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 vessel in the shape of a hollow hemisphere surmounted by a cone is held with the axis vertical and vertex uppermost. If it be filled with a liquid so as to submerge half the axis of the cone in the liquid, and the height of the cone be double the radius of its base, find the value of $x$, where the resultant downward thrust of the liquid on the vessel is $x$ times the weight of the liquid that the hemisphere can hold.

  1. $15/8$
  2. $1/8$
  3. $5/8$
  4. $15/2$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The resultant downward thrust is calculated by integrating the pressure over the wetted surface area. For a cone with height h=2r, submerging half the axis means the liquid height is r. The calculation involves the weight of the displaced liquid and the pressure at the base, leading to the ratio 5/8.

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

What is the area of the smallest block of ice $0.5\ m$ thick that will just support a man of mass $100\ kg$? The block of ice is floating in fresh water.$\left(SG\ of\ ice\ =0.9\right)$.

  1. $2m^{2}$
  2. $4m^{2}$
  3. $10m^{2}$
  4. $0.25m^{2}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

For the ice block to support the man, the total upward buoyant force from the displaced water must equal the combined weight of the ice and the man. Using the specific gravity of ice (0.9) and thickness 0.5 m, we solve for the area to find 2 square meters.

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

A wooden cube floating in water supports a mass m = 0.2 kg on its stop. When the mass is removed the cube rises by 2 cm. The side of the cube is - (density of water $10^3 kg/m^3$)

  1. 6 cm

  2. 12 cm

  3. 8 cm

  4. 10 cm
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
Cube is floating, therefore
Weight of liquid displaced $=$ Weight of cube
let  the volume of liquid displaced $= V$
Density of liquid $= \rho$
Mass of cube $= M$

Then,

$(M+m)g=\rho v g$

$\implies M+m=\rho v$       ............(1)

When mass is removed, cube reise by $l=2\,cm$

Therefore,

$Mg=(V-lA)\rho g$

where, $AS$ is the are of face of cube

$\implies M=\rho(V-lA)$          .................(2)

Eliminating $M$ by subtracting (2) from (1)

$m=\rho l A$

$\implies A=\dfrac{m}{\rho l}$

Let side of cube $=a$

Then,

$a^2=A=\dfrac{m}{\rho l}$

$\implies a=\sqrt{\dfrac{m}{\rho l}}$

If liquid is water, then $\rho=1000\,kg/m^3$

$I=0.02\,m$ (given)

$m=0.2\,kg$ (given)

Putting these value, we get $a=0.1\,m=10\,cm$
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 body is floating in liquid with  $50\%$  of its volume out side the liquid. When the entire system accelerates upwards with an acceleration  $g / 3 ,$  the percentage of its volume outside the liquid is

  1. $33 \%$
  2. $50 \%$
  3. $25 \%$
  4. $66 \%$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

When a system accelerates vertically, both the effective gravity and the buoyant force scale by the same factor (g + a). Since the ratio of forces remains unchanged, the fraction of the body submerged or outside the liquid remains unaffected at 50 percent.

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

A body floats in a liquid if the buoyant force is

  1. zero

  2. less than its weight

  3. equal to its weight

  4. none of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

According to the principle of flotation, a body floats in equilibrium when the upward buoyant force exerted by the fluid is exactly equal to the downward gravitational force (weight) of the body.

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.