Mechanics

31.Water pipes burst due to extreme cold in winter nights because-
 
A. The density of water decreases after freezing
 
B. Water becomes hard after freezing, so its hardness breaks the pipe.
 
C. The volume of water increases after freezing
 
D. Water pipe metal melts after freezing
 
Option”C” is correct.
In the anomalous expansion, water contracts on heating and expands on cooling in the range 0∘Cto4∘C. Therefore, water pipes sometimes burst.
32.A stoppered bottle filled with water will break on freezing because-
 
A. Bottle shrinks when frozen
 
B. The volume of water decreases on freezing
 
C. The volume of water increases on freezing
 
D. Glass is a bad conductor of heat
 
Option”C” is correct.
When water freezes, the ice formed expands and occupies more volume than the volume initially occupied by water. Hence, the same mass of water takes up more volume upon freezing and therefore has less density than water, i.e ice. This expansion causes the bottle of water to crack on freezing.
33.What will be the change in the volume of water if the temperature is dropped from 9°C to 3°C?
 
A. Volume will not change
 
B. Volume will first increase and then decrease
 
C. Volume will first decrease and then increase
 
D. Water will freeze
 
Option”C” is correct.
34.What is the volume of water displaced by the boat for a steel boat floating in a lake?
 
A. Below boat weight
 
B. Over load
 
C. Equal to the weight of the part of the boat that is below the surface of the water in the lake
 
D. Tonnage
 
Option”C” is correct.

  • When the object is submerged in water then the buoyancy force (Fb) act on the object in upward direction.
  • Here the boat is floating in the water. So buoyancy force balances the weight of the boat.

Buoyancy force (Fb) = Weight (W)

35.Which rule applies if a carpet is beaten with a stick to clean it?
 
A. First law of motion
 
B. Second law of motion
 
C. Third law of motion
 
D. Combination of first and second laws of motion
 
Option”A” is correct.
The dust particles get separated out from the carpet by beating it with a stick due to their inertia of rest which is explained by Newton’s first law of motion.
36.It is more difficult to walk on snow than to walk on the road because-
 
A. Snow hardens off the road
 
B. The road hardens with snow
 
C. When we push with our feet, the snow does not react
 
D. Friction is less on the snow than on the road
 
Option”D” is correct.

  • When your shoes push back on the ground the force of friction between the ground and your shoes pushes you in the opposite direction, allowing you to move forward.
  • Whereas when two surfaces in contact are very smooth this or have very less friction it becomes difficult to walk on such surface.
  • Thus because of this there’s very little friction between the bottoms of your shoes and the ice and the person will slip because of this.
  • Hence, it is difficult to walk on the ice than on the road because Ice has a lesser friction than the road.

37.It is more difficult to walk on snow than on a concrete road because-
 
A. Snow is soft and spongy while concrete road is hard.
 
B. The force of friction between the foot and the snow is less than the force of friction between the road and the foot.
 
C. The force of friction on ice is greater than on concrete road.
 
D. None of the above.
 
Option”B” is correct.
Friction is a type of contact force that opposes motion. Ice floor is smooth. The force of friction is very less between the feet and an icy floor when compared to a concrete floor. Thus, people can skid easily on ice surfaces.
38.It is easier to roll a wooden cylinder instead of pulling it. Because-
 
A. The weight of the object is acting when it is pulled
 
B. The frictional force in the rolling state is much less than the frictional force in the sliding state.
 
C. The surface area of ​​the barrel in contact with the road is more in the towed state.
 
D. Any option other than the above options.
 
Option”B” is correct.
During pulling we have to oppose the kinetic friction whereas during rolling we have to oppose the rolling friction.
Rolling friction being smaller than the kinetic friction causes less force in rolling the cylinder.
39.When a ship sailing in a river comes to the sea, then the level of the ship-
 
A. As before
 
B. Will rise up a little
 
C. Will come down a bit
 
D. Up or down depending on the load it carries
 
Option”B” is correct.
The correct answer is It rises a little. A ship rises a little when enters the sea from a river. Because of salinity, seawater has more density than river water. In the sea, the more buoyant force is exerted upon the ship than in the river
40.Why does an iron nail float in mercury while it sinks in water?
 
A. Due to the tendency of chemical action of iron in mercury to be less than that of water
 
B. Iron weighs more than water and less than mercury
 
C. The density of iron is greater than that of water and less than that of mercury.
 
D. Mercury is heavier than water
 
Option”C” is correct.
An iron nail floats on mercury because the density of mercury is greater than the density of iron. But the density of water is less than that of iron so it sinks in water.

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