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# Lakhmir Singh solutions for Class 9 Science chapter 3 - Gravitation

## Chapter 3 : Gravitation

#### Pages 100 - 104

Q 1 | Page 100

What is the value of gravitational constant G (i) on the earth, and (ii) on the moon ?

Q 2 | Page 100

Which force is responsible for the moon revolving round the earth ?

Q 3 | Page 100

Does the acceleration produced in a freely falling body depend on the mass of the body ?

Q 4 | Page 100

Name the scientist who gave the three laws of planetary motion.

Q 5 | Page 100

Name the scientist who explained the motion of planets on the basis of gravitational force between the sun and planets.

Q 6 | Page 100

State the Kepler’s law which is represented by the relation r3 ∝ T2.

Q 7 | Page 100

Which of the Kepler’s laws of planetary motion led Newton to establish the inverse-square rule for gravitational force between two bodies ?

Q 8 | Page 100

Name the property of earth which is responsible for extremely small acceleration being produced in it as a result of attraction by other small objects.

Q 9 | Page 100

What is the acceleration produced in a freely falling body of mass 10 kg ? (Neglect air resistance)

Q 10 | Page 100

When an object is dropped from a height, it accelerates and falls down. Name the force which accelerates the object.

Q 11 | Page 100

Give the formula for the gravitational force F between two bodies of masses M and m kept at a distance d from each other.

Q 12 | Page 100

What force is responsible for the earth revolving round the sun ?

Q 13 | Page 100

What name has been given to the force with which two objects lying apart attract each other ?

Q 14 | Page 100

What type of force is involved in the formation of tides in the sea ?

Q 15 | Page 100

Which force is responsible for holding the solar system together ?

Q 16 | Page 100

What is the weight of a 1 kilogram mass on the earth ? (g = 9.8 m/s2).

Q 17 | Page 100

On what factor/factors does the weight of a body depend ?

Q 18 | Page 100

As the altitude of a body increases, do the weight and mass both vary ?

Q 19 | Page 100

If the same body is taken to places having different gravitational field strength, then what will vary: its weight or mass ?

Q 20 | Page 100

If the mass of an object be 10 kg, what is its weight ? (g = 9.8 m/s2).

Q 21 | Page 100

The weight of a body is 50 N. What is its mass ? (g = 9.8 m/s2).

Q 22 | Page 100

A body has a weight of 10 kg on the surface of earth. What will be its weight when taken to the centre of the earth ?

Q 23 | Page 100

Write down the weight of a 50 kg mass on the earth, (g = 9.8 m/s2).

Q 24 | Page 100

If the weight of a body on the earth is 6 N, what will it be on the moon ?

Q 25.1 | Page 100

State whether the following statement is true or false :
A falling stone also attracts the earth.

Q 25.2 | Page 100

State whether the following statement is true or false :

The force of gravitation between two objects depends on the nature of medium between them.

Q 25.3 | Page 100

State whether the following statement is true or false :

The value of G on the moon is about one-sixth (1/6)of the value of G on the earth.

Q 25.4 | Page 100

State whether the following statement is true or false :

The acceleration due to gravity acting on a freely falling body is directly proportional to the mass of the body.

Q 25.5 | Page 100

State whether the following statement is true or false :

Tire weight of an object on the earth is about one-sixth of its weight on the moon.

Q 26.1 | Page 101

Fill in the following blank with suitable word :

The acceleration due to gravity on the moon is about ………………of that on the earth.

Q 26.2 | Page 101

Fill in the following blank with suitable word :

In order that the force of gravitation between two bodies may become noticeable and cause motion, one  of the bodies must have an extremely large……………………………..

Q 26.3 | Page 101

Fill in the following blank with suitable word :

The weight of an object on the earth is about……………….. of its weight on the moon.

Q 26.4 | Page 101

Fill in the following blank with suitable word :

The weight of an object on the moon is about …………………….. of its weight on the earth.

