Fluid Mechanics - JEE Main Previous Year Questions with Solutions
Fluid Mechanics carries 1–2 questions in JEE Main every year, typically from buoyancy, the continuity equation, Bernoulli's theorem, surface tension, and viscosity. Previous-year questions show a consistent pattern of numerical + conceptual mixing. Practising these PYQs with solutions is the fastest way to secure full marks in this chapter.
JEE Main Previous Year Question of Physics with Solutions are available here. Practicing JEE Main Previous Year Papers Questions of Physics will help all the JEE aspirants in realizing the question pattern as well as help in analyzing their weak & strong areas. Get detailed Class 11th &12th Physics Notes to prepare for Boards as well as competitive exams like IIT JEE, NEET etc. eSaral helps the students in clearing and understanding each topic in a better way. eSaral is providing complete chapter-wise notes of Class 11th and 12th both for all subjects. Besides this, eSaral also offers NCERT Solutions, Previous year questions for JEE Main and Advance, Practice questions, Test Series for JEE Main, JEE Advanced and NEET, Important questions of Physics, Chemistry, Math, and Biology and many more. Download eSaral app for free study material and video tutorials.
Q. A ball is made of a material of density r where $\rho_{\text {oil }}<\rho<\rho_{\text {water }}$ with $\rho_{\text {oil }}$ and $\rho_{\text {water }}$ representing the densities of oil and water, respectively. The oil and water are immiscible. If the above ball is in equilibrium in a mixture of this oil and water, which of the following pictures represents its equilibrium position ? [AIEEE-2010]
Ans. (3) In equilibrium ball will remain at the interface of water and oil.
Q. Water is flowing continuously from a tap having an internal diameter $8 \times 10^{-3}$m. The water velocity as it leaves the tap is 0.4 ms–1. The diameter of the water stream at a distance $2 \times 10^{-1}$m below the tap is close to :- (1) $9.6 \times 10^{-3} \mathrm{m}$ (2) $3.6 \times 10^{-3} \mathrm{m}$ (3) $5.0 \times 10^{-3} \mathrm{m}$ (4) $7.5 \times 10^{-3} \mathrm{m}$ [AIEEE-2011]
Ans. (2) According to equation of continuity $\mathrm{A}_{1} \mathrm{V}_{1}=\mathrm{A}_{2} \mathrm{V}_{2}$ or $\mathrm{r}_{2}=\sqrt{\frac{\mathrm{r}_{1}^{2} \mathrm{v}_{1}}{\mathrm{v}_{2}}}$ Velocity of stream at 0.2 m below tap. $\mathrm{V}_{2}^{2}=\mathrm{V}_{1}^{2}+2 \mathrm{as}=0.16+2 \times 10 \times 0.2=4.16 \mathrm{m} / \mathrm{s}$ $\mathrm{r}_{2}=\sqrt{\frac{\mathrm{r}_{1}^{2} \mathrm{v}_{1}}{\mathrm{v}_{2}}}=\sqrt{\frac{16 \times 10^{-6} \times 0.4}{2}}=\sqrt{3.2} \times 10^{-3} \mathrm{m}$ so diameter $=2 \times \sqrt{3.2} \times 10^{-3} \mathrm{m}$ $=2 \times 1.8 \times 10^{-3}=3.6 \times 10^{-3} \mathrm{m}$
Q. Work done in increasing the size of a soap bubble from a radius of 3 cm to 5cm is nearly (Surface tension of soap solution = 0.03 Nm–1) :- (1) $2 \pi \mathrm{mJ}$ (2) $0.4 \pi \mathrm{mJ}$ (3) $4 \pi \mathrm{mJ}$ (4) $0.2 \pi \mathrm{mJ}$ [AIEEE-2011]
Q. Two merucry drops (each of radius 'r') merge to form a bigger drop. The surface energy of the bigger drop, ifs T is the surface tension, is : (1) $2^{5 / 3} \pi r^{2} T$ (2) $4 \pi r^{2} \mathrm{T}$ (3) $2 \pi r^{2} T$ (4) $2^{8 / 3} \pi r^{2} \mathrm{T}$ [AIEEE-2011]
