Floatation: Previous Year s’ Questions of 3 marks for class IX

1. An object is made to fall from different heights 20 cm, 40 cm and 60 cm on wet sand.

 (a) What do you observe on the sand?  (b) Explain the reasons of the observations. CBSE Questions

2. A solid object of mass 50 g and volume 100 cm3 is put in water. Will the object float or sink? Give reasons for your answer.

3. Name the instrument used to determine the density of liquid in which balance and graduated cylinder are not required. Why this instrument is made heavy near the bottom?

4. A sphere of mass 5 kg and volume 2.2 × 10^–4 m3 is completely immersed in water. Find the buoyant force exerted by water on the sphere. Density of water = 1000 kgm^–3. (Given: g =9.8m/s²)

5. What happens when :(a) Buoyant Force exerted by the fluid is less than the weight of the body?

(b) Buoyant Force exerted by the fluid is equal to the weight of the body?

6. What is Relative Density? The relative density of a substance is greater than 1, w hat does it signify?

7. If relative density of aluminum is 2.7 and density of water is 1000 kg/m³. What is the density of aluminum in SI unit?

8. The volume of a solid block is 300 cm³, find the mass of water displaced when it is immersed in water? (Density of water is 1 g/cm³)

9. (a) A floating boat displaces water weighing 6000 N.

(i) What is the buoyant force on the boat?  (ii) What is the weight of boat?

(b) What happens to the buoyant force as more and more volume of a solid object is immersed in a liquid?

10. (a) Define relative density.

(b) Relative density of gold is 19.3. The density of water is 10.3 kg/m3. What is the density of gold in S.I. units?
More Questions are solved Here:  Floatation Thrust-and-pressure

IX Thrust and Pressure, Archimedes’ Principle, Relative Density Related posts:
CBSE Class 9 - Science - Chapter 10: Flotation: Notes and Quest	Read
MCQ: Flotation: Thrust, Pressure, Buoyancy and Density	Read
Thrust and Pressure, Archimedes’ Principle, RD	Read
Notes : Flotation: Thrust, Pressure, Buoyancy and Density	Read
Flotation Term-II Class IX  Buoyant force Detail Study	Read
9th Physics Solved Numerical Floating bodies	Read

IX Floatation Physics: Very Short answer Questions

IX Floatation Very Short answer Questions (1 Mark)

1. Give reason why, a block of plastic when released under water comes up to the surface of water.

Ans: It is because of buoyancy force

2. Define density. What is the SI unit of density?

Ans: Mass per unit volume of a substance is called density.

The SI unit of density is kg/m3 or gm/cm3

3. Name and define SI unit of pressure.

Ans: Pascal or n/m2

1 Pascal is the pressure act when 1 N  force acts perpendicular on unit area.

4. What do you understand by the term buoyancy?

Ans: Tendency of fluid to exert an upward force when a body is immersed in to it is called buoyancy

5. Why does a mug full of water feel lighter inside water?

Ans: a mug full of water feels lighter inside water because of buoyancy force

6. A perpendicular force of 50 N acting on a surface generates a pressure of 250 Pa. Calculate the area of cross-section of the surface on which pressure is acting.

Ans: p = F/A Þ A = F/p Þ 50 N/250 Pa = 0.2 m2

7. What is a lactometer used for?

Ans: lactometer used for finding purity of milk

8. State the unit of density and relative density in SI system.

Ans: The unit of density is  kg/m3

There is no unit of relative as it is ratio of same physical quantity.  

9. State Archimedes’ principle.

Ans: When a body is immersed partially or completely in a fluid (liquid or gas), it experiences an up thrust or buoyant force which is equal to the weight of the fluid displaced by the body.

10. What is meant by buoyant force?

Ans:  if a body is immersed partially or completely in a fluid, it experiences an up thrust called buoyant force.

11. A cork floats while the nail sinks in water. Give reason.

Ans: Weight of cork is  less than buoyant force acting on it so it floats Where as : Weight of the nail  is  more than buoyant force acting on it so it sink

12. What is the principle of floatation?

Ans: A body floats is a liquid if weight of the liquid displaced by the immersed portion of the body is equal to the weight of the body.

13. A body of mass 400 g has a volume of 300 cm³. Will body sink in water or not? [Density of water = 1000 kgm–3]

Ans:  density of body =m/v= 400/300=1.3gm/cm3 which is greater than density of water 1gm/cm3

Hence, Body will  sink in water.

14. When we stand on loose sand, our feet go deep into the sand. But when we lie down on the sand our body does not go that deep in the sand. Why?

Ans: when we lie down on the sand our body exert same force on large are that reduce pressure and our body does not go that deep in the sand.

15. Why is it easy to walk on sand with flat shoes, then with high heel shoes?

Ans: flat shoes exert force on large surface area and less force act on sand this make easy to walk on sand

CBSE Class 9 - Science - Chapter 10: FLOATATION

CBSE Class 9 - Science - Ch10: FLOATATION: Buoyancy

BUOYANCY AND FORCE OF BUOYANCY

When a body is immersed fully or partially in a fluid, it displaced the fluid whose volume is equal to the volume of the body immersed in the fluid. This displaced fluid exerts an upward force on the body called the force of buoyancy.

