NCERT class 9 Floatation notes pdf | NCERT class 9 Floatation Questions and Answers

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The NCERT class 9 Floatation notes that you will find in the forthcoming section are very important for CBSE, NCERT and other board exams. In the following section, you will understand the concepts like fluids, thrust, pressure, buoyant force, Archimedes principle.


Floatation Class 9 Science Notes

This section is very important from your examination point of view. So go through these Floatation questions and answers very carefully.

What are fluids?

The substances which have a capacity to flow are called as fluids. Fluid word refers to flow.

So any substance that has a capacity to flow when a net external force is applied on it is a fluid. Liquids and gases are examples of fluids.


A body always exerts a force equal to its weight on any surface supporting it. The effect of this force on the surface depends on the area on which it is spread. This, force acting normally on a surface is called thrust.

Thrust is a vector quantity and is measured in the unit of force i.e., newton (N).

Pressure: The pressure is defined as the force exerted by a body per unit area and is represented as:

P= F/A•••• (1)
where P is pressure, F is force or thrust and A is area.

S.I Units of thrust is Newton (N) and S.I unit of area is m2. Therefore S.I unit of Pressure is N/m2.

1 N/m2 is also called 1 pascal (Pa).

One Pascal is defined as the pressure exerted on a surface area of Im? by a thrust or force of 1N.

Now if A = 1m2, then from equation (1), P = F.

Thus the thrust exerted by a body on a surface having area equal to 1m2, is called pressure.

Or

Pressure is the thrust exerted by a body on unit area of the surface.


Applications of the concept of pressure.

Since P = F/A

=> P ∝ 1/A

So more area of contact leads to less pressure and vice versa.

  1. Sleepers are laid down under rails of train. This is to provide more area of contact and less pressure. High pressure may cause land to yield.
  2. Trucks that carry heavy loads are provided with additional tyres to decrease effect of the force by increasing the area on which it acts.
  3. This fact is also used in the foundation of buildings and dams.
  4. A sharp knife is more effective in cutting than a blunt knife because the area of contact in case of a sharp knife is less.

Pressure exerted by the fluids.

The particles of gasses and liquids are in a state of random motion. When these molecules colide with the walls of the container or with the surface of any object placed in it, the molecules suffer a change in momentum.

Because of this change in momentum, the molecules exert a force on the wall or surface.

The force exerted per unit area is called pressure exerted by fluids.

Change in momentum = Pf – Pi = mv – (-mv)

=> Change in momentum = 2mv.

Pressure is exerted normal to the surface.


How much pressure is exerted by a liquid at a point inside it?

Consider a liquid at rest in a container. In a liquid, all points at the same depth are at the same pressure.

To show it, let us consider a cylindrical column of the liquid of height h, and cross sectional area a.

If d is the density of the liquid.

The volume of liquid = a x h

So, Mass of liquid in column (m) = a x h x d

If g is the acceleration due to gravity, then

Weight of liquid in the column = mass of liquid (a x h x d) x acceleration due to gravity(g).

Weight of liquid in the column = ahdg.

Now Pressure exerted by the liquid at depth of h (at point p) = thrust/area

That is Pressure= ahdg/a =hdg

i.e. pressure at a point in a liquid is proportional to its depth density and acceleration due to gravity.


What is Buoyant Force?

When a object is immersed in a liquid, it experiences an upward force. This upward force is called buoyant force.

It is due to the upward buoyant force exerted by a liquid that the weight of an object appears to be less in the liquid than its actual weight in air.

The magnitude of buoyant force acting on an object immersed in a liquid depends on two factors:

  1. Volume of an object immersed in the liquid.
  2. Density of the liquid.

On what factors does the buoyant force depends?

The buoyant force acting upon a body depends upon:

  1. Size or volume of the body immersed in the fluids: The buoyant force is directly proportional to the size or volume of the body immersed in a fluid. Thus greater the volume greater is the buoyant force.
  2. Density of the fluid in which the body is immersed: The buoyant force is directly proportional to the density of the fluid in which it is immersed. Thus greater the density of the fluid greater is the buoyant force acting on the body.
  3. Acceleration due to gravity: The buoyant force is directly proportional to the acceleration due to gravity at the place of measurement. Thus, a body when immersed in the same liquid will experience lesser buoyant force on the moon than on the earth.
  4. Temperature: Buoyant force is inversely proportional to the temperature of the fluid in which the body is immersed.

State Archimedes Principle. What is the relation between actual weight, apparent weight and the buoyant force?

Archimedes was a Greek mathematician, who lived in the third century B. C. Archimedes had formulated a law known as Archimedes principle.

This principle states that when a body is immersed in a liquid, partially or wholly, it experiences a loss of weight.

This loss of weight is equal to the weight of the liquid displaced by the immersed part of the body.

