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PHYSICS [3 Mark Question Answer]

Fluids · ICSE STD 9 (ICSE - English Medium). 32 questions.

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[3 Mark Question Answer]

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32 questions · timed · auto-graded

Question 13 Marks
What are the factors on which the pressure of a liquid depends?
Answer
The pressure at a point in a liquid depends upon the following three factors:
(i) It depends on the point below the free surface (h)
(ii) It depends on the density of the liquid (ρ)
(iii) It depends upon acceleration due to gravity (g) of the place.
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Question 23 Marks
Explain the following:Icebergs floating in the sea are very dangerous for ships.
Answer
Relative density of Ice is$ = 0.9 cm^{-3}$
Relative density of sea water is $= 1 cm^{-3}$​​​​​​​
(Density of ice / Density of sea water) = fraction submerged of iceberg
0.9/1 = fraction of iceberg submerged
The fraction of the iceberg submerged$ = 9/10.$
Thus in colder countries where there are icebergs in oceans, only about $1/10$ is seen above water and the remaining water 9/10 remains submerged. Hence, there is a danger of these icebergs to the ships sailing in these oceans.
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Question 33 Marks
Explain the following :

A balloon filled with hydrogen rises to a certain height and then stops rising further.

Answer
The reason is that the density of air decreases with altitude. Therefore, as the balloon gradually goes up, the weight of the displaced air (i.e. uphrust) decreases. It keeps on rising as long as the upthrust exceeds its weight. When upthrust becomes equal to its weight, it stops rising.
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Question 43 Marks
State the principle of floatation.
Answer
“When a solid is floating in a fluid, the weight of whole solid acting vertically downward at its "Centre of Gravity", is equal to the weight of the fluid displaced by the "Immersed" part of solid acting upward, at its "centre of Buoyancy" or at the centre of the "Bulk of liquid" displaced.”

OR

“The weight of a floating body is equal to the weight of the liquid displaced by its "Submerged" part.”