This tendency of the displaced fluid to exert an upward force) is called buoyancy.     OR,

The tendency of an object to float in a liquid or the power of liquid to make an object float is called buoyancy

Up thrust or buoyant force depends on:

(i) The size or volume of the body immersed in a liquid.

(ii) The density of the liquid in which the body is immersed.

Quest Time  (Answer these questions)

Q. What are the factors the buoyant force depends on?

Answer: 1. Volume of the object immersed in the liquid. 2. Density of the liquid.

Q. Name the factors the buoyant force does not depend on?

Ans:  Buoyant force does not change with depth.

Q .In what direction does the buoyant force on an object immersed in a liquid act?

Ans: in the upward direction

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

Ans: It is because  the upward buoyant force acting on plastic block  is greater than the downward gravitational force

Q. The volume of 50 g of a substance is 20 cm³. If the density of water is 1 g cm⁻³, will the substance float or sink?

Ans: Density of substance: masss/volume = 50g/20cm³= 2.5g/cm³

The density of the substance is more than the density of water (1 g cm−3). Hence, the substance will sink in water.

Q. The volume of a 500 g sealed packet is 350 cm3. Will the packet float or sink in water if the density of water is 1 g cm−3? What will be the mass of the water displaced by this packet?

Ans: Density of substance: masss/volume =500/350 =1.428

The density of the substance is more than the density of water (1 g/cm³). Hence, it will sink in water.

The volume of water displaced by the packet =  the volume of the packet= 350 cm³

The mass of water displaced by the packet  = dxv = 1 g/cm³ x 350 cm³  =350g

Floating and Sinking: if density of object is more than the density of liquid object float unless sink

Statement of Archimedes principle:

When a body is immersed partially or completely in a fluid (liquid or gas), it experiences an up thrust or buoyant force which is equal to the weight of the fluid displaced by the body.

Applications based on Archimedes’ principle are:

a. Designing of ships and submarines

b. Lactometers, which are used to determine the purity of a sample of milk

c. Hydrometers used for determining density of liquids

Proof of Archimedes Principe :

Consider a cylindrical body of cross-sectional area ‘a’ submerged in a liquid of density r . Let the upper face of the body is at a depth h1 below the surface of the liquid and the lower face is at a depth h2 below the surface of the liquid. The pressure exerted by the liquid on the upper surface of the body is given by p1 = h1dg .

Downward thrust on the upper surface of the body is ,

F1 = p1 × a = h1 d g × a

Pressure exerted by the liquid at the lower surface of the body,

P2 = h2 d g

Upward thrust on the lower surface of the body is,

F2 = P2 × a = h2 d g × a

The horizontal thrusts acting on the vertical sides of the body being equal and opposite from all the sides cancels out.

Resultant upthrust or buoyant force acting on the body

F = F2 - F1 = h2 d g a - h1 d g a = (h2 - h1) d g a  Since volume of the body, V = (h2 - h1)a

F = V dg

Thus, when a body is submerged in a liquid, it experiences an upward thrust equal to the weight of the liquid displaced by the body.

Flotation Term-II Class IX Force :Pressure:Thrust:Atmospheric pressure:Buoyant force

Science CBSE Physics Flotation Term-II Class IX Force :

Pressure :Thrust : Atmospheric pressure: Buoyant force

Thrust – The force acting on a body perpendicular to its surface is called thrust.
The S.I. unit of thrust is Newton (N).
 e.g. For fixing a poster on a bulletin board one has to press drawing pins with the thumb.

When pressing a drawing pin, force is applied on the surface area of its head. 

The force is directed perpendicular to the surface of the board. This force is called thrust.
Pressure – The thrust per unit area is called pressure.
Pressure = Thrust/ Area
The S.I. unit of pressure is Newton per square metre (N/m2) which is also called pascal (Pa). 

Many times a bigger unit of pressure called kilopascal (kPa) is used.
The pressure depends on two factors:
(a). Force applied              (b). Area over which force acts.

The same force can produce different pressures depending on the area over which it acts 

e.g. when a force acts over a large area of an object, it produces a small pressure. 

But if the same force acts over a small area of the object, it produces a large pressure.
Let we take  two similar bricks lying on the ground, one in the lying position and another in the standing position. The two bricks exert the same force on the ground because they have the same weight. 

But the two bricks exert different pressures on the ground because their areas in contact with the ground are different. 

The brick in the lying position has a large area in contact with the ground. So, the force of the weight of the brick falls on a large area of the ground and the ‘force per unit area ‘ or pressure on the ground is less. 

The brick in the standing position has a small area in contact with the ground. So, the force of the weight of the brick falls on a smaller area of the ground and the pressure on the ground is more.

Read full notes by CBSE ADDA
CBSE PHYSICS: 9th CBSE PHYSICS Gravitation Class IX