#### Chapters

## Chapter 3: Gravitation

#### Lakhmir Singh solutions for Science for Class 9 Physics Chapter 3 GravitationExercise Very Short Answers, Short Answers, Long Answers, MCQ, Hots Questions [Pages 100 - 104]

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

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

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

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

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

State the Kepler’s law which is represented by the relation r^{3} ∝ T^{2}.

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

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.

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

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

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

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

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

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

Which force is responsible for holding the solar system together ?

What is the weight of a 1 kilogram mass on the earth ? (g = 9.8 m/s^{2}).

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

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

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

If the mass of an object be 10 kg, what is its weight ? (g = 9.8 m/s^{2}).

The weight of a body is 50 N. What is its mass ? (g = 9.8 m/s^{2}).

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 ?

Write down the weight of a 50 kg mass on the earth, (g = 9.8 m/s^{2}).

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

State whether the following statement is true or false :

A falling stone also attracts the earth.

State whether the following statement is true or false :

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

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.

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.

State whether the following statement is true or false :

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

Fill in the following blank with suitable word :

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

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……………………………..

Fill in the following blank with suitable word :

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

Fill in the following blank with suitable word :

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

Fill in the following blank with suitable word :

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

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.

Explain what is meant by the equation :

`g= Gxxm/r^2`

where the symbols have their usual meanings.

What do you mean by the term free fall ?

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

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

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.

Describe how the gravitational force between two objects depends on the distance between them.

What happens to the gravitational force between two objects when the distance between them is :

(1) doubled ?

(2) halved ?

State two applications of universal law of gravitation.

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

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} Nm^{2} kg^{-2})

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 10^{24} kg; Radius of earth = 6.4 x 10^{6} m; G = 6.7 x 10^{-11} Nm^{2}/kg^{2})

The mass of sun is 2 ✕ 10^{30} kg and the mass of earth is 6 ✕ 10^{24} kg. If the average distance between the sun and the earth be 1.5 ✕ 10^{8} km, calculate the force of gravitation between them.

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/s^{2} directed towards the ground, calculate the initial velocity of the stone with which it is thrown upwards

A stone falls from a building and reaches the ground 2.5 seconds later. How high is the building ? (g =9.8 m/s^{2})

A stone is dropped from a height of 20 m.

(i) How long will it take to reach the ground?

(ii) What will be its speed when it hits the ground? (g = 10 m/s^{2})

A stone is thrown vertically upwards with a speed of 20 m/s. How high will it go before it begins to fall ? (g =9.8 m/s^{2})

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})

Write the differences between mass and weight of an object.

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

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}).

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/s^{2}).

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/s^{2}).

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

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

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

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

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

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

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

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

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

State the SI unit of acceleration due to gravity.

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

Calculate the acceleration due to gravity on the surface of a satellite having a mass of 7.4 x 10^{22} kg and a radius of 1.74 x 10^{6} m (G = 6.7 x 10^{-11} Nm^{2}/kg^{2}). Which satellite do you think it could be ?

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

The mass of a planet is 6 x 10^{24} kg and its diameter is 12.8 x 10^{3} If the value of gravitational constant be 6.7 x 10^{-11} Nm^{2}/kg^{2}, calculate the value of acceleration due to gravity on the surface of the planet. What planet could this be

An object is thrown vertically upwards with a velocity u, the greatest height h to which it will rise before falling back is given by :

u/g

u

^{2}/2gu

^{2}/gu/2g

The mass of moon is about 0.012 times that of earth and its diameter is about 0.25 times that of earth. The value of G on the moon will be:

less than that on the earth

more than that on the earth

same as that on the earth

about one-sixth of that on the earth

The value of g on the surface of the moon :

is the same as on the earth

is less than that on the earth

is more than that on the earth

keeps changing day by day

The atmosphere consisting of a large number of gases is held to the earth by :

winds

clouds

earth's magnetic field

gravity

The force of attraction between two unit point masses separated by a unit distance is called :

gravitational potential

acceleration due to gravity

gravitational field strength

universal gravitational constant

The weight of an object at the centre of the earth of radius R is :

Zero

R times the weight at the surface of the earth

Infinite

1/R

^{2}times the weight at the surface of the earth

Two objects of different masses falling freely near the surface of moon would :

have same velocities at any instant

have different accelerations

experience forces of same magnitude

undergo a change in their inertia

The value of acceleration due to gravity of earth :

is the same on equator and poles

is the least on poles

is the least in equator

increases from pole to equator

The law of gravitation gives the gravitational force between :

the earth and a point mass only

the earth and the sun only

any two bodies having some mass

any two charged bodies only

The value of quantity G in the formula for gravitational force :

depends on mass of the earth only

depends on the radius of earth only

depends on both mass and radius of earth

depends neither on mass nor on radius of earth

Two particles are placed at some distance from each other. If, keeping the distance between them unchanged, the mass of each of the two particles is doubled, the value of gravitational force between them will become :

