Mechanical Properties Of Fluids(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

MECHANICAL PROPERTIES OF FLUIDS MCQs

Total Questions : 29 | Page 3 of 3 pages

Question 21. Two bodies are in equilibrium when suspended in water from the arms of a balance. The mass of one body is 36 g and its density is 9 g / $c m^{3}$ . If the mass of the other is 48 g, its density in g / $c m^{3}$ is

Discuss Question

Answer: Option C. -> 3
:
C
Apparent weight = V

(ρ - σ) g

\frac{m}{ρ} (ρ - σ) g

where m = mass of the body,

ρ

= density of the body

σ

= density of water
If two bodies are in equilibrium then their apparent weight must be equal.

∴ \frac{m_{1}}{ρ_{1}} (ρ_{1} - σ)

\frac{m_{2}}{ρ_{2}} (ρ_{2} - σ)

\Rightarrow \frac{36}{9} (9 - 1)

\frac{48}{ρ_{2}} (ρ_{2} - 1)

By solving we get

ρ_{2}

= 3

Question 22. Density of ice is

ρ

and that of water is

σ

. What will be the decrease in volume when a mass M of ice melts

Mσ−ρ
σ−ρM
M[1ρ−1σ]
1M[1ρ−1σ]

Discuss Question

Answer: Option C. -> M[1ρ−1σ]
:
C
Volume of ice =

\frac{M}{ρ}

, volume of water =

\frac{M}{σ}

∴

Change in volume =

\frac{M}{ρ} - \frac{M}{σ} = M ⟮ \frac{1}{ρ} - \frac{1}{σ} ⟯

Question 23. A body of density

d_{1}

is counterpoised by Mg of weights of density

d_{2}

in air of density d. Then the true mass of the body is

M
M⟮1−dd2⟯
M⟮1−dd1⟯
M(1−dd2)(1−dd1)

Discuss Question

Answer: Option D. -> M(1−dd2)(1−dd1)
:
D
Let

M_{O}

=mass of body in vacuum.
Apparent weight of the body in air = Apparent weight of standard weights in air
Actual weight of the body - upthrust due to displaced air
= Actual weight of the weights- upthrust due to displaced air

\Rightarrow M_{0} g - ⟮ \frac{M_{0}}{d_{1}} ⟯ d g = M g - ⟮ \frac{M}{d_{2}} ⟯ d g \Rightarrow M_{0} = \frac{M [1 - \frac{d}{d_{2}}]}{[1 - \frac{d}{d_{1}}]}

Question 24. A long cylindrical rod of radius

R_{1}

is placed co-axially inside a cylindrical tube of radius

R_{2} (> R_{1})

filled withviscous liquid and pulled along the axis with a constant velocity. In steady state the velocity gradient

(\frac{d v}{d r})

depends on the distance from the common axis 'r' as proportional to

r∘
r−1
r−2
r−3

Discuss Question

Answer: Option B. -> r−1
:
B

f = η A \frac{d v}{d x} = c o n s t η 2 π r l \frac{d v}{d x} = c o n s t \frac{d v}{d x} α \frac{1}{r}

Question 25. Air is streaming past a horizontal air plane wing such that its speed is 120m/s over the upper surface and 90 m/s at the lower surface. If the density of air is 1.3 kg per

m e t r e^{3}

and the wing is 10 m long and has an average width of 2 m, then the difference of the pressure on the two sides of the wing is

4095.0 Pascal
409.50 Pascal
40.950 Pascal
4.0950 Pascal

Discuss Question

Answer: Option A. -> 4095.0 Pascal
:
A
From the Bernoulli's theorem

P_{1} - P_{2} = \frac{1}{2} ρ (v_{2}^{2} - v_{1}^{2}) = \frac{1}{2} \times 1.3 \times [(120)^{2} - (90)^{2}]

= 4095 N / m^{2}

or Pascal

Question 26. A spherical ballof radius

3.0 \times 10^{- 4} m

and density

10^{4}

k g / m^{3}

falls freely under gravity through a distance h it before entering a tank of water (coefficient of viscosity =

η

9.8 \times 10^{- 6} N s m^{- 2}

and density =

10^{3} k g / m^{3}

. If after entering the water, the velocity of the ball does not change, find~h.(Take g =9.8

m s^{- 2}

)

10.24 m
1.65 km
32.36 m
50.12 m

Discuss Question

Answer: Option B. -> 1.65 km
:
B
Terminal velocity,

u = \sqrt{2 g h} = \frac{2 r^{2} (ρ - σ) g}{9 η} \Rightarrow h = \frac{2 g r^{4} (ρ - r)^{2}}{81 η^{2}} = 1.65 k m

Question 27. An ice berg of density 900 Kg/

m^{3}

is floating in water of density 1000 Kg/

m^{3}

. The percentage of volume of ice-cube outside the water is

Discuss Question

Answer: Option C. -> 10%
:
C
Let the total volume of ice-berg is V and its density is

ρ

. If this ice-berg floats in water with volume

V_{i n}

inside
it then

V_{i n} σ g = V ρ g \Rightarrow V_{i n} = ⟮ \frac{ρ}{σ} ⟯ V

[

σ

= density of water]
or

V_{o u t} = V - V_{i n} = ⟮ \frac{σ - ρ}{σ} ⟯ V

\Rightarrow \frac{V_{o u t}}{V} = ⟮ \frac{σ - ρ}{σ} ⟯ = \frac{1000 - 900}{1000} = \frac{1}{10}

∴ V_{o u t} = 10 %

of V

Question 28. For which of the two pairs, the angle of contact is same

Water and glass; glass and mercury
Pure water and glass; glass and alcohol
Silver and water; mercury and glass
Silver and chromium; water and chromium

Discuss Question

Answer: Option B. -> Pure water and glass; glass and alcohol
:
B
Both liquids water and alcohol have same nature (i.e. wet the solid). Hence angle of contact for both is acute.

Question 29. In the following fig. is shown the flow of liquid through a horizontal pipe. Three tubes A, B and C are connected to the pipe. The radii of the tubes A, B and C at the junction are respectively 2 cm, 1 cm and 2 cm. It can be said that the

Height of the liquid in the tube A is maximum
Height of the liquid in the tubes A and B is the same
Height of the liquid in all the three tubes is the same
Height of the liquid in the tubes A and C is the same

Discuss Question

Answer: Option D. -> Height of the liquid in the tubes A and C is the same
:
D
As cross-section areas of both the tubes A and C are same and tube is horizontal. Hence according to equation of continuity