Selina Textbook Solutions: Upthrust in Fluids, Archimedes` Principle and Floatation | Physics Class 9 ICSE PDF Download

 Page 1


Chapter 5. Upthrust in Fluids, Archimedes’ Principle 
and Floatation
Exercise 5(A)
Solution 1S.
When a body is partially or wholly immersed in a liquid, an upward force acts on it. This 
upward force is known as an upthrust.
Upthrust can be demonstrated by the following experiment:
Take an empty can and close its mouth with an airtight stopper. Put it in a tub filled with 
water. It floats with a large part of it above the surface of water and only a small part of 
it below the surface of water. Push the can into the water. You can feel an upward force 
and you find it difficult to push the can further into water. It is noticed that as the can is 
pushed more and more into the water, more and more force is needed to push the can 
further into water, until it is completely immersed. When the can is fully inside the water, 
a definite force is still needed to keep it at rest in that position. Again, if the can is 
released in this position, it is noticed that the can bounces back to the surface and starts 
floating again.
Solution 2S.
Buoyant force on a body due to a liquid acts upwards at the centre of buoyancy.
Solution 3S.
The property of a liquid to exert an upward force on a body immersed in it is called 
buoyancy.
Solution 4S.
The upward force exerted on a body by the fluid in which it is submerged is called the 
upthrust. Its S.I. unit is ‘newton’.
Solution 5S.
A liquid contained in a vessel exerts pressure at all points and in all directions. The 
pressure at a point in a liquid is the same in all directions – upwards, downwards and 
sideways. It increases with the depth inside the liquid. 
Page 2


Chapter 5. Upthrust in Fluids, Archimedes’ Principle 
and Floatation
Exercise 5(A)
Solution 1S.
When a body is partially or wholly immersed in a liquid, an upward force acts on it. This 
upward force is known as an upthrust.
Upthrust can be demonstrated by the following experiment:
Take an empty can and close its mouth with an airtight stopper. Put it in a tub filled with 
water. It floats with a large part of it above the surface of water and only a small part of 
it below the surface of water. Push the can into the water. You can feel an upward force 
and you find it difficult to push the can further into water. It is noticed that as the can is 
pushed more and more into the water, more and more force is needed to push the can 
further into water, until it is completely immersed. When the can is fully inside the water, 
a definite force is still needed to keep it at rest in that position. Again, if the can is 
released in this position, it is noticed that the can bounces back to the surface and starts 
floating again.
Solution 2S.
Buoyant force on a body due to a liquid acts upwards at the centre of buoyancy.
Solution 3S.
The property of a liquid to exert an upward force on a body immersed in it is called 
buoyancy.
Solution 4S.
The upward force exerted on a body by the fluid in which it is submerged is called the 
upthrust. Its S.I. unit is ‘newton’.
Solution 5S.
A liquid contained in a vessel exerts pressure at all points and in all directions. The 
pressure at a point in a liquid is the same in all directions – upwards, downwards and 
sideways. It increases with the depth inside the liquid. 
 
When a body is immersed in a liquid, the thrusts acting on the side walls of the body are 
neutralized as they are equal in magnitude and opposite in direction. However, the 
magnitudes of pressure on the upper and lower faces are not equal. The difference in 
pressure on the upper and lower faces cause a net upward force (= pressure x area) or 
upthrust on the body. 
It acts at the centre of buoyancy.
Solution 6S.
Upthrust due to water on block when fully submerged is more than its weight. Density of 
water is more than the density of cork; hence, upthrust due to water on the block of cork 
when fully submerged in water is more than its weight.
Solution 7S.
A piece of wood if left under water comes to the surface of water because the upthrust on 
body due to its submerged part is equal to its own weight.
Solution 8S.
Page 3