Ans. (4) By volume conservation $\frac{4}{3} \pi \mathrm{R}^{3}=2\left(\frac{4}{3} \pi \mathrm{r}^{3}\right)$ $\mathrm{R}=2^{1 / 3} \mathrm{r}$ Surface energy E = T (A) $=\mathrm{T}\left(4 \pi \mathrm{R}^{2}\right)=\mathrm{T}\left(4 \pi 2^{2 / 3} \mathrm{r}^{2}\right)=2^{8 / 3} \pi \mathrm{r}^{2} \mathrm{T}$
Q. If a ball of steel (density $\left.\rho=7.8 \mathrm{g} \mathrm{cm}^{-3}\right)$ attains a terminal velocity of 10 cm s–1 when falling in a tank of water (coefficient of viscosity $\left.\eta_{\text {water }}=8.5 \times 10^{-4} \mathrm{Pa.s}\right)$ then its terminal velocity in glycerine $\left(\rho=1.2 \mathrm{g} \mathrm{cm}^{-3}, \eta=13.2 \mathrm{Pa.s}\right)$ would be nearly :- (1) $1.6 \times 10^{-5} \mathrm{cm} \mathrm{s}^{-1}$ (2) $6.25 \times 10^{-4} \mathrm{cm} \mathrm{s}^{-1}$ (3) $6.45 \times 10^{-4} \mathrm{cm} \mathrm{s}^{-1}$ (4) $1.5 \times 10^{-5} \mathrm{cm} \mathrm{s}^{-1}$ [AIEEE-2011]
Q. A wooden cube (density of wood 'd') of side '$\ell$' floats in a liquid of density 'r' with its upper and lower surfaces horizontal. If the cube is pushed slightly down and released, it performs simple harmonic motion of period 'T'. Then, 'T' is equal to :- (1) $2 \pi \sqrt{\frac{\ell \rho}{(\rho-\mathrm{d}) \mathrm{g}}}$ (2) $2 \pi \sqrt{\frac{\ell \mathrm{d}}{\rho \mathrm{g}}}$ (3) $2 \pi \sqrt{\frac{\ell \rho}{\mathrm{dg}}}$ (4) $2 \pi \sqrt{\frac{\ell \mathrm{d}}{(\rho-\mathrm{d}) \mathrm{g}}}$ [AIEEE-2011]
Ans. (2) By using $\mathrm{T}=2 \pi \sqrt{\frac{\mathrm{m}}{\mathrm{A} \rho \mathrm{g}}}$ Where $\mathrm{m}=\ell^{3} \mathrm{d}$ and $\mathrm{A}=\ell^{2}$ $\mathrm{T}=2 \pi \sqrt{\frac{\ell^{3} \mathrm{d}}{\ell^{2} \rho \mathrm{g}}} \Rightarrow \mathrm{T}=2 \pi \sqrt{\frac{\ell \mathrm{d}}{\rho \mathrm{g}}}$
Q. A thin liquid film formed between a U-shaped wire and a light slider supports a weight of $1.5 \times 10^{-2} \mathrm{N}$ (see figure). The length of the slider is 30 cm and its weight negligible. The surface tension of the liquid film is :- [AIEEE-2012]
Ans. (2) weight $=\mathrm{mg}=1.5 \times 10^{-2} \mathrm{N}(\text { given })$ length $=\ell=30 \mathrm{cm}(\text { given })$ = 0.3 m $2 \mathrm{T} \ell=\mathrm{mg}$ $\mathrm{T}=\frac{\mathrm{mg}}{2 \ell}=\frac{1.5 \times 10^{-2}}{2 \times 0.3}=0.025 \mathrm{N} / \mathrm{m}$
Q. A uniform cylinder of length L and mass M having cross- sectional area A is suspended, with its length vertical, form a fixed point by a massless spring, such that it is half submerged in a liquid of density $\sigma$ at equilibrium position. The extension $\mathrm{X}_{0}$ of the spring when it is in equilibrium is : (1) $\frac{\mathrm{Mg}}{\mathrm{k}}$ ( 2)$\frac{\mathrm{Mg}}{\mathrm{k}}\left(1-\frac{\mathrm{LA\sigma}}{\mathrm{M}}\right)$ (3) $\frac{\mathrm{Mg}}{\mathrm{k}}\left(1-\frac{\mathrm{LA\sigma}}{2 \mathrm{M}}\right)$ (4)$\frac{\mathrm{Mg}}{\mathrm{k}}\left(1+\frac{\mathrm{LA\sigma}}{\mathrm{M}}\right)$ (Here k is spring constant) [AIEEE-2013]
Ans. (1) At equilibrium $\mathrm{Kx}_{0}=\mathrm{Mg}-\mathrm{B}$ $\mathrm{Kx}_{0}=\mathrm{Mg}-\frac{\sigma \mathrm{AL}}{2}$ $\mathrm{X}_{0}=\frac{\left(\mathrm{Mg}-\frac{\sigma \mathrm{AL}}{2} \mathrm{g}\right)}{\mathrm{K}}$ $=\left[1-\frac{\sigma \mathrm{AL}}{2 \mathrm{M}}\right]$ .