According to Archimedes principle, apparent weight of a body in the liquid = Actual weight of the body in air – weight of the liquid displaced by the body.

Applications of Archimedes principle

  1. In designing ships and submarines
  2. Lactometer is based on the Archimedes principle. It is used to determine the purity of a sample of milk.
  3. Hydrometer is also based on Archimedes principle. It is used to determine the denisty of a liquid.

Define the terms “Density” and “Relative Density”. How are these terms related to each other?

Density of a substance may be defined as its mass per unit volume.

Thus, density’D’ = Mass (M)/ Volume (V)

or D = M/V

Since the mass ‘M’ is measured in kilogram (kg) and the volume ‘V’ is measured in (meter)3,

Therefore D = M/V= Kg/m3

Thus the unit of density is Kg/m3.

Relative Density:- In many cases, instead of dealing with density of a substance, it is preferable to consider the number of times the substance is as dense as water. This is called relative density.

Relative density of a substance is defined as the ratio of its density to that of water at 4°C.

Thus, Relative density = Density of substance/ Density of water at 4°C.

Knowing the relative density of a substance, its density can be determined e.g,

relative density of silver is 10.8 and the density of water is 103 kg m3.

Thus, the density of silver is,

Density of Silver = Relative density of Silver * density of water = 10.8 x 10 3 kg/m3.


What is the Significance of relative density?

If the relative density of a solid or liquid is more than 1, then that substance will sink in water.

If the relative density of a solid or liquid is less than 1, then the substance would float in water.


What is specific gravity?

Specific gravity of the body is defined as the ratio of weight of body in air to the loss of weight in water at 4°C.

That is, Specific gravity of a body =Weight of body in air / Loss of weight in water at 4°C.


Why do we use sharp knife instead of a blunt one?

A sharp knife has a very thin blade while as the blunt one has a thicker edge.

So, in case of sharp knife, force from our hand falls over a very small area of the object producing a large pressure, cutting the objects easily.

On the other hand, in case of blunt knife, the force from our hand falls over a very large area of the object (thicker edge) and produces less pressure. This less pressure cuts the objects with difficulty.

Thus we prefer to use sharp knife instead of a blunt one.


What is principle of floatation?

When an object is placed in a liquid, two forces acting on it are:

  • Weight (w1)of the object acting vertically downwards.
  • Buoyant force (w2) acting vertically upwards.

The object will move in the direction of the force which is greater.

Depending upon the magnitude of the two forces, the following three situations are possible:

  1. When weight of object is greater than the buoyant force i.e. w1 > w2, the object sinks to the bottom.
  2. When weight of the object is equal to the buoyant force i.e. w1 = w2, then the body will completely immerse and float.
  3. When weight of object is less than the buoyant force i.e. w1 < w2, the object floats partially immersed in the liquid such that the weight of the liquid displaced by the immersed portion of the object is equal to the weight of the object.

Whether an object will float or sink in aliquid, can be predicated from the density of the object.

  1. If the density of object is greater than the density of liquid, the object will sink e.g. density of stone, iron is more than water, that is why they sink in water.
  2. If the density of object is equal to the density of the liquid, the object will float wholly immersed in water e.g. a drop of olive oil when placed in a mixture of water and alcohol floats wholly immersed in it.
  3. If density of the object immersed in a liquid is less than the density of liquid, the object will float partially immersed in the liquid or totally above the surface of the liquid e.g. wood has less density than water that is why it floats on water.

What is hydrometer? Explain its principle and construction?

A direct reading instrument used for measuring the density or relative density of a liquid is called hydrometer.

Principle of hydrometer: Hydrometer is based on the principle of floatation i.e. weight of the liquid displaced by the immersed portion of the hydrometer is equal to the weight of the hydrometer.

Construction of a Hydrometer: Hydrometer is a glass instrument. It consists of a cylindrical spherical bulb at its lower end and a long stem at its upper end.

The lower spherical bulb is filled with lead shots. This helps hydrometer to float (or stand) vertical in liquids. The stem has markings so that the relative density of a liquid can be read off directly.

The hydrometer is placed in the liquid taken in a glass jar. The reading against the level of liquid gives the relative density of the liquid.


So these were NCERT class 9 floatation notes. We are sure that you must have found the section very useful Now let’s jump to Floatation Text Questions and Answers.


Textual Questions of Floatation chapter Class 9

In this section you will get NCERT Solutions for Class 9 Science Chapter Floatation textual questions. Check them out.

Why is it difficult to hold a school bag having strap made of thin and strong string?

We know that Pressure = Force / Area.

Case1: When the Strap of the school bag is thin, that means its area is small. Thus the pressure exerted by the bag on the shoulders is large and the weight of the bag is concentrated on a small area of the shoulder.