Chapter 5. Upthrust in Fluids, Archimedes’ Principle 
and Floatation
Exercise 5(A)
Solution 1S.
When a body is partially or wholly immersed in a liquid, an upward force acts on it. This 
upward force is known as an upthrust.
Upthrust can be demonstrated by the following experiment:
Take an empty can and close its mouth with an airtight stopper. Put it in a tub filled with 
water. It floats with a large part of it above the surface of water and only a small part of 
it below the surface of water. Push the can into the water. You can feel an upward force 
and you find it difficult to push the can further into water. It is noticed that as the can is 
pushed more and more into the water, more and more force is needed to push the can 
further into water, until it is completely immersed. When the can is fully inside the water, 
a definite force is still needed to keep it at rest in that position. Again, if the can is 
released in this position, it is noticed that the can bounces back to the surface and starts 
floating again.
Solution 2S.
Buoyant force on a body due to a liquid acts upwards at the centre of buoyancy.
Solution 3S.
The property of a liquid to exert an upward force on a body immersed in it is called 
buoyancy.
Solution 4S.
The upward force exerted on a body by the fluid in which it is submerged is called the 
upthrust. Its S.I. unit is ‘newton’.
Solution 5S.
A liquid contained in a vessel exerts pressure at all points and in all directions. The 
pressure at a point in a liquid is the same in all directions – upwards, downwards and 
sideways. It increases with the depth inside the liquid. 
 
When a body is immersed in a liquid, the thrusts acting on the side walls of the body are 
neutralized as they are equal in magnitude and opposite in direction. However, the 
magnitudes of pressure on the upper and lower faces are not equal. The difference in 
pressure on the upper and lower faces cause a net upward force (= pressure x area) or 
upthrust on the body. 
It acts at the centre of buoyancy.
Solution 6S.
Upthrust due to water on block when fully submerged is more than its weight. Density of 
water is more than the density of cork; hence, upthrust due to water on the block of cork 
when fully submerged in water is more than its weight.
Solution 7S.
A piece of wood if left under water comes to the surface of water because the upthrust on 
body due to its submerged part is equal to its own weight.
Solution 8S.
Experiment to show that a body immersed in a liquid appears lighter: 
 
Take a solid body and suspend it by a thin thread from the hook of a spring balance as 
shown in the above figure (a). Note its weight. Above figure (a) shows the weight as 0.67 
N. 
Then, take a can filled with water. Immerse the solid gently into the water while hanging 
from the hook of the spring balance as shown in figure (b). Note its weight. Above figure 
(b) shows the weight as 0.40 N. 
The reading in this case (b) shall be less than the reading in the case (a), which proves 
that a body immersed in a liquid appears to be lighter.
Solution 9S.
The readings in the spring balance decreases. 
As the cylinder is immersed in the jar of water, an upward force acts on it, which is in 
opposition to the weight component of the cylinder. Hence the cylinder appears to be 
lighter.
Solution 10S.
A body shall weigh more in vacuum because in vacuum, i.e. in absence of air, no upthrust 
will act on the body.
Solution 11S.
Upthrust on a body depends on the following factors:
1. Volume of the body submerged in the liquid or fluid.
2. Density of liquid or fluid in which the body is submerged.
Solution 12S.
Larger the volume of body submerged in liquid, greater is the upthrust acting on it.
Page 4


Chapter 5. Upthrust in Fluids, Archimedes’ Principle 
and Floatation
Exercise 5(A)
Solution 1S.
When a body is partially or wholly immersed in a liquid, an upward force acts on it. This 
upward force is known as an upthrust.
Upthrust can be demonstrated by the following experiment:
Take an empty can and close its mouth with an airtight stopper. Put it in a tub filled with 
water. It floats with a large part of it above the surface of water and only a small part of 
it below the surface of water. Push the can into the water. You can feel an upward force 
and you find it difficult to push the can further into water. It is noticed that as the can is 
pushed more and more into the water, more and more force is needed to push the can 
further into water, until it is completely immersed. When the can is fully inside the water, 
a definite force is still needed to keep it at rest in that position. Again, if the can is 
released in this position, it is noticed that the can bounces back to the surface and starts 
floating again.
Solution 2S.
Buoyant force on a body due to a liquid acts upwards at the centre of buoyancy.
Solution 3S.
The property of a liquid to exert an upward force on a body immersed in it is called 
buoyancy.
Solution 4S.
The upward force exerted on a body by the fluid in which it is submerged is called the 
upthrust. Its S.I. unit is ‘newton’.
Solution 5S.
A liquid contained in a vessel exerts pressure at all points and in all directions. The 
pressure at a point in a liquid is the same in all directions – upwards, downwards and 
sideways. It increases with the depth inside the liquid. 
 