Q. Assume that a drop of liquid evaporates by decrease in its surface energy, so that its temperature remains unchanged. What should be the minimum radius of the drop for this to be possible? The surface tension is T, density of liquid is and L is its latent heat of vaporization. (1) $\frac{\rho \mathrm{L}}{\mathrm{T}}$ (2) $\sqrt{\frac{\mathrm{T}}{\rho \mathrm{L}}}$ ( 3)$\frac{\mathrm{T}}{\rho \mathrm{L}}$ (4) $\frac{2 \mathrm{T}}{\rho \mathrm{L}}$ [AIEEE-2013]
Ans. (3) If a layer of thickness dr is evaporates then change in surface energy = (change in surface area) T $=\left(\mathrm{d}\left(4 \pi \mathrm{r}^{2}\right)\right) \mathrm{T}=8 \pi \mathrm{rdr} \mathrm{T}$ energy required to evaporate layer of thickness dr $=\left(4 \mathrm{pr}^{2} \mathrm{dr}\right) \rho . \mathrm{L}$ The process of evaporation only starts only if change in surface energy is just sufficient to evaporate the water layer $\Rightarrow\left(4 \pi r^{2} d r\right) L \rho=(8 \pi r d r) T$ $\Rightarrow \mathrm{r}=\frac{2 \pi}{\rho \mathrm{L}}$
Q. On heating water, bubbles being formed at the bottom of the vessel detatch and rise. Take the bubbles to be spheres of radius R and making a circular contact of radius r with the bottom of the vessel. If r << R, and the surface tension of water is T, value of r just before bubbles detatch is:- (dencity of water is $\rho_{\mathrm{w}}$) (1) $\mathrm{R}^{2} \sqrt{\frac{\rho_{\mathrm{w}} \mathrm{g}}{\mathrm{T}}}$ (2) $\mathrm{R}^{2} \sqrt{\frac{3 \rho_{\mathrm{w}} \mathrm{g}}{\mathrm{T}}}$ (3) $\mathrm{R}^{2} \sqrt{\frac{\rho_{\mathrm{w}} \mathrm{g}}{3 \mathrm{T}}}$ (4) $\mathrm{R}^{2} \sqrt{\frac{\rho_{\mathrm{w}} \mathrm{g}}{6 \mathrm{T}}}$ [JEE Mains-2014]
Ans. (4) Force due to Surface Tenstion $=\mathrm{T}(2 \pi \mathrm{r}) \sin \theta=\mathrm{T}(2 \pi \mathrm{r}) \times \frac{\mathrm{r}}{\mathrm{R}}$ This force will balance the force of Bouyancy $\mathrm{T}(2 \pi \mathrm{r}) \times \frac{\mathrm{r}}{\mathrm{R}}=\rho_{\mathrm{w}} \times \frac{4}{3} \pi \mathrm{R}^{3} \mathrm{g}$ $r=R^{2} \sqrt{\frac{2 \rho_{\mathrm{w}} g}{3 T}}$
Q. There is a circular tube in a vertical plane. Two liquids which do not mix and of densities $\mathrm{d}_{1}$ and $\mathrm{d}_{2}$ are filled in the tube. Each liquid subtends $90^{\circ}$ angle at centre. Radius joining their interface makes an angle with vertical. Ratio $\frac{\mathrm{d}_{1}}{\mathrm{d}_{2}}$ is : [JEE Mains-2014]
Ans. (1) Let Radius of circular tube is R
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Frequently Asked Questions
Find answers to common questions.
What is the most important topic in Fluid Mechanics for JEE Main?
Surface tension and surface energy appear most frequently (5–6 times from 2010–2024), followed by buoyancy and equation of continuity. If time is limited, prioritise surface tension, Archimedes' principle, and terminal velocity — these three sub-topics cover roughly 60% of all Fluid Mechanics questions asked since 2010.
Is Fluid Mechanics difficult for JEE Main?
Fluid Mechanics is rated medium difficulty overall. Buoyancy and surface tension questions are generally easier and formula-direct. Bernoulli's theorem and combined concept questions (e.g., surface tension + energy conservation) are harder. The chapter rewards students who understand why formulas work, not just what they are.
How many questions come from Fluid Mechanics in JEE Main?
JEE Main typically includes 1–2 questions from Fluid Mechanics per session. Over the 2010–2024 period, the chapter has contributed 1 question in most sessions, occasionally 2. Given that JEE Main is now held across multiple sessions per year, a student sitting two sessions has a high probability of encountering at least one Fluid Mechanics question.
How are AIEEE questions different from JEE Main questions in Fluid Mechanics?
AIEEE questions (pre-2013) and JEE Main questions (2013 onwards) follow the same conceptual framework and difficulty range for Fluid Mechanics. The format shifted from paper-based to computer-based and integer-type questions were added post-2019, but the underlying physics tested remains identical. All AIEEE PYQs on this page are fully relevant for current JEE Main preparation.
Should I memorise all Fluid Mechanics formulas for JEE Main?
You should understand and derive the key formulas rather than rote-memorise them. The six core formulas (continuity, Bernoulli, terminal velocity, surface energy, capillary rise, Stokes' law) are sufficient for 95% of JEE Main questions. Deriving them once from scratch ensures you won't confuse factors of 2, π, or ρ under exam pressure.
Do JEE Main Fluid Mechanics questions come from NCERT?
Yes — most Fluid Mechanics questions in JEE Main are based on NCERT Class 11 Physics concepts. The formulas, definitions, and even some numerical values (e.g., surface tension problems) mirror NCERT back exercises. Completing NCERT Solutions for Class 11 Physics is a non-negotiable first step.
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[AIEEE-2010] 
[AIEEE-2012] 
This force will balance the force of Bouyancy $\mathrm{T}(2 \pi \mathrm{r}) \times \frac{\mathrm{r}}{\mathrm{R}}=\rho_{\mathrm{w}} \times \frac{4}{3} \pi \mathrm{R}^{3} \mathrm{g}$ $r=R^{2} \sqrt{\frac{2 \rho_{\mathrm{w}} g}{3 T}}$ 
[JEE Mains-2014] 