Q 26.5 | Page 101

Fill in the following blank with suitable word :

The value of g on the earth is about………………. of that on the moon.

Q 26.6 | Page 101

Fill in the following blank with suitable word :

If the weight of a body is 6 N on the moon, it will be about……………………… on the earth.

Q 27 | Page 101

Explain what is meant by the equation :
g= Gxxm/r^2
where the symbols have their usual meanings.

Q 28.1 | Page 101

What do you mean by the term free fall ?

Q 28.2 | Page 101

During a free fall, will heavier objects accelerate more than lighter ones ?

Q 29 | Page 101

Can we apply Newton’s third law to the gravitational force ? Explain your answer.

Q 30 | Page 101

Give reason for the following :
The force of gravitation between two cricket balls is extremely small but that between a cricket ball and the earth is extremely large.

Q 31 | Page 101

Give the formula for the gravitational force F between two bodies of masses M and m kept at a distance d from each other.

Q 32 | Page 101

What happens to the gravitational force between two objects when the distance between them is :
(1) doubled ?
(2) halved ?

Q 33 | Page 101

State two applications of universal law of gravitation.

Q 34 | Page 101

Explain why, if a stone held in our hand is released, it falls towards the earth.

Q 35 | Page 101

Calculate the force of gravitation between two objects of masses 50 kg and 120 kg respectively kept at a distance of 10 m from one another. (Gravitational constant, G = 6.7 x 10-11 Nm2 kg-2)

Q 36 | Page 101

What is the force of gravity on a body of mass 150 kg lying on the surface of the earth? (Mass of earth = 6 x 1024 kg; Radius of earth = 6.4 x 106 m; G = 6.7 x 10-11 Nm2/kg2)

Q 37 | Page 101

The mass of sun is 2 x 1030 kg and the mass of earth is 6 x 1024 If the average distance between the sun and the earth be 1.5 x 108 km, calculate the force of gravitation between them.

Q 38 | Page 101

A piece of stone is thrown vertically upwards. It reaches the maximum height in 3 seconds. If the acceleration of the stone be 9.8 m/s2 directed towards the ground, calculate the initial velocity of the stone with which it is thrown upwards

Q 39 | Page 101

A stone falls from a building and reaches the ground 2.5 seconds later. How high is the building ? (g =8 m/s2)

Q 40 | Page 101

A stone is dropped from a height of 20 m.
(1) How long will it take to reach the ground ?
(2) What will be its speed when it hits the ground ? (g =10 m/s2)

Q 41 | Page 101

A stone is thrown vertically upwards with a speed of 20 m/s. How high will it go before it begins to fall ? (g =8 m/s2)

Q 42 | Page 101

When a cricket ball is thrown vertically upwards, it reaches a maximum height of 5 metres.
(a) What was the initial speed of the ball ?
(b) How much time is taken by the ball to reach the highest point ? (g=10 ms-2)

Q 43 | Page 101

Write the differences between mass and weight of an object.

Q 44 | Page 101

Can a body have mass but no weight ? Give reasons for your answer.

Q 45 | Page 102

A force of 20 N acts upon a body whose weight is 9.8 N. What is the mass of the body and how much is its acceleration ? (g = 9.8 m s-2).

Q 46 | Page 102

A stone resting on the ground has a gravitational force of 20 N acting on it. What is the weight of the stone ? What is its mass ? (g = 10 m/s2).

Q 47 | Page 102

An object has mass of 20 kg on earth. What will be its (1) mass, and (2) weight, on the moon ? (g on moon = 1.6 m/s2).

Q 48 | Page 102

Which is more fundamental, the mass of a body or its weight ? Why ?

Q 49 | Page 102

How much is the weight of an object on the moon as compared to its weight on the earth ? Give reason for your answer

Q 50.1 | Page 102

Define mass of a body. What is the SI unit of mass ?

Q 50.2 | Page 102

Define weight of a body. What is the SI unit of weight ?

Q 50.3 | Page 102

What is the relation between mass and weight of a body ?