Case 2: When the strap of the school bag is broad that means its area is more than 1st case. So the pressure exerted by the bag on the shoulders is small and the weight of the bag is disturbed over a large area of the shoulder

Therefore it is easy to hold the bag having broad strap rather than small strap.


What do you mean by buoyancy?

When a object is immersed in a liquid, it experiences an upward force. This upward force is called buoyant force.

It is due to the upward buoyant force exerted by a liquid that the weight of an object appears to be less in the liquid than its actual weight in air.

The magnitude of buoyant force acting on an object immersed in a liquid depends on two factors

  1. Volume of an object immersed in the liquid.
  2. Density of the liquid.

Why does an object float or sink when placed on the surface of water?

Whenever the upthrust due to a liquid in which an object is immersed is less than the weight, of the object it sinks in the liquid.

In case, the upthrust due to a liquid in which an object is immersed of a body is more than or equal to the weight of the object, it floats.


You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?

In fact, a weighing machine is a sort of spring balance which measures the weight and not the mass.

When we stand on the weighing machine, our weight, which is due to the gravitational attraction of the earth acts vertically downwards. But the buoyancy due to air on our body acts vertically upwards. As a result of this, our apparent weight (true weight – buoyant force) is less than the true weight.

Since the weighing machine measures the apparent weight, our true weight is more than 42 kg.


You have a bag of cotton and an iron bag, each containing a mass of 100kg when measured on a weighting machine. In reality, one is heavier than the other. Can you say which one is heavier and why?

Apparent weight is the weight measured in a weighing machine. True weight is always more than the apparent weight because there is always weight loss due to up word thrust of air.

Apparent Weight = true weight – upward thrust.

Or true weight = apparent weight + upward thrust.

Thus weight loss depends upon the upward thrust of air which in turn depends up on the volume of air displaced by the body. As the volume of air displaced by cotton bag will be more than the volume of air displaced by iron bag, therefore the upward thrust of air will be more in case of cotton than in case iron.

We know that true weight is equal to apparent weight, plus upward thrust as apparent weight is equal in both the cases i.e 100 kg and upward thrust is more in case of cotton so cotton bag will be heavier than iron bag.


In which direction does the buoyant force on an object immersed in a liquid act?

The buoyant force on an object immersed in a liquid always acts vertically upwards. i.e, in a direction opposite to the weight of the object.


Why does a block of plastic released under water come up to the surface of water?

As the density of plastic is less than the density of water therefore when a block of plastic is immersed in water the weight of water displaced by it will be greater than the weight of plastic block. Thus the upward thrust of water will be more than the weight of the plastic block.

As we know that when upward thrust due to a liquid in which an object is immersed is more than the weight of the object, it floats. As in this case the upward thrust of water is more than the weight of the plastic block, it (the plastic block) comes up to the surface of water when released under water.


The volume of 50g of a substance is 20cm’ ‚if the density of water is 1g/cm. Will the substance float or sink?

Here mass of the substance= 50 g

Volume of the substance=20cm3

Therefore, density of the substance = 50/20 = 2.5g/cm3

Since the density of the substance (2.5g/cm3 is more than that of water (1g/cm3), the substance will sink.


The volume of 500g sealed packet is 350cm3. Will the packet float or sink if the density of water is 1g/cm3. What will be the mass of water displaced by it?

Here, Mass of bucket M = 500 g

Volume of the bucket, V=350cm3

Therefore, density of bucket d = M/V

=500/350 = 1.42g/cm3

Since the density of bucket is more than density of water, it will sink. The bucket is fully submerged in water,

Mass of water displaced by the packet = volume of the packet x density of water

= 350 x 1 = 350 g


Also Check: Sources of energy Class 10 Science notes.

Also Check: Why do we fall ill Class 9 Science notes pdf.

These were the Questions and Answers of Floatation chapter of Class 9 science book. I am very sure that these Class 9 floatation notes pdf will help you a lot in your competitive exams.

MCQs on floatation Class 9 Science

These are some MCQs on Class 9 Science Floatation chapter.

A balloon filled with Helium rises upwards. It is because;

  1. density of Hydrogen is less than the density of air.
  2. density of Hydrogen is more than the density of air.
  3. density of Hydrogen is equal to the density of air.
  4. none

density of Hydrogen is less than the density of air.

When a body is placed in a fluid, it experiences;

  1. upthrust
  2. weight
  3. both of the above
  4. none of the above

both of the above

The upward force experienced by an object is equal to the product of;

  1. density of fluid, volume of object and acceleration due to gravity
  2. density of object, volume of object and acceleration due to gravity
  3. density of fluid, total volume of fluid and acceleration due to gravity
  4. none of the above

density of fluid, volume of object and acceleration due to gravity

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