Physics

Fluid Mechanics

673 Questions

Fluid mechanics is a core physics topic that evaluates the principles of liquid pressure, buoyancy, density, and viscosity through complex numerical problems. The questions require calculating the volume of submerged objects, understanding hydraulic jumps, and applying fundamental fluid statics principles. It is a highly scoring subject for candidates preparing for technical and engineering competitive exams.

Liquid pressure and depthBuoyancy and densityVolume expansionHydraulic jump calculationsSurface tension mechanics

Fluid Mechanics Questions

Multiple choice physics upthrust in fluids, archimedes' principle and floatation upthrust is equal to the weight of displaced liquid pressure in fluids buoyancy

A cube of wood floats in water, with $42$% of its volume is submerged, then the density of the wood is

  1. $42\ g\ cm^{-3}$
  2. $0.42\ g\ cm^{-3}$
  3. $0.58\ kg\ cm^{-3}$
  4. $600\ g\ cm^{-3}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

By Archimedes Principle, the buoyant force on a body partially or fully immersed in a fluid is given by the weight of the fluid displaced.


Let the volume of wood be $V$
Thus, volume of wood submerged is $0.42V$

Thus, the buoyant force acting on the wood is $B = 0.42\rho gV$
Weight of wood is $\rho _\textrm{wood}gV$

Thus, in equilibrium, $0.42\rho gV = \rho _\textrm{wood}gV \Rightarrow \rho _\textrm{wood} = 0.42\rho$

As Density of water is $\rho = 1\textrm{ g cm}^{-3}$, we have $\rho _\textrm{wood} = 0.42 \textrm{ g cm}^{-3}$

Multiple choice physics upthrust in fluids, archimedes' principle and floatation upthrust is equal to the weight of displaced liquid pressure in fluids buoyancy

A: Diver dives in and reaches a depth of 100m.
B: Diver swims up from the depth of 100m to the surface.
Choose the correct alternative:

  1. A is easier than B

  2. B is easier than A

  3. A and B are equally hard

  4. A is easier than B if speed of descent and ascent is same and more.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Assume:
$F _d: \text{Force applied by diver during descent in downward direction}$
$F _u: \text{Force applied by diver during ascent in upward direction}$
$U: \text{Upthrust}$
$m: \text{Mass of the diver}$
$g: \text{Acceleration due to gravity}$
Uniform ascent and descent.

Diver has more density than water. Hence, weight of diver is more than the upthrust and without any effort, the diver sinks.
i.e. $mg>U..................(1)$

During upward motion, $F _u+U-mg=0............(2)$
During downward motion, $F _d-U+mg=0...............(3)$

From (1),(2) and (3), 
$F _u-F _d=2mg-2U$
$F _u-F _d>0$
$F _u>F _d$
Multiple choice physics upthrust in fluids, archimedes' principle and floatation upthrust is equal to the weight of displaced liquid pressure in fluids buoyancy

Mathematical proof of upthrust is based on 

  1. Definition of pressure

  2. Weight of object

  3. Pressure exerted by a column of fluid

  4. Viscosity
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Mathematical proof:
Consider a cylinder of cross section area $A$ and height $L$ completely submerged in water. Let depth of upper surface be $h$.
Using, pressure exerted by fluid column:
Force on the upper face of the cylinder = $hρgA$
Force on the lower face of the cylinder = $[h + L]ρgA$
Difference in force = $LAρg$

But $LA$ is the volume of liquid displaced by the cylinder, and $LrgA$ is the weight of the liquid displaced by the cylinder.

Therefore there is a net upward force on the cylinder equal to the weight of the fluid displaced by it.
Multiple choice botany water absorption and ascent of sap in plants factors affecting the transport of water and minerals in plants ascent of sap long distance transport of water

Cohesive force of water molecules is of the magnitude of 

  1. 1 - 10 atm

  2. 10 - 15 atm

  3. 45 - 200 atm

  4. 15 - 45 atm
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Cohesive force is called as the tensile strength of water. It is the force which acts between the water molecules. The cohesive strength of 47-207 atm. in xylem sap is sufficient to meet the stress of transpiration pull so that water column does not break.
Thus, the correct answer is option (C). 
Multiple choice biology option c: ecology and conservation hotspots and methods of conservation biodiversity: conservation conservation of biodiversity

What will be percentage of water absorbed?