Q 51.1 | Page 102

State the universal law of gravitation. Name the scientist who gave this law.

Q 51.2 | Page 102

Define gravitational constant. What are the units of gravitational constant ?

Q 52.1 | Page 102

What do you understand by the term acceleration due to gravity of eart ?

Q 52.2 | Page 102

What is the usual value of the acceleration due to gravity of earth ?

Q 52.3 | Page 102

State the SI unit of acceleration due to gravity ?

Q 53.1 | Page 102

Is the acceleration due to gravity of earth ‘g’ a constant ? Discuss.

Q 53.2 | Page 102

Calculate the acceleration due to gravity on the surface of a satellite having a mass of 7.4 x 1022 kg and a radius of 1.74 x 106 m (G = 6.7 x 10-11 Nm2/kg2). Which satellite do you think it could be ?

Q 54 | Page 102

State and explain Keplers laws of planetary motion. Draw diagrams to illustrate these laws.

Q 55 | Page 102

The mass of a planet is 6 x 1024 kg and its diameter is 12.8 x 103 If the value of gravitational constant be 6.7 x 10-11 Nm2/kg2, calculate the value of acceleration due to gravity on the surface of the planet. What planet could this be

Q 71 | Page 103

If the distance between two masses is increased by a factor of 5, by what factor would the mass of one of them have to be altered to maintain the same gravitational force ? Would this be an increase or decrease in the mass ?

Q 72 | Page 103

Universal law of gravitation states that every object exerts a gravitational force of attraction on every other object. If this is true, why don’t we notice such forces ? Why don’t the two objects in a room move towards each other due to this force ?

Q 73 | Page 103

Suppose a planet exists whose mass and radius both are half those of the earth. Calculate the acceleration due to gravity on the surface of this planet.

Q 74 | Page 104

A coin and a piece of paper are dropped simultaneously from the same height. Which of the two will touch the ground first ? What will happen if the coin and the piece of paper are dropped in vacuum ? Give reasons for your answer.

Q 75 | Page 104

A stone and the earth attract each other with an equal and opposite force. Why then we see only the stone falling towards the earth but not the earth rising towards the stone ?

Q 76 | Page 104

What is the actual shape of the orbit of a planet around the sun ?

Q 76 | Page 104

What assumption was made by Newton regarding the shape of an orbit of a planet around the sun for deriving his inverse square rule from Kepler’s third law of planetary motion ?

Q 77 | Page 104

The values of g at six distances A, B, C, D, E and F from the surface of the earth are found to be 3.08 m/s2, 9.2.3 m/s2, 0.57 m/s2, 7.34 m/s2, 0.30 m/s2 and 1.49 m/s2, respectively.
(a) nArrange these values of g according to the increasing distances from the surface of the earth (keeping the value of g nearest to the surface of the earth first)
(b) If the value of distance F be 10000 km from the surface of the earth, state whether this distance is deep . inside the earth or high up in the sky. Give reason for your answer.

#### Pages 123 - 128

Q 1 | Page 123

Write the common unit of density.

Q 2 | Page 123

What is the density of water in SI units ?

Q 3 | Page 123

What is the value of relative density of water ?

Q 4 | Page 123

Name the quantity whose one of the units is pascal (Pa).

Q 5 | Page 123

State the units in which pressure is measured.

Q 6.1 | Page 123

State whether the following statements is true or false :

The buoyant force depends on the nature of object immersed in the liquid

Q 6.2 | Page 123

State whether the following statements is true or false :

Archimedes’ principle can also be applied to gases.

Q 7 | Page 123

In which direction does the buoyant force on an object due to a liquid act ?

Q 8 | Page 123

What is the other name of buoyant force ?

Q 9 | Page 123

Name the force which makes heavy objects appear light when immersed in a liquid.

Q 10 | Page 123

What is upthrust  ?

Q 11 | Page 123

Name the principle which gives the magnitude of buoyant force acting on an object immersed in a liquid.