When a body is immersed in a liquid, the thrusts acting on the side walls of the body are 
neutralized as they are equal in magnitude and opposite in direction. However, the 
magnitudes of pressure on the upper and lower faces are not equal. The difference in 
pressure on the upper and lower faces cause a net upward force (= pressure x area) or 
upthrust on the body. 
It acts at the centre of buoyancy.
Solution 6S.
Upthrust due to water on block when fully submerged is more than its weight. Density of 
water is more than the density of cork; hence, upthrust due to water on the block of cork 
when fully submerged in water is more than its weight.
Solution 7S.
A piece of wood if left under water comes to the surface of water because the upthrust on 
body due to its submerged part is equal to its own weight.
Solution 8S.
Experiment to show that a body immersed in a liquid appears lighter: 
 
Take a solid body and suspend it by a thin thread from the hook of a spring balance as 
shown in the above figure (a). Note its weight. Above figure (a) shows the weight as 0.67 
N. 
Then, take a can filled with water. Immerse the solid gently into the water while hanging 
from the hook of the spring balance as shown in figure (b). Note its weight. Above figure 
(b) shows the weight as 0.40 N. 
The reading in this case (b) shall be less than the reading in the case (a), which proves 
that a body immersed in a liquid appears to be lighter.
Solution 9S.
The readings in the spring balance decreases. 
As the cylinder is immersed in the jar of water, an upward force acts on it, which is in 
opposition to the weight component of the cylinder. Hence the cylinder appears to be 
lighter.
Solution 10S.
A body shall weigh more in vacuum because in vacuum, i.e. in absence of air, no upthrust 
will act on the body.
Solution 11S.
Upthrust on a body depends on the following factors:
1. Volume of the body submerged in the liquid or fluid.
2. Density of liquid or fluid in which the body is submerged.
Solution 12S.
Larger the volume of body submerged in liquid, greater is the upthrust acting on it.
Solution 13S.
A stone falls faster. 
Because the volume of stone is less than the volume of bunch of feathers of the same 
mass, the upthrust due to air on stone is less than that on the bunch of feathers, and 
hence, the stone falls faster. 
However, in vacuum, both shall fall together because there will be no upthrust.
Solution 14S.
F
2 
> F
1
; Sea water is denser than river water; therefore, the upthrust due to sea water 
will be greater than that due to river water at the same level. This shall make the body to 
appear lighter in the sea water.
Solution 15S.
Observation: Volume of a block of wood immersed in glycerine is smaller as compared 
to the volume of block immersed in water. 
Explanation: Density of glycerine is more than that of water. Hence, glycerine exerts 
more upthrust on the block of wood than water, causing it to float in glycerine with a 
smaller volume.
Solution 16S.
Solution 17S.
Page 5


Chapter 5. Upthrust in Fluids, Archimedes’ Principle 
and Floatation
Exercise 5(A)
Solution 1S.
When a body is partially or wholly immersed in a liquid, an upward force acts on it. This 
upward force is known as an upthrust.
Upthrust can be demonstrated by the following experiment:
Take an empty can and close its mouth with an airtight stopper. Put it in a tub filled with 
water. It floats with a large part of it above the surface of water and only a small part of 
it below the surface of water. Push the can into the water. You can feel an upward force 
and you find it difficult to push the can further into water. It is noticed that as the can is 
pushed more and more into the water, more and more force is needed to push the can 
further into water, until it is completely immersed. When the can is fully inside the water, 
a definite force is still needed to keep it at rest in that position. Again, if the can is 
released in this position, it is noticed that the can bounces back to the surface and starts 
floating again.
Solution 2S.
Buoyant force on a body due to a liquid acts upwards at the centre of buoyancy.
Solution 3S.
The property of a liquid to exert an upward force on a body immersed in it is called 
buoyancy.
Solution 4S.
The upward force exerted on a body by the fluid in which it is submerged is called the 
upthrust. Its S.I. unit is ‘newton’.
Solution 5S.
A liquid contained in a vessel exerts pressure at all points and in all directions. The 
pressure at a point in a liquid is the same in all directions – upwards, downwards and 
sideways. It increases with the depth inside the liquid. 
 
