Mechanics

21.When a solid body is immersed in water, then its weight decreases, how much is this loss?
 
A. Equal to the weight of water displaced
 
B. Less than the weight of displaced water
 
C. More than the weight of displaced water
 
D. Not related to the weight of displaced water
 
Option”A” is correct.
Loss in weight = W – W’ = Fb (Buoyancy force) = weight of the water displaced. Thus loss in weight is equal to the weight of the water displaced.
22.Ice floats in water but sinks in alcohol because-
 
A. Water is more transparent than alcohol
 
B. Ice is formed by freezing of water
 
C. Ice is solid while liquid is
 

D. Ice is lighter than water and heavier than alcohol

Option”D” is correct.
Materials less dense than ethyl alcohol will float in it, while materials more dense will sink. Therefore as observed in this demonstration, ice with a density of 0.92 g/cm3 will float in water but will sink in ethyl alcohol.

23.Steel ball floats in mercury because-
 
A. Nothing can sink in mercury
 
B. The density of mercury is higher than that of steel
 
C. The density of steel is greater than that of mercury
 
D. Bullet can’t swim
 
Option”B” is correct.
The steel ball floats in the mercury because it’s less dense than mercury. Here a part of it is in the mercury while a part is in water. due to the density of mercury, it floats on it as it gets some upthrust and when water is poured over the setup, then it gets some additional upthrust from water too.
24.An iron ball is dropped in a bucket full of mercury, then –
 
A. It will sit in the bottom of the vault
 
B. It will float on the surface of mercury
 
C. It will dissolve
 
D. None of the above
 
Option”B” is correct.
The ball will float. It is because the density of the iron ball is lesser than the density of mercury.
25.At what temperature is the density of water maximum?
 
A. 0°C
 
B. 1°C
 
C. 2°C
 
D. 4°C’
 
Option”D” is correct.
The density of water is maximum at 𝟒°𝐂 temperature.
26.The quantities that could remain constant, if an object’s speed is changing, is/are
 
A. Displacement
 
B. Velocity
 
C. Acceleration
 
D. Linear momentum
 
Option”D” is correct.

Displacement,  , when v changing also S will change. 
speed and velocity are same in magnitude but speed is scalar and velocity is a vector means velocity is direction-aware. 

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27.Why does a swimmer find it easier to swim in sea water than in a river?
 
A. Pollution in sea water is less
 
B. Sea waves help swimmers to swim
 
C. The density of sea water is higher than that of ordinary water.
 
D. The volume of water in the sea is more
 
Option”C” is correct.
The reason is that the sea water contains salt and so its density is more than the density of river water. The weight of a man gets balanced by the less immersed part of his body in sea water as compared to river water. Thus, it is easier for a person to swim in sea water than in river water.
28.The reason for the floating of clouds in the atmosphere is
 
A. low pressure
 
B. Low density
 
C. Low viscosity
 
D. Low temperature
 
Option”B” is correct.
Even though a cloud weighs tons, it doesn’t fall on you because the rising air responsible for its formation keeps the cloud floating in the air. The air below the cloud is denser than the cloud, thus the cloud floats on top of the denser air nearer the land surface.
29.How much part of the floating iceberg remains above the surface of the ocean?
 
A. 1/
 
B. 1/10
 
C. 1/6
 
D. 1/9
 
Option”B” is correct.
Because the difference in relative density between ice and sea water is small, only some of the iceberg floats above the water. In fact, on average only 1/10th of an iceberg is above the surface of the water.
30.The bottom wall of the dam is made thick because-
 
A. Fluid pressure increases with increase in depth
 
B. Fluid pressure decreases with increase in depth
 
C. The density of liquid increases with increasing depth
 
D. The density of liquid decreases with increase in depth
 
Option”A” is correct.
The pressure exerted by liquids increases with depth. So, as the depth increases, more pressure is applied to the walls. That is why walls are made thicker at the bottom, so that they can handle the pressure exerted by water.

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