Q 12 | Page 123

lire relative density of mercury is 13.6. What does this statement mean ?

Q 13 | Page 123

What name is given to thrust per unit area ?

Q 14 | Page 123

What is the ss ientific name of the upward force acting on an object immersed in a liquid ?

Q 15 | Page 123

What is meant by the term ‘buoyancy’ ?

Q 16 | Page 123

What causes buoyant force (or upthrust) on a boat ?

Q 17 | Page 123

Why does ice float in water ?

Q 18 | Page 123

What force acting on an area of 0.5 m2 will produce a pressure of 500 Pa ?

Q 19 | Page 123

An object of weight 200 N is floating in a liquid. What is the magnitude of buoyant force acting on it ?

Q 20 | Page 123

Name the scientist who gave he magnitude of buoyant force acting on a solid object immersed in a liquid.

Q 21 | Page 123

The density of gold is 19 g/cm3. Find the volume of 95 g of gold.

Q 22 | Page 123

What is the mass of 5 m3 of cement of density 3000 kg/m3 ?

Q 23 | Page 123

What is the density of a substance of mass 100 g and volume 10 cm3 ?

Q 24 | Page 123

Why does a block of wood held under water rise to the surface when released ?

Q 25 | Page 123

The density of a body is 800 kg/m3. Will it sink or float when dipped in a bucket of water ? (Density’ of water = 1000 kg/m3).

Q 26.1 | Page 123

Fill in the following blank with suitable word :

Force acting on a unit area is called…………………..

Q 26.2 | Page 123

Fill in the following blank with suitable word :

It is the……………. force which makes objects appear lighter in water.

Q 26.3 | Page 123

Fill in the following blank with suitable word :
A heavy ship floats in water because its……………………. density is less than that of water.

Q 26.4 | Page 123

Fill in the following blank with suitable word :

In fluids (liquids and gases), pressure acts in……………………….. directions, and pressure………………… as the depth increases.

Q 26.5 | Page 123

Fill in the following blank with suitable word :

In order to sink in a fluid, the density of an object must be……………………… than the………………… of the fluid.

Q 26.6 | Page 123

Fill in the following blank with suitable word :

Snow shoes work by spreading out a person’s……………………… over a much bigger………………………..

Q 26.7 | Page 123

Fill in the following blank with suitable word :

If the area of a snow shoe is five times……………………. than the area of an ordinary shoe, then the pressure of a snow shoe on the snow is five times……………………

Q 27.1 | Page 124

What is the difference between the density and relative density of a substance ?

Q 27.2 | Page 124

If the relative density of a substance is 7.1, what will be its density in SI units ?

Q 28 | Page 124

Define thrust. What is its unit ?

Q 29 | Page 124

A mug full of water appears light as long as it is under water in the bucket than when it is outside water.
Why ?

Q 30 | Page 124

What happens to the buoyant force as more and more volume of a solid object is immersed in a liquid ? When does the buoyant force become maximum ?

Q 31 | Page 124

Why do we feel light on our feet when standing in a swimming pool with water up to our armpits ?

Q 32 | Page 124

Explain why, big boulders can be moved easily by flood.

Q 33 | Page 124

An iron nail sinks in water but it floats in mercury. Why ?

Q 34 | Page 124

Explain why, a piece of glass sinks in water but it floats in mercury.

Q 35 | Page 124

Steel sinks in water but a steel boat floats. Why ?

Q 36 | Page 124

Explain why, school bags are provided with wide straps to carry them.

Q 37 | Page 124

Why does a sharp knife cut objects more effectively than a blunt knife ?

Q 38 | Page 124

Explain why, wooden (or concrete) sleepers are kept below the railway line.

Q 39 | Page 124

Explain why, a wide steel belt is provided over the wheels of an army tank.

Q 40 | Page 124

Explain why, the tip of a sewing needle is sharp.

Q 41 | Page 124

When is the pressure on the ground more—when a man is walking or when a man is standing ? Explain.

