1. Continuity: 40 x 3 = 20 x vQ
2. vQ = 6 m/s

Teacher's Note:
1. Apply the standard formula and check the units.

Question 19:
Assertion (A): When the temperature of a liquid is increased, its surface tension decreases.
Reason (R): Kinetic energy and oscillations of molecules increase, so the intermolecular force of attraction decreases.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Higher thermal motion weakens the attraction
2. So surface tension decreases

Teacher's Note:
1. Decide whether each statement is true, then whether R explains A.

Question 20: Copper of fixed volume V is drawn into a wire of length l. When this wire is subjected to a constant force F, the extension produced is delta l. Which graph is a straight line?
(a) delta l versus 1/l
(b) delta l versus l^2
(c) delta l versus 1/l^2
(d) delta l versus l
Show Answer & Explanation

Answer: (b) delta l versus l^2

Explanation:
1. A = V/l, so delta l = F l^2/(V Y)
2. Delta l is proportional to l^2, so it gives a straight line against l^2

Teacher's Note:
1. Apply the standard formula and check the units.

Question 21: A number of spherical drops of a liquid of radius r coalesce to form a single drop of radius R and volume V. If T is the surface tension, then:
(a) \( \text{energy}=4VT(1/r-1/R) \) is released
(b) \( \text{energy}=3VT(1/r+1/R) \) is absorbed
(c) \( \text{energy}=3VT(1/r-1/R) \) is released
(d) energy is neither released nor absorbed
Show Answer & Explanation

Answer: (c) \( \text{energy}=3VT(1/r-1/R) \) is released

Explanation:
1. Surface area falls, so energy is released
2. Energy released = T(n 4 pi r^2 - 4 pi R^2) with n r^3 = R^3 = 3V/4pi, which gives 3VT(1/r - 1/R)

Teacher's Note:
1. Apply the standard formula and check the units.

Question 22: The approximate depth of an ocean is 2700 m. The compressibility of water is \( 45.4\times10^{-11} \) Pa-1 and the density of water is 1000 kg/m3. The fractional compression of water at the bottom of the ocean is:
(a) \( 1.0\times10^{-2} \)
(b) \( 1.2\times10^{-2} \)
(c) \( 1.4\times10^{-2} \)
(d) \( 0.8\times10^{-2} \)
Show Answer & Explanation

Answer: (b) \( 1.2\times10^{-2} \)

Explanation:
1. Pressure = 1000 x 10 x 2700 = 2.7e7 Pa
2. Fractional compression = 2.7e7 x 45.4e-11 = 1.2e-2

Teacher's Note:
1. Apply the standard formula and check the units.

Question 23: A wind with speed 40 m/s blows parallel to the roof of a house. The area of the roof is 250 m2. Assuming that the pressure inside the house is atmospheric pressure, the force exerted by the wind on the roof and its direction are (density of air = 1.2 kg/m3):
(a) \( 4.8\times10^5 \) N, upwards
(b) \( 2.4\times10^5 \) N, upwards
(c) \( 2.4\times10^5 \) N, downwards
(d) \( 4.8\times10^5 \) N, downwards
Show Answer & Explanation

Answer: (b) \( 2.4\times10^5 \) N, upwards

Explanation:
1. Pressure outside is lower by (1/2) rho v^2 = 0.5 x 1.2 x 1600 = 960 Pa
2. Force = 960 x 250 = 2.4e5 N, directed upwards

Teacher's Note:
1. Apply the standard formula and check the units.

Question 24:
Assertion (A): The surface tension of water is larger than that of most fluids.
Reason (R): There is hydrogen bonding in water, so its surface tension is larger than that of other fluids.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Hydrogen bonds give strong cohesion
2. So R explains A

Teacher's Note:
1. Decide whether each statement is true, then whether R explains A.

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Multiple Choice Questions (MCQs) for JEE Physics Mechanical Properties Of Fluids

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