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Question 53 Marks
State the principle of floatation. What can you say about the average density of a ship floating on water?
Answer
Principle of floatation states that a floatating body displaces an amount of fluid equal to its own weight.
Ship is designed in such a manner that it encloses large quantity of air in air tight bags and in rooms and corridors which makes the average density of ship less than that of water.
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Question 63 Marks
A piece of metal weighs 44.5 gf in air, 39.5 gf in water. What is the R.D. of the metal?
Answer
Piece of metal weighs in air = 44.5 f
Piece of metal weighs in liquid = 39.5 f.
Loss of weight of metal in liquid = 44.5 - 39.5 = 5f.
Relative density = weight of solid in air/ loss of weight of solid in water.
Relative density of liquid =44.5f/5f =8.9
Relative density of liquid = 8.9
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Question 73 Marks
Calculate the mass of a body whose volume is $2m^3$​​​​​​​ and relative density is $0.52.$
Answer
Relative density of body $= 0.52$
Density of water at $4^oC = 1000 Kgm^{-3}.$
Density of body $= 0.52 \times 1000 Kgm^{-3}= 520 Kgm^{-3}$
We know density = mass $\times$ volume.
Mass = density $\times$ volume
Mass $= 520 \times 2 =1040 Kg.$
Mass of given body is $1040 Kg.$
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Question 83 Marks
A solid weighs 2.10 N in air. It has a relative density of 8.4. How much will the body weigh if placed: In a liquid of relative density 1.2
Answer
Relative density of liquid =1.2
We know
Relative density of liquid = Loss of weight of solid in liquid/loss of weight of solid in water.
Loss of weight of solid in liquid = Relative density X loss of weight of solid in water.
Loss of weight of solid in liquid = 1.2 X 0.25 = 0.3 N.
Weight of solid in liquid = weight of solid in the air - loss of weight of solid in a liquid.
Weight of solid in liquid = 2.10 - 0.3 = 1.8 N.
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Question 93 Marks
A solid weighs 2.10 N in air. It has a relative density of 8.4. How much will the body weigh if placed: In water
Answer
Weight of solid in air = 2.10 N
Relative density of solid = 8.4
Now, Relative density = weight of solid in air/ loss of weight of solid in water.
Loss of weight of solid in water = weight of solid in air/ Relative density.
Loss of weight of solid in water = 2.10/8.4 = 0.25 N.
Weight of solid in water = weight in air - loss of weight in water
Weight of solid in water = 2.10 - 0.25 =1.85 N.
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Question 103 Marks
What is the purpose of a barometer? State two major defects of a simple barometer.
Answer
Barometer is a device used for measuring atmospheric pressure.
Simple barometer has two main defects:
(i) It is not suitable for making an accurate measurements of atmospheric pressure as any change in the level of mercury in the tube changes the level of the free surface of mercury is a trough and fixed scale cannot be used with it.
(ii) Simple barometer is not portable. So, it cannot be used by airmen, navigators, mountaineers, who need a portable barometer. >
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Question 113 Marks
How does the atmospheric pressure change with altitude?
Answer
Atmospheric pressure decreases with increase in height. Our atmosphere comprises of a large number of parallel layers. The pressure on a layer is equal to the thrust or weight of the gaseous column on the unit area of that layer. Hence, as we go up, the weight of the gaseous column decreases, which decrease the pressure of the gaseous column.
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Question 123 Marks
A dam has broader walls at the bottom than at the top. Explain.
Answer
The pressure exerted by a liquid increases with its depth. Thus as depth increases, more and more pressure is exerted by water on wall of the dam. A thicker wall is required to withstand greater pressure, therefore, the thickness of the wall of dam increases towards the bottom.
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Question 133 Marks
State Pascal's law.
Answer
Pascal's law states that pressure applied to an enclosed liquid, is transmitted equally to every part of the liquid or in other words when pressure is applied at a point in a confined fluid, it is transmitted undiminished and equally in all directions throughout the liquid.
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Question 143 Marks
A body dipped into a liquid experience an upthrust. State two factors on which upthrust on the body depend.
Answer
Upthrust or buoyant force depends on the following factors:
(i) The volume of the body submerged in the liquid.
(ii) The density of the liquid.
(iii) Acceleration due to gravity.
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Question 153 Marks
Will a body weigh more in the air or in the water when weighed with a spring balance? Why?
Answer
A body will weigh more in the air as the weight of the body acts in a downward direction and there is no force in an upward direction while the body submerged in water weigh less because an upthrust act on the body in an upward direction so the resultant weight of the body decreases.
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Question 163 Marks
A piece of wood when left underwater again comes to the surface. Explain with reason.
Answer
Wood has density less than water so the volume of water displaced by it is more than the volume of wooden block submerged so the force of upthrust is greater than the weight of wood which pushes wooden block on the surface. Hence, a piece of wood when left underwater again comes to the surface.
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Question 173 Marks
How are the (i) Mass, (ii) Volume and (iii) Density of a metallic piece affected, if at all, with an increase in temperature?
Answer
(i) Mass of a metallic body remains unchanged with increase in temperature.
(ii) Volume of metallic body increases with an increase in temperature.
(iii) Density (= Mass/volume) of a metallic body decreases with an increase in temperature.
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Question 183 Marks
A body of mass $70 kg $, when completely immersed in water, displaces $20,000 cm^3$ of water. Find the relative density of the material of the body.
Answer
When a body is completely immersed in water then it displaces an equal volume of water to its own weight.
So, the volume of the body $=20000 cm^3$.
Mass of body $=70 kg=70000 gm$
Density of body $=$ mass $/$ volume $=70000 / 20000=3.5 gm cm ^{-3}$.
Density of water in C.G.S system $=1 g cm ^{-3}$.
Relative density of body $=$ density of body $/$ density of water $=3.5 gm cm ^{-3} / 1 g cm ^{-3}$.
Relative density $=3.5$.
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Question 193 Marks
If the density of ice is $0.9 g cm ^{-3}$, then what portion of an iceberg will remain below the surface of water in sea? (Density of sea water $=1.1 g cm ^{-3}$ )
Answer
Density of ice $\left(\rho_1\right)=0.9 gcm ^{-3}$
Density of sea water $\left(\rho_s\right)=1.1 gcm ^{-3}$
Let the total volume of the iceberg be $V$ and the volume of immersed portion be $v$.
According to the law of floatation,
$
\frac{ v }{ V }=\frac{\rho_1}{\rho_s}
$
or , $\frac{ v }{ V }=\frac{0.9}{1.1}=\frac{9}{11}$
or , $v =\frac{9}{11} V$
Thus, ice floats with $\frac{9}{11}$ th part of its volume above the surface sea water.
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Question 203 Marks
A man first swims in sea water and then in river water. (i) Compare the weights of sea water and river water displaced by him.
(ii) Where does he find it easier to swim and why?
Answer

(i) 1:1; The weight of the water displaced by the man in sea and river will be same and will be equal to his own weight.

(ii) He finds it easier to swim in the sea because the density of sea water is more than the density of river water. So his weight is balanced in sea water with a part of his body submerged in the water.