Q 42 | Page 124

Explain why, snow shoes stop you from sinking into soft snow.

Q 43 | Page 124

Explain why, when a person stands on a cushion, the depression is much more than when he lies down on it.

Q 44 | Page 124

Use your ideas about pressure to explain why it is easier to walk on soft sand if you have flat shoes rather than shoes with sharp heels.

Q 45 | Page 124

Explain why, a nail has a pointed tip.

Q 46 | Page 125

Explain why, buildings and dams have wide foundations.

Q 47 | Page 125

Why does a ship made of iron and steel float in water whereas a small piece of iron sinks in it ?

Q 48 | Page 125

Why do camels have large flat feet ?

Q 49.1 | Page 125

Name the force the upward push of water on a submerged object .

Q 49.2 | Page 125

Name the force which wears away two surfaces as they move over one another.

Q 49.3 | Page 125

Name the force which pulled the apple off Isaac Newton’s tree.

Q 49.4 | Page 125

Name the force which stops you falling through the floor.

Q 50 | Page 125

A pressure of 10 Pa acts on an area of 3.0 m2. What is the force acting on the area ? What force will be exerted by the application of same pressure if the area is made one-third ?

Q 51 | Page 125

A girl is wearing a pair of flat shoes. She weighs 550 N. The area of contact of one shoe with the ground is 160 cm2. What pressure will be exerted by the girl on the ground if she stands on two feet ?

Q 51 | Page 125

A girl is wearing a pair of flat shoes. She weighs 550 N. The area of contact of one shoe with the ground is 160 cm2. What pressure will be exerted by the girl on the ground if she stands on one foot ?

Q 52 | Page 125

Calculate the density of an object of volume 3 m3 and mass 9 kg. State whether this object will float or sink in water. Give reason for your answer.

Q 53 | Page 125

An object weighs 500 grams in air. This object is then fully immersed in water. State whether it will weigh less in water or more in water. Give reason for your answer.

Q 54.1 | Page 125

Write down an equation that defines density.

Q 54.2 | Page 125

5 kg of material A occupy 20 cm3 whereas 20 kg of materia! B occupy 90 cm3. Which has the greater density : A or B ? Support your answer with calculations.

Q 55.1 | Page 125

Define buoyant force. Name two factors on which buoyant force depends.

Q 55.2 | Page 125

What is the cause of buoyant force ?

Q 55.3 | Page 125

When a boat is partially immersed in water, it displaces 600 kg of water. How much is the buoyant force acting on the boat in newtons ? (g = 10 m s-2)

Q 56.1 | Page 125

State the principle of flotation.

Q 56.2 | Page 125

A floating boat displaces water weighing 6000 newton.

What is the buoyant force on the boat ?

Q 56.2 | Page 125

A floating boat displaces water weighing 6000 newton.
What is the weight of the boat ?

Q 57.1 | Page 125

Define density. What is the SI unit of density ?

Q 57.2 | Page 125

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

Q 57.3 | Page 125

The density of turpentine is 840 kg/m3. What will be its relative density ? (Density of water = 1000 kg/m3)

Q 58.1 | Page 125

Define pressure.

Q 58.2 | Page 125

What is the relation between pressure, force and area ?

Q 58.3 | Page 125

Calculate the pressure when a force of 200 N is exerted on an area of 5 m .

Q 58.3 | Page 125

Calculate the pressure when a force of 200 N is exerted on an area of 10 m2

Q 59.1 | Page 126

What are fluids ?

Q 59.1 | Page 126

Name two common fluids.

Q 59.2 | Page 126

State Archimedes principle.

Q 59.3 | Page 126

When does an object float or sink when placed on the surface of a liquid ?

Q 60.1 | Page 126

How does a boat float in water ?

Q 60.2 | Page 126

A piece of steel has a volume of 12 cm3, and a mass of 96 g. What is its density in kg/m3 ?

Q 60.2 | Page 126

A piece of steel has a volume of 12 cm3, and a mass of 96 g. What is its density in g/cm3 ?