  1. $\displaystyle\frac {U - V}{100} X 50$
  2. $\displaystyle\frac {V - U}{50} X 100$
  3. $\displaystyle\frac {U - V}{50} X100$
  4. $\displaystyle\frac {V - U}{100} X50$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

U - V ? 50 * 100 is the percentage of water absorbed where, 

U is the initial volume of water and V is the final volume of water. 
So, option C is the correct answer.

Multiple choice physics heat and temperature thermal expansion in liquids anomalous expansion of water thermal expansion in solids, liquids and gases

Assertion(A): Real expansion of liquid does not depend up on material of container.

Reason (R): Liquids have no definite shape. They acquire the mouth of the containers in which they are taken.

  1. Both A and R are true and R is the correct explanation of A

  2. Both A & R are true but R is not the correct explanation of A.

  3. A is true but R is false

  4. A is false but R istrue
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Real expansion would not be affected by the size and shape of container. Because of liquid has no any definate shape.
Both assertion and reason are true and reason is correct explanation.

Multiple choice physics heat and temperature thermal expansion in liquids anomalous expansion of water thermal expansion in solids, liquids and gases

(1): when a liquid with a coefficient of $\gamma $ is heated in a vessel of a coefficient of linear expansion $\frac{\gamma }{3}$, the level of liquid in the vessel remains unchanged.

 (2):$\gamma _{a}=\gamma _{r}-\gamma _{g}=\gamma _{r}-3\alpha =0$

  1. Both 1 and 2 are true and 2 is the correct explanation of 1

  2. Both 1 & 2 are true but 2 is not the correct explanation of 1.

  3. 1 is true but 2 is false

  4. 1 is false but 2 is true
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Coefficient of volume expansion is thrice the coefficient of linear expansion. Hence the coefficient of expansion of both liquid and vessel is same. Hence they expand equally with rise in temperature. Therefore no rise in the level of liquid in vessel is observed.

Multiple choice physics heat and temperature thermal expansion in liquids anomalous expansion of water thermal expansion in solids, liquids and gases

The co-efficient of apparent expansion of a liquid in a vessel A is $18\times 10^{-4}/^oC$ and in vessel B is $21\times 10^{-4}/^oC$. The difference in the coefficients of linear expansions of A and B is :

  1. $1\times 10^{-3}/^oC$
  2. $1\times 10^{-6}/^oC$
  3. $1\times 10^{-5}/^oC$
  4. $1\times 10^{-4}/^oC$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

${\gamma} _r={\gamma} _a +{\gamma} _g={\gamma} _a +3{\alpha} _g$
${\gamma} _r={\gamma} _{a1} +3{\alpha} _1={\gamma} _{a2} +3{\alpha} _2$ for both vessels A and B
$ \therefore 18 \times 10^{-4} + 3{\alpha} _1= 21 \times 10^{-4}+ 3{\alpha} _2$
$ \therefore3( {\alpha} _1- {\alpha} _2) = 3 \times 10^{-4}$
$ \therefore {\alpha} _1- {\alpha} _2 = 1 \times 10^{-4}$/ $^{0}$C

Multiple choice physics heat and temperature thermal expansion in liquids anomalous expansion of water thermal expansion in solids, liquids and gases

A specific gravity bottle is filled up to the brim with mercury of 400g, at $0^{0}$C. When heated to $90^{0}$C, the mass of the mercury that overflows from the specific gravity bottle is : (Coefficient of apparent expansion of mercury in glass is $\frac{1}{6500}/^{0}C$

  1. 5.46g

  2. 6.54g

  3. 10.92g

  4. 13.08 g
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Coefficient of apparent expansion is the ratio of liquid spilled out to the liquid left in the bottle per unit degree change in temperature.

$\gamma _{a}=\dfrac{mass   spilled }{mass   left \times \Delta \theta  }$
Let the mass spilled be x.
$\gamma _{a}=\dfrac{x}{(400-x)(90)}$
$\implies x=5.46g$

Multiple choice physics heat and temperature thermal expansion in liquids anomalous expansion of water thermal expansion in solids, liquids and gases

The coefficient of apparent expansion of a liquid when determined using two different vessels A and B are $\gamma _{1}$ and $\gamma _{2}$ respectively. If the coefficient of linear expansion of the vessel A is $\alpha $, the coefficient of linear expansion of the vessel B is