When a body is immersed in a liquid, the thrusts acting on the side walls of the body are 
neutralized as they are equal in magnitude and opposite in direction. However, the 
magnitudes of pressure on the upper and lower faces are not equal. The difference in 
pressure on the upper and lower faces cause a net upward force (= pressure x area) or 
upthrust on the body. 
It acts at the centre of buoyancy.
Solution 6S.
Upthrust due to water on block when fully submerged is more than its weight. Density of 
water is more than the density of cork; hence, upthrust due to water on the block of cork 
when fully submerged in water is more than its weight.
Solution 7S.
A piece of wood if left under water comes to the surface of water because the upthrust on 
body due to its submerged part is equal to its own weight.
Solution 8S.
Experiment to show that a body immersed in a liquid appears lighter: 
 
Take a solid body and suspend it by a thin thread from the hook of a spring balance as 
shown in the above figure (a). Note its weight. Above figure (a) shows the weight as 0.67 
N. 
Then, take a can filled with water. Immerse the solid gently into the water while hanging 
from the hook of the spring balance as shown in figure (b). Note its weight. Above figure 
(b) shows the weight as 0.40 N. 
The reading in this case (b) shall be less than the reading in the case (a), which proves 
that a body immersed in a liquid appears to be lighter.
Solution 9S.
The readings in the spring balance decreases. 
As the cylinder is immersed in the jar of water, an upward force acts on it, which is in 
opposition to the weight component of the cylinder. Hence the cylinder appears to be 
lighter.
Solution 10S.
A body shall weigh more in vacuum because in vacuum, i.e. in absence of air, no upthrust 
will act on the body.
Solution 11S.
Upthrust on a body depends on the following factors:
1. Volume of the body submerged in the liquid or fluid.
2. Density of liquid or fluid in which the body is submerged.
Solution 12S.
Larger the volume of body submerged in liquid, greater is the upthrust acting on it.
Solution 13S.
A stone falls faster. 
Because the volume of stone is less than the volume of bunch of feathers of the same 
mass, the upthrust due to air on stone is less than that on the bunch of feathers, and 
hence, the stone falls faster. 
However, in vacuum, both shall fall together because there will be no upthrust.
Solution 14S.
F
2 
> F
1
; Sea water is denser than river water; therefore, the upthrust due to sea water 
will be greater than that due to river water at the same level. This shall make the body to 
appear lighter in the sea water.
Solution 15S.
Observation: Volume of a block of wood immersed in glycerine is smaller as compared 
to the volume of block immersed in water. 
Explanation: Density of glycerine is more than that of water. Hence, glycerine exerts 
more upthrust on the block of wood than water, causing it to float in glycerine with a 
smaller volume.
Solution 16S.
Solution 17S.
Solution 18S.
(a) Both have equal volumes. 
(b) Bounce back to the surface. 
(c) More than
Solution 19S.
Consider a cylindrical body PQRS of cross-sectional area A immersed in a liquid of 
density ? as shown in the figure above. Let the upper surface PQ of the body is at a 
depth h
1
 while its lower surface RS is at depth h
2
 below the free surface of liquid.
At depth h
1
, the pressure on the upper surface PQ, 
P
1
 = h
1 
?g.
Therefore, the downward thrust on the upper surface PQ, 
F
1
 = Pressure x Area = h
1
 ?gA ……………….(i)
At depth h
2
, pressure on the lower surface RS, 
P
2
 = h
2
 ?g
Therefore, the upward thrust on the lower surface RS, 
F
2
 = Pressure x Area = h
2
 ?gA …………………(ii)
The horizontal thrust at various points on the vertical sides of body get balanced because 
the liquid pressure is the same at all points at the same depth.
From the above equations (i) and (ii), it is clear that F
2
 > F
1
 because h
2
 > h
1
 and 
therefore, body will experience a net upward force.
Resultant upward thrust or buoyant force on the body,
F
B
 = F
2
 – F
1 
 = h
2
 ?gA – h
1
 ?gA