Q 61 | Page 126

An elephant weighing 40,000 N stands on one foot of area 1000 cm2 whereas a girl weighing 400 N is standing on one ‘stiletto’ heel of area 1 cm2.

(a) Which of the two, elephant or girl, exerts a larger force on the ground and by how much ?
(b) What pressure is exerted on the ground by the elephant standing on one foot ?
(c) What pressure is exerted on the ground by the girl standing on one heel ?
(d) Which of the two exerts larger pressure on the ground : elephant or girl ?
(e) What is the ratio of pressure exerted by the girl to the pressure exerted by the elephant ?

Q 72 | Page 127

If two equal weights of unequal volumes are balanced in air, what will happen when they are completely dipped in water ? Why ?

Q 73 | Page 127

Two different bodies are completely immersed in water and undergo the same loss in weight. Is it necessary that their weights in air should also be the same ? Explain.

Q 74 | Page 127

A body floats in kerosene of density 0.8 x 103 kg/m3 up to a certain mark. If the same body is placed in water of density 1.0 x 103 kg/m3, will it sink more or less ? Give reason for your answer.

Q 75 | Page 127

Giving reasons state the reading on a spring balance when it is attached to a floating block of wood which weighs 50 g in air.

Q 76 | Page 127

If a fresh egg is put into a beaker filled with water, it sinks. On dissolving a lot of salt in the water, the egg begins to rise and then floats. Why ?

Q 77.1 | Page 128

A beaker full of water is suspended from a spring balance. Will the reading of the balance change if a cork is placed in water ?

Q 77.2 | Page 128

A beaker full of water is suspended from a spring balance. Will the reading of the balance change if a piece of heavy metal is placed in it ?

Q 78 | Page 128

When a golf ball is lowered into a measuring cylinder containing water, the water level rises by 30 cm3 when the ball is completely submerged. If the mass of ball in air is 33 g, find its density.

Q 79.1 | Page 128

A boy get into a floating boat .

What happens to the boat ?

Q 79.2 | Page 128

A boy get into a floating boat.

What happens to the weight of water displaced ?

Q 79.3 | Page 128

A boy get into a floating boat.

What happens to the buoyant force on the boat ?

Q 80 | Page 128

A y kg sheet of tin sinks in water but if the same sheet is converted into a box or boat, it floats. Why ?

## Lakhmir Singh solutions for Class 9 Science chapter 3 - Gravitation

Lakhmir Singh solutions for Class 9 Science chapter 3 (Gravitation) include all questions with solution and detail explanation. This will clear students doubts about any question and improve application skills while preparing for board exams. The detailed, step-by-step solutions will help you understand the concepts better and clear your confusions, if any. Shaalaa.com has the CBSE Science for Class 9 Part 1 Physics solutions in a manner that help students grasp basic concepts better and faster.

Further, we at shaalaa.com are providing such solutions so that students can prepare for written exams. Lakhmir Singh textbook solutions can be a core help for self-study and acts as a perfect self-help guidance for students.

Concepts covered in Class 9 Science chapter 3 Gravitation are Universal Law of Gravitation, Relative Density, Archimedes' Principle, Thrust and Pressure - Why Objects Float Or Sink When Placed on the Surface of Water?, Thrust and Pressure - Pressure in Fluids, Thrust and Pressure - Buoyancy, Mass and Weight, Free Fall - Motion of Objects Under the Influence of Gravitational Force of the Earth, Free Fall - to Calculate the Value of g, Concept of Free Fall, Importance of the Universal Law of Gravitation, Gravitation.

Using Lakhmir Singh Class 9 solutions Gravitation exercise by students are an easy way to prepare for the exams, as they involve solutions arranged chapter-wise also page wise. The questions involved in Lakhmir Singh Solutions are important questions that can be asked in the final exam. Maximum students of CBSE Class 9 prefer Lakhmir Singh Textbook Solutions to score more in exam.

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