Loading....
Coupon Accepted Successfully!

 

Question-1

Why does a ship made of iron float in water although a needle sinks?

Solution:
This is because the weight of water displaced by the ship is more than its own weight. On the other hand, the weight of water displaced by the needle is less than its own weight, Hence it sinks.

Question-2

It is easier to lift a heavy stone under water than on the surface of the Earth. Why?

Solution:
The upthrust of water decreases the weight of the stone in water. Hence a heavy stone can be lifted easily under water than on the surface of the Earth.

Question-3

A balloon filled with hydrogen rises up in air where as a balloon filled with air settles down. Why?

Solution:
The density of hydrogen is less than air. The upthrust of air on the balloon is more than the weight of the balloon filled with hydrogen. Thus the hydrogen balloon rises up. In case of air balloon, the weight of balloon is more than the upthrust and hence it settles down.

Question-4

Why does an air bubble in water grow in size as it rises from bottom to top?

Solution:
We know that fluids move from higher pressure to lower pressure and in a fluid pressure increases with depth. Hence pressure at the top (=P0) is less than that at the bottom (=P0+hg) and so the air bubble moves from bottom to top. The bubble cannot move side ways as the pressure at the same level in a fluid is the same. Further more, when bubbles moves from bottom to top, the pressure decreases and in accordance with Boyle’s law (PV=constant) volume V will increase i.e., the bubble will grow in size.

Question-5

A man is sitting in a boat, which is floating in a pond. If the man drinks some water from the pond, what will happen to the level of water in the pond.

Solution:
When a man sitting in a boat drinks m gram of water from the pond, the weight of (boat + man) the system will increase by mg and so the system will displace m gram more water for floating. Due to removal of water from the pond, the water level in the pond will fall. But due to water displaced by the floating system the water level in the pond will rise. Since the water removed from the pond is equal to the water displaced by the system, the level of water in the pond remains unchanged.

Question-6

A beaker containing a liquid is kept inside the jar is continuously pumped out, what will be the pressure in the liquid near the bottom of the liquid.

Solution:
The pressure in a liquid at the bottom of the liquid will decrease. It is so because the total pressure at a point near the bottom of the liquid is equal to the sum of the pressure due to column the of liquid and due to air inside the jar. When the air inside the jar is pumped out, the pressure decreases.

Question-7

A block of ice is floating in a liquid of specific gravity 1.2 contained in the beaker. What will be the effect on the level of liquid in the beaker when all the ice melts?

Solution:
The level of water in the beaker will rise when all the ice in it melts. It is so because the density of water formed by melting of ice is less than the density of liquid in a beaker. Due to this, the volume of the water formed by melting the ice will be more than the volume of the portion of the ice in a liquid while floating in it.

Question-8

A piece of ice is floating in a vessel containing water and inside the ice is a bubble of air. What will be the effect on the level of water, when the ice melts?

Solution:
The level of water remains the same. It is so because the weight of ice is equal and opposite to the weight of the water displaced. When the ice melts, the water formed must be of volume which was displaced by the immersed part of the ice.

Question-9

A hydraulic lift is designed to life cars with a maximum mass of 4 ×103 Kg. The areas of cross-section of the piston carrying the load is 5 ×103 m2. How much pressure will the smaller piston bear?

Solution:
Maximum pressure =

Maximum mass, m = 4 × 103kg
Maximum force = mg = 4
× 103 ×9.8
                              = 39.2
× 103N
area = 5
× 10-2
and Maximum pressure = = 7.84
× 105 Nm-2.

Question-10

How is Pascal’s law modified in the presence of gravity?

Solution:
Consider a vessel containing fluid. As the fluid is in equilibrium, so every volume element of the fluid is also in equilibrium. Consider that one volume element in the form of a cylindrical column of fluid is also in equilibrium. Consider one volume element in the form of a cylindrical column of fluid of height h and of area of cross section A.

                       

The various forces acting on the cylindrical column of fluid are:

(1) Force, F1= p1 A acting vertically downward on the top face of the column. p1 is the pressure of the fluid on the top face of the column.

(2) Force, F2 = p2 A acting vertically upward at the bottom face of the cylindrical column, p2 is the pressure of the fluid on the bottom face of the column.

(3) Weight, W = mg of the cylindrical column of the fluid acting vertically downward. Since the cylindrical column of the fluid is in equilibrium, so the net force acting on the column is zero.

(4) I.e., F1 + W - F2 = 0
p1A + mg - p2A = 0
p2= p1+ g
Now, mass of the cylindrical column of the fluid is,
M = volume
×density of the fluid
=Area of cross section
×height ×density
=Ah
Eqn. (i) becomes
p2 = p1+
p2 = p1+ h
ρ g.

Question-11

Why is the blood pressure in humans greater at the feet than at the brain?

Solution:
The height of the blood column is quite large at feet than at the brain. Therefore, the blood pressure in humans is greater at the feet than at the brain.

Question-12

Why are sleepers used below the rails?

Solution:
The cross-sectional area of the rails is increased by placing sleepers below them. We know that pressure is force/area, hence when the cross-sectional area of the rails is increased it reduces the pressure due to the weight of the train on the rails.

Question-13

Suppose a few drops of water are introduced in the barometer tube. What would be the effect on the barometric height?

Solution:
The barometer height would decrease due to the pressure of water vapours.

Question-14

Why is the force required by a man to move his limbs immersed in water smaller than the force for the same movement in air?

Solution:
The up thrust of water apparently reduces the weight of man in water. So it is easier for him to move the limbs in water than in air.

Question-15

Why are two holes made to empty an oil tin?

Solution:
If only one hole is made, then pressure inside the tin would be less than the atmospheric pressure. Therefore, the oil would not come out.

Question-16

A drop of olive oil is introduced in a mixture of alcohol and water. The density of the mixture is the same as that of olive oil. What is the upward thrust on the drop?

Solution:
The upward thrust is equal to the weight of the drop.

Question-17

A block of wood is floating in a lake. What is the apparent weight of the floating block?

Solution:
The apparent weight of the block is equal to zero because the weight of the block acting vertically downwards is balanced by the buoyant force acting on the block upwards.

Question-18

(c) Pins and nails are made to have pointed ends.

 


Solution:
(a) It is painful to walk bare footed on a road covered with edge pebbles.

(b) It is difficult to walk bare footed on a sandy ground.

(c) Pins and nails are made to have pointed ends.

Question-19

We can cut an apple easily with a sharp knife as compared to with a blunt knife. Explain why?

Solution:
A cutting knife has a very small area at its edge hence even a small force will create a very high pressure. This high pressure will help us to cut of an apple or any other objects easily. In the case of a blunt knife, the area of the cutting edge is large and therefore large force has to be applied to create a pressure required to cut an apple.

Question-20

The pressure produced on the ground is less when we stand on both our feet as compared to the pressure when we stand on one foot. Explain why?

Solution:
Pressure = Force/area
When we stand on both the feet, the force exerted by our body is very small because the area on which the force applied is large. When we stand on one foot the force exerted is very large because the area on which the force applied is small when compared to the previous case.




Test Your Skills Now!
Take a Quiz now
Reviewer Name