Physics

Fluid Mechanics

637 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

Three cubes of volume $V , V / 4$ and $V / 8$ respectively are immersed in thesame liquid. If the buoyant force exerted on the $3 ^ { rd }$ cube is 40 units. Then 

  1. Buoyant force on $1 ^ { st }$ cube is 80 units.
  2. Buoyant force on $1 ^ { st }$ cube is 320 units.
  3. Buoyant force on $2 ^ { nd }$ cube is 80 units.
  4. Buoyant force on $2 ^ { nd }$ cube is 320 units.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Buoyant force is equal to the weight of the displaced liquid, which is proportional to the volume of the object immersed. If the 3rd cube (V/8) experiences 40 units, the 2nd cube (V/4) has twice the volume, so it experiences 40 * 2 = 80 units.

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

A block of wood floats in water with (2/3)rd its volume submerged. The density of wood is 

  1. ${ 67gm/cm^{ 3 } }$
  2. ${ 6.7gm/cm^{ 3 } }$
  3. ${0. 67gm/cm^{ 3 } }$
  4. $7\times { 10 }^{ 3 }kg/{ m }^{ 3 }$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

For a floating object, the ratio of the submerged volume to the total volume is equal to the ratio of the density of the object to the density of the liquid. Here, 2/3 = rho_wood / rho_water. Since rho_water = 1 g/cm^3, rho_wood = 0.67 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

Initially A cubical block of density  $\rho$  is floating in water. in a vessel Out of its height  $L ,$  length  $x$  is submerged in water. Suddenly vessel is kept in an elevator accelerating upwards with acceleration  $a.$   What is the length immersed in water?

  1. $L - X$
  2. $2 x$
  3. $x$
  4. $L$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The fraction of the block submerged is determined by the ratio of the density of the block to the density of the liquid. This ratio is independent of the acceleration of the system, as both the weight and the buoyant force are scaled by the same factor (g + a).

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

A ball falls through a liquid at a constant speed. It is acted upon by three forces: an upthrust, a drag-force and its weight.
Which statement is correct?

  1. The drag-force increases with increasing depth

  2. The drag-force is equal to the sum of the upthrust and weight

  3. The upthrust is constant with increasing depth

  4. The weight is greater than the sum of the drag-force and the upthrust
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

A ball falls through a liquid at a constant speed .

It acted upon by the three force ,
1. up thrust : it is constant
2. drag force
3, weight ; it is downward
Hence, C is correct option.

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

A boat having a length of 3 metre and breadth 2 metre is floating on a lake. The boat sinks by one cm when a man gets on it. The mass of the man is:

  1. 60 kg

  2. 62 kg

  3. 72 kg

  4. 128 kg
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Amount of water displaced is $V=3\times 2\times 0.01=0.06m^3$
$\therefore F=1000\times 0.06 \times g=60g$
hence the man weighs 60kgs

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

A piece of iron of density $7800kg/m^3$ and volume $100cm^3$ is completely immersed in water. Calculate the upthrust on the iron piece.

[Take, $g=10m/s^2$.]

  1. $1N$
  2. $2.8N$
  3. $7.8N$
  4. $6.8N$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Upthrust (buoyant force) = weight of displaced liquid = density_water * volume_immersed * g. Density_water = 1000 kg/m^3, Volume = 100 cm^3 = 10^-4 m^3, g = 10 m/s^2. Upthrust = 1000 * 10^-4 * 10 = 1 N.

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

if a block of wood is floating in a river water, then the apparent weight of the floating block is

  1. equal to the weight of the displaced water

  2. zero.

  3. greater than the weight of the displaced water

  4. equal to the actual weight of the block
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Apparent weight in calculated as
Apparent weight$=$(weight of body)$-$(Buoyancy force)
Where
weight of body$=$(mass of body)$\times$(Gravitational force)
Buoyancy force$=$weight of fluid displaced
for the floating body:-
weight of body$=$Buoyancy force
$\therefore$ The body floating and
apparent weight equals to zero.
$\therefore$ Answer is B.
Multiple choice physics upthrust in fluids, archimedes' principle and floatation upthrust is equal to the weight of displaced liquid pressure in fluids buoyancy

Calculate the upthrust acting on a ball of volume $\displaystyle 0.2{ m }^{ 3 }$ immersed in a liquid having density $\displaystyle { 1kg }/{ { m }^{ 3 } }$.

  1. 0.2f

  2. 2f

  3. 0.2g f

  4. 2g f
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Upthrust = weight of the liquid displaced by the submerged part of the body Upthrust = mass of liquid displaced x Acceleration due to gravity Upthrust = volume of liquid displaced x density of liquid displaced x Acceleration due to gravity,Since volume of solid immersed is equal to the volume of the liquid displaced, Upthrust = volume of solid immersed x density of liquid displaced x Acceleration due to gravity Upthrust = 0.2 x 1 x g = 0.2g f.

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.