9th Standard Science Solution in English – Lesson 3 Fluids

Lesson 3 Fluids

9th Science Guide: Hello, students! Welcome to this page where you will find the 9th Standard Science Solutions for the unit called ‘Fluids,‘ We have taken great care to provide solutions for almost all the book-back questions in the Third unit of Class 9 Science, specifically focusing on the chapter on Fluids. Furthermore, we have included some additional questions that we believe could be relevant to the final examination

Fluids Book Back Answers

Here, you will find answers for various sections related to the lesson on Fluids, including “Choose the Correct Answer,” “Fill in the Blanks,” “State Whether True or False,” and more.

I. Choose the correct answer

1. The size of an air bubble rising up in the water

  1. decreases
  2. increases
  3. remains same
  4. may increase or decrease

Ans: Increases

2. Clouds float in the atmosphere because of their low

  1. density
  2. pressure
  3. velocity
  4. mass

Ans: density

3. In a pressure cooker, the food is cooked faster because

  1. increased pressure lowers the boiling point
  2. increased pressure raises the boiling point
  3. decreased pressure raises the boiling point
  4. increased pressure lowers the melting point

Ans: increased pressure raises the boiling point

4. An empty plastic bottle closed with an airtight stopper is pushed down into a bucket filled with water. As the bottle is pushed down, there is an increasing force on the bottom as shown in the graph. This is because Depth of immersion

Fluids Book Back Answers

  1. more volume of liquid is displaced
  2. more weight of the liquid is displaced
  3. pressure increases with depth
  4. all the above

Ans: pressure increases with depth

II. Fill in the blanks

1. The weight of the body immersed in a liquid appears to be _________ than its actual weight.

Ans: lesser

2. The instrument used to measure atmospheric  pressure is _________

Ans: barometer

3. The magnitude of buoyant force acting on an object immersed in a liquid depends on ________ of the liquid.

Ans: density

4. A drinking straw works on the existence of _________ 

Ans: atmospheric pressure

5. In a fluid, buoyant force exists because the pressure at the _____________ of an object is greater than the pressure at the top. Force on bottle Depth of immersion

Ans: depth

III. True or False

1. The weight of the fluid displaced determines the buoyant force on an object.

Ans: True

2. The shape of an object helps to determine whether the object will float.

Ans: True

3. The foundations of high-rise buildings are kept wide so that they may exert more pressure on the ground.

Ans: False 

4. Archimedes’ principle can also be applied to gases.

Ans: True

5. Hydraulic press is used in the extraction of oil from oil seeds.

Ans: True

IV. Match the Following

1. Densityhρg
2. 1 gwtMilk
3. Pascal’s lawMass/Volume
4. Pressure exerted by a fluidPressure
5. Lactometer980 dyne
Ans : 1 – C, 2 – E, 3 – D, 4 – A, 5 – B

V. Answer in brief

1. On what factors the pressure exerted by the liquid depends on?

Pressure exerted by a liquid at a point is determined by,

  1. depth (h)
  2. density of the liquid (ρ)
  3. acceleration due to gravity (g).

2. Why does a helium balloon float in the air?

Hydrogen, helium, and hot air are much less dense than ordinary air and this gives them buoyancy.

3. Why it is easy to swim in river water than in seawater?

Saltwater provides more buoyant force than fresh water. Because buoyant force depends as much on the density of fluids as on the volume displaced.

4. What is meant by atmospheric pressure?

Earth is surrounded by a layer of air up to a certain height (nearly 300 km) and this layer of air around the Earth is called an atmosphere of the Earth. Since air occupies space and has weight, it also exerts pressure. This pressure is called atmospheric pressure.

5. State Pascal’s law.

Pascal’s law states that the external pressure applied on an incompressible liquid is transmitted uniformly throughout the liquid.

VI. Answer in detail

1. With an appropriate illustration prove that the force acting on a smaller area exerts a greater pressure.

Stand on loose sand. our feet go deep into the sand. we lie down on the sand. we will find that our body will not go that deep into the sand.