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Question 213 Marks
An iron nail sinks in the water while an iron ship floats on water. Explain the reason.
What can you say about the average density of a ship floating on water?
Answer
The density of iron is more than the density of water so it sinks down in the water but in case of a ship, it is designed in such a manner that it encloses large quantity of air in airtight bags and in rooms and corridors which makes the average density of ship less than that of water and ship floats on the surface of the water.
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Question 223 Marks
When a piece of wood is suspended from the hook of a spring balance, it reads 90 gf. The wood is now lowered into the water. What reading do you expect on the scale of the spring balance?
Answer
Wood has density less than water so the volume of water displaced by it is more than the volume of wooden block submerged so the force of upthrust is greater than the weight of wood which makes it float on the water surface. And the apparent weight of the piece of the wood would be zero.
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Question 233 Marks
It is easier to lift a heavy stone under water than in air. Explain.
Answer
It is easier to lift a heavy stone under water than in air because in water, it experiences an upward buoyant force which balances the actual weight of the stone acting downwards. Thus, due to upthrust there is an apparent loss in the weight of the heavy stone, which makes it lighter in water, and hence easy to lift.
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Question 243 Marks
A body weighs 300 gf in air and 280 gf when completely immersed in water. Calculate:
(i) The loss in weight of the body,
(ii) The upthrust on the body.
Answer
Weight of the body in air = 300 gf.
Apparent Weight of the completely immersed body in water = 280 gf.
(i) Loss in weight of the body = Weight of body in air - apparent weight of the immersed body.
Loss in weight = 300 gf - -280 gf = 20 gf.
(ii) As upthrust on the body = loss in weight
(iii) So uptrust = 20 gf.
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Question 253 Marks
Why can't water be used in place of mercury in a barometer?
Answer
Water can't be used in place of mercury in a barometer because of its low density. It would require a 10.34 m long tube to measure 1 atmospheric pressure which is not practically possible while mercury having high density (13.6 g/cc) would require only 0.76 m long pipe which is practically possible.
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Question 263 Marks
Why is an aneroid barometer called so?
Answer
Aneroid means containing no liquid and the aneroid barometer is evacuated so it tends to collapse under the pressure of air. The stout spring balances the thrust on the metal box due to normal air pressure and prevents the box from collapsing. This type of barometer doesn't contain any liquid so it got its name aneroid barometer.
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Question 273 Marks
How does atmosphere pressure vary with height?
Answer
Atmospheric pressure decreases with an increase in height. our atmosphere comprises a large number of parallel layers. The pressure on a layer is equal to the thrust or weight of the gaseous column on the unit area of that layer. Hence, as we go up, the weight of the gaseous column decreases, which decreases the pressure of the gaseous column.
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Question 283 Marks
The atmospheric pressure is $75 \ cm$ of mercury. Express it in $Nm^{-2}.$
Answer
The pressure is given by $P = h Xp Xg$.
Where h is the height of the liquid column, p is the density of the liquid, g is the acceleration due to gravity.
Density of mercury is $=1.36 \times 10^4 kg / m ^3$.
$h =$ height of mercury column which is given $=75 cm=0.75 m$.
So pressure $=0.75 \times 1.36 \times 10^4 \times 9.8=9.996 \times 104 Nm ^{-2}$.
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Question 293 Marks
Deduce an expression for the pressure at a depth inside the liquid.
Answer
For calculating the total pressure in a liquid at depth, we have to add the atmospheric pressure that acts on the free surface of the liquid.

Total pressure inside a liquid at depth 'h' = atmospheric pressure + pressure due to the liquid column.

So Total pressure inside a liquid at depth = Po + hpg. Here Po is atmospheric pressure acting on the free surface of liquid.

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Question 303 Marks
State three factors on which the pressure at a point in a liquid depends.
Answer
Pressure at a point in a liquid depends upon the following three factors:
(i) Depth of the point below the free surface.
(ii) Density of liquid.
(iii) Acceleration due to gravity.
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Question 313 Marks
A dam has broader walls at the bottom than at the top. Explain.
Answer
The pressure exerted by a liquid increases with its depth. Thus as depth increases, more and more pressure is exerted by water on wall of the dam. A thicker wall is required to withstand greater pressure, therefore, the thickness of the wall of dam increases towards the bottom.
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Question 323 Marks
Explain, why a gas bubble released at the bottom of a lake grows in size as it rises to the surface of the lake.
Answer
Lake has greater pressure at the bottom than the surface as pressure increases with depth. So when the gas bubble is released at the bottom of the lake it experiences more pressure and is small in size but as it rises upwards the pressure experienced by it decreases. So it grows in size as it moves towards the surface from bottom.
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[3 Mark Question Answer] - PHYSICS STD 9 Questions - Vidyadip