  1. $\dfrac{\alpha \gamma _{1}\gamma _{2}}{\gamma _{1}+\gamma _{2}}$
  2. $\dfrac{\gamma _{1}-\gamma _{2}}{2\alpha }$
  3. $\dfrac{\gamma _{1}-\gamma _{2}+\alpha }{3}$
  4. $\dfrac{\gamma _{1}-\gamma _{2}}{3}+\alpha $
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$\gamma _r = \gamma _l + \gamma _g=\gamma _r + 3\alpha _g$
Given,
$\gamma _r = \gamma _1 + 3\alpha _1$ ....(1)
$\gamma _r = \gamma _2 + 3\alpha _2$ ....(2)
Equating (1) and (2),
$\gamma _1 + 3\alpha _1= \gamma _2 + 3\alpha _2$
$ \therefore \alpha _2 =\frac{ \gamma _1 - \gamma _2}{3} + \alpha _1$

Multiple choice physics heat and temperature thermal expansion in liquids anomalous expansion of water thermal expansion in solids, liquids and gases

The coefficient of apparent expansion of mercury in a glass vessel is $153 \times 10^{-6}/^{\circ}C$ and in a steel vessel is $144 \times 10^{-6} / ^{\circ}C$. If $\alpha$ for steel is then, that of glass is $12 \times 10^{-6} / ^{\circ}C$.then that of glass is

  1. $ 9\times 10^{-6} / ^{\circ}C$.
  2. $ 6\times 10^{-6} / ^{\circ}C$.
  3. $36 \times 10^{-6} / ^{\circ}C$.
  4. $27 \times 10^{-6} / ^{\circ}C$.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$We\quad know\quad that\quad γ _{ real }=γ _{ apparent }+γ _{ vessel }\ So,(γ _{ app }+γ _{ vessel }) _{ glass }=(γ _{ app }+γ _{ vessel }) _{ steel }\ (∴γ _{ real }\quad is\quad same\quad in\quad both\quad cases)\quad or\quad \ 153×{ 10 }^{ -6 }+(γ _{ vessel }) _{ glass }=144×{ 10 }^{ -6 }+(γ\ _ { vessel} )\ _ { steel} \ Further(\gamma \ _ { vessel })\ _ { steel} =3α=3×(12×{ 10 }^{ -6 })=36×{ 10 }^{ -6 }/∘C\ ∴153×10−6+(γ _{ vessel }) _{ glass }=144×{ 10 }^{ -6 }+36×{ 10 }^{ -6 }\ Solving\quad we\quad get\quad (γ\ _ { vessel} )\ _ { glass} =27×10−6/∘C(γ\ _ { vessel} )\ _ { glass} \quad or\\quad α=\frac{ γ _{ glass } }{ 3 }=9×10−6/∘C$

Multiple choice physics pressure in fluids and atmospheric pressure hydraulic equipment applications of pascals law examples of hydraulic press

The principle of fluid pressure that is used in hydraulic brakes or lifts is that.

  1. pressure is the same at all levels in a fluid.

  2. increases of pressure are transmitted equally to all parts of a fluid.

  3. the pressure at a point in a fluid is due to the weight of the fluid above it.

  4. increases of pressure can only be transmitted through fluids.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Answer is B.
Hydraulics is the use of a liquid under pressure to transfer force or motion, or to increase an applied force. The pressure on a liquid is called HYDRAULIC PRESSURE. And the brakes which are operated by means of hydraulic pressure are called HYDRAULIC BRAKES. These brakes are based on the principle of Pascals law. 
The Pascal law states that 'The pressure exerted anywhere in a mass of confined liquid is transmitted undiminished in all directions throughout the liquid'.

Multiple choice physics pressure in fluids and atmospheric pressure hydraulic equipment applications of pascals law examples of hydraulic press

The center of pressure of surface subjected to fluid pressure is the point

  1. on the surface at which resultant pressure acts

  2. on the surface at which gravitational force acts

  3. equal to volume of liquid displaced

  4. none
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The center of pressure of surface subjected to fluid pressure is the point on the surface at which gravitational force acts

Multiple choice physics pressure in fluids and atmospheric pressure hydraulic equipment applications of pascals law examples of hydraulic press

An enclosed fluid under pressure exerts that pressure throughout its volume and against any surface containing it. Is the statement true or false

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

It is true that an enclosed fluid under pressure exerts that pressure throughout its volume and against any surface containing it.

Multiple choice physics pressure in fluids and atmospheric pressure hydraulic equipment applications of pascals law examples of hydraulic press

When two cylinder with different size like larger and smaller are kept together then though the pressure is same in both, still the force required is more for larger cylinder. Due to increase in which property?

  1. Area

  2. height

  3. Weight

  4. Volume
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Force required        $F = PA$    

$\implies$    $F\propto A$                          $(\because P = constant)$
Thus option A is correct.