In both cases of the above activity, the force exerted on the sand is the weight of your body which is the same. This force acting perpendicular to the surface is called thrust. When you stand on loose sand, the force is acting on an area equal to the area of your feet. When you lie down, the same force acts on an area of your whole body, which is larger than the area of your feet. Therefore the effect of thrust, that is, pressure depends on the area on which it acts

2. Describe the construction and working of the mercury barometer.

The instrument used to measure atmospheric pressure is called a barometer. A mercury barometer, first designed by an Italian Physicist Torricelli, consists of a long glass tube (closed at one end, open at the other) filled with mercury and turned upside down into a container of mercury. This is done by closing the open end of the mercury-filled tube with the thumb and then opening it after immersing it into a trough of mercury. The barometer works by balancing the mercury in the glass tube

Fluids Book Back Answers Mercury Barometer

against the outside air pressure. If the air pressure increases, it pushes more of the mercury up into the tub and if the air pressure decreases, more of the mercury drains from the tube. As there is no air trapped in the space between mercury and the closed end, there is a vacuum in that space. The vacuum cannot exert any pressure. So the level of mercury in the tube provides a precise measure of air pressure which is called atmospheric pressure. This type of instrument can be used in a lab or weather station.

3. How does an object’s density determine whether the object will sink or float in water?

Whether an object will sink or float in a liquid is determined by the density of the object compared to the density of the liquid. If the density of a substance is less than the density of the liquid it will float. For example, a piece of wood that is less dense than water will float on it. Any substance having more density than water (for example, a stone), will sink into the water

4. Explain the construction and working of a hydrometer with a diagram.


A direct-reading instrument used for measuring the density or relative density of the liquid is called a hydrometer. Hydrometer is based on the principle of flotation, i.e., the weight of the liquid displaced by the immersed portion of the hydrometer is equal to the weight of the hydrometer

Fluids Book Back Answers Hydrometer

Hydrometer consists of a cylindrical stem having a spherical bulb at its lower end and a narrow tube at its upper end. The lower spherical bulb is partially filled with lead shots or mercury. This helps the hydrometer to fl at or stand vertically in liquids. The narrow tube has markings so that the relative density of a liquid can be read directly.

The liquid to be tested is poured into the glass jar. The hydrometer is gently lowered into the liquid until it floats freely. The reading against the level of liquid touching the tube gives the relative density of the liquid.

Hydrometers may be calibrated for different uses such as lactometers for measuring the density (creaminess) of milk, saccharometers for measuring the density of sugar in a liquid, and alcoholometers for measuring higher levels of alcohol in spirits.

5. State the laws of flotation.

State the laws of flotation

1. Te weight of a floating body in a fluid is equal to the weight of the fluid displaced by the body.

2. The center of gravity of the floating body and the center of buoyancy are in the same vertical line.

3. The point through which the force of buoyancy is supposed to act is known as a center of buoyancy.

VII. Assertion and Reason

Directions: In each of the following questions, a statement of Assertion (A) is given followed by a corresponding statement of Reason (R) just below it. Of the statements, mark the correct answer as

  1. If both assertion and reason are true and the reason is the correct explanation of assertion.
  2. If both assertion and reason are true but reason is not the correct explanation of assertion.
  3. If the assertion is true but the reason is false.
  4. If the assertion is false but the reason is true.

1. Assertion: To float, the body must displace liquid whose weight is equal to the actual weight.
    Reason: The body will experience no net downward force in that case.

Ans: If both assertion and reason are true and the reason is the correct explanation of assertion.

2. Assertion: Pascal’s law is the working principle of a hydraulic lift.
    Reason: Pressure is thrust per unit area.

Ans: If both assertion and reason are true and the reason is the correct explanation of assertion.

3. Assertion: The force acting on the surface of a liquid at rest, under gravity, in a container is always horizontal.
    Reason: The forces acting on a fluid at rest have to be normal to the surface.

Ans: If the assertion is false but the reason is true.

4. Assertion: A sleeping mattress is so designed that when you lie on it, a large area of your body comes into its contact.
   Reason: This reduces the pressure on the body and sleeping becomes comfortable.

Ans: If both assertion and reason are true and the reason is the correct explanation of assertion.

5. Assertion: Wide wooden sleepers are kept below railway lines to reduce pressure on railway tracks and prevent them from sinking into the ground.
Reason: Pressure is directly proportional to the area in which it is acting.

Ans: If the assertion is true but the reason is false.

VIII. Numerical Problems

1. A block of wood of weight 200 g floats on the surface of the water. If the volume of the block is 300 cm3, calculate the upthrust due to water.

Weight of wood block, m =200 g
Volume of the wood block, V = 300 cm3
Upthrust = Weight of the fluid displaced
= Volume of the wood block
Upthrust =300 cm3

2. Density of mercury is 13600 kg m–3. Calculate the relative density.

Density of Mercury =13600 kg m–3
Density of water at 4°C = 1000 kg m–3
Relative density =Density of mercury / Density of water at 4°C
= 13600 kg m–3 / 1000 kg m
Relative Density = 13.6

3. The density of water is 1 kg cm–3. What is its density in S.I. units?

Density of water in SI units = 1000 kg / m3

4. Calculate the apparent weight of wood floating on water if it weighs 100g in air.

Mass of wood = 100 g.

As the wood floats on the water, water will not be displaced.
So, actual weight of wood is equal to Apparent weight of wood.

IX. Higher Order Thinking Skills

1. How high does the mercury barometer stand on a day when atmospheric pressure is 98.6 kPa?

Pressure of Atmosphere Patm = 98.6 kPa.
Density of Mercury, ρHg =13.6 × 103 kg/cm3
Acceleration due to gravity, g =9.8 m/s3
Pressure, Patm =h × ρHg × g
h =Patm / ρHg x g
= 98.6 kPa / (13.6×103)×(9.8ms-2)
= 98.6 103Pa / (13.6×103)×(9.8ms-2)
Height of Barometer, h = 0.7397 m = 739.7 mm

2. How does a fish manage to rise up and move down in water?

  1. Fish manages to rise up in the water by reducing their density by filling oxygen in the bladder via the gills. Thus volume will be increased to support its ascending motion.
  2. Fish moves down by decreasing its volume by releasing oxygen from bladder. Thus volume will be decreased so it will sink in the water.

3. If you put one ice cube in a glass of water and another in a glass of alcohol, what would you observe? Explain your observations.

Ice cube in water: As the density of the ice cube is less than water, the ice cube floats in water.
Ice cube in alcohol: As the density of the ice cube is greater than alcohol, the ice cube will sink into the alcohol.

[Note : Density : Water = 1.00, Ice cube = 0.917, Alcohol = 0.78].

4. Why does a boat with a hole in the bottom would eventually sink?

  1. The water entered through a hole will increase the weight of the boat.
  2. The boat becomes heavier so it cannot displace more water. So the boat sinks

X. Comprehension type

1. While passing nearby a pond, some students saw a drowning man screaming for help. They alerted another passerby, who immediately threw an inflated rubber tube in the pond. The man was saved. Respond to the given questions using the information provided above.

a. Why the passerby did use an inflated rubber tube to save the drowning man?

The buoyancy force of an inflated rubber tube is height and it. also capable of balancing the weight of that drowning ma

b. Write the principle involved herein.

Archmedes principle

c. Which qualities shown by the students and the passerby do you identify that helped in saving the drowning man.

    1. Helping Tendency
    2. Knowledge about the fluids.

2. A balloon displaces air and it results in buoyant force. This buoyant force is more than the weight of the balloon and hence the balloon moves up.

a. As the balloon moves up what happens to the density of it?

Density of the balloon never change.

b. Write the condition for floating of balloon.

    1. Weight of the balloon is equal to the weight of the air displaced.
    2. The density of the balloon lesser than the density of circles that place

c. The buoyant force depends on the density of ___________ Air

3. Two different bodies A and B are completely immersed in water and undergo the same loss in weight.

a. Will the weight of the body A and body B in air be the same?


b. If 4 kg of material occupy 20 cm3 and 9 kg of material be occupy 90 cm3, which has greater density A or B?

Density of the object A
Mass of the object A= 4kg
Volume of the object A=20Cm3
= 20 x 10-5m3
Density of the object A= mass/Volume
= 4/2 x 10-5
= 2 x 105Kg m-3
Density of the object B
Mass of the object B= 9kg
Volume of the object B= 90cm3
= 90 x 10-5m3
Density of the object B= mass/Volume
= 9/9 x 10-5
= 1 x 105Kg m-3

c. What vertical height of mercury will exert a pressure of 99960 Pa? (Density of mercury = 136000 kg m-3).

P= 99960 Pa
density (ρ)= 13600 Kg m-3
g= 9.8 ms-2
P= hρg
= 99960/13600 x 9.8
= 0.75m (OR) = 75 cm

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Last Updated: 30th June 2023