1/4 times

1/2 times

4 times

2 times

In the relation F = G ✕ M ✕ m/d^{2}, the quantity G:

depends on the value of g at the place of observation

is used only when the earth is one of the two masses

is the greatest on the surface of the earth

is of the same value irrespective of the place of observation

The gravitational force of attraction between two objects is x. Keeping the masses of the objects unchanged, if the distance between the objects is halved, then the magnitude of gravitational force between them will become :

x/4

x/2

2x

4x

An apple of mass 100 g falls from a tree because of gravitational attraction between the earth and the apple. If the magnitude of force exerted by the earth on the apple be F_{1} and the magnitude for force exerted by the apple on the earth be F_{2}, then :

F

_{1}_{ }is very much greater than F_{2}F

_{2}is very much greater than F_{1}F

_{1}is only a little grater than F_{2}F

_{1}and F_{2}are exactly equal

According to one of the Kepler's laws of planetary motion :

r

^{2}∝ T^{3}r ∝ T

^{2}r

^{3}∝ T^{2}`r^3 ∝ 1/T^2`

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 ?

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 ?

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.

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.

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 ?

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

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/s^{2}, 9.23 m/s^{2}, 0.57 m/s^{2}, 7.34 m/s^{2}, 0.30 m/s^{2} and 1.49 m/s^{2}, respectively.

(a) Arrange 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.

#### Lakhmir Singh solutions for Science for Class 9 Physics Chapter 3 GravitationExercise Very Short Answers, Short Answers, Long Answers, MCQ, Hots Questions [Pages 123 - 128]

Write the common unit of density.

What is the density of water in SI units ?

What is the value of relative density of water ?

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

State the units in which pressure is measured.

State whether the following statement is true or false :

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

State whether the following statement is true or false :

Archimedes’ principle can also be applied to gases.

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

What is the other name of buoyant force ?

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

What is upthrust ?

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

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

What name is given to thrust per unit area ?

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

What is meant by the term ‘buoyancy’ ?

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

Why does ice float in water ?

What force acting on an area of 0.5 m^{2} will produce a pressure of 500 Pa ?

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

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

The density of gold is 19 g/cm^{3}. Find the volume of 95 g of gold.

What is the mass of 5 m^{3} of cement of density 3000 kg/m^{3} ?

What is the density of a substance of mass 100 g and volume 10 cm^{3} ?

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

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/m^{3}).

Fill in the blank with suitable word :

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

Fill in the blank with suitable word :

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

Fill in the blank with suitable word :

A heavy ship floats in water because its....................density is less than that of water.

Fill in the blank with suitable word :

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

Fill in the blank with suitable word :

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

Fill in the blank with suitable word :

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

Fill in the 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...............

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

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

Define thrust. What is its unit ?

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

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 ?

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

Explain why, a nail has a pointed tip.

Explain why, buildings and dams have wide foundations.

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

Why do camels have large flat feet ?

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

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

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

Name the force which stops you falling through the floor.

A pressure of 10 Pa acts on an area of 3.0 m^{2}. 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 ?

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 cm^{2}. What pressure will be exerted by the girl on the ground if she stands on two feet ?

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 cm^{2}. What pressure will be exerted by the girl on the ground if she stands on one foot ?

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

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.

Write down an equation that defines density.

5 kg of material A occupy 20 cm^{3} whereas 20 kg of materia! B occupy 90 cm^{3}. Which has the greater density : A or B ? Support your answer with calculations.

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

What is the cause of buoyant force ?

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})

State the principle of flotation.

A floating boat displaces water weighing 6000 newton.

What is the buoyant force on the boat ?

A floating boat displaces water weighing 6000 newton.

What is the weight of the boat ?

Define density. What is the SI unit of density ?

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

The density of turpentine is 840 kg/m^{3}. What will be its relative density ? (Density of water = 1000 kg/m^{3})

Define pressure.

What is the relation between pressure, force and area ?

Calculate the pressure when a force of 200 N is exerted on an area of 10 m^{2}

Calculate the pressure when a force of 200 N is exerted on an area of 5 m^{2} .

What are fluids ? Name two common fluids.

State Archimedes principle.

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

How does a boat float in water ?

A piece of steel has a volume of 12 cm^{3}, and a mass of 96 g. What is its density in g/cm^{3} ?

A piece of steel has a volume of 12 cm^{3}, and a mass of 96 g. What is its density in kg/m^{3} ?

An elephant weighing 40,000 N stands on one foot of area 1000 cm^{2} whereas a girl weighing 400 N is standing on one ‘stiletto’ heel of area 1 cm^{2}.

Which of the two, elephant or girl, exerts a larger force on the ground and by how much?

An elephant weighing 40,000 N stands on one foot of area 1000 cm^{2} whereas a girl weighing 400 N is standing on one 'stiletto' heel of area 1 cm^{2}.

What pressure is exerted on the ground by the elephant standing on one foot?

An elephant weighing 40,000 N stands on one foot of area 1000 cm^{2} whereas a girl weighing 400 N is standing on one ‘stiletto’ heel of area 1 cm^{2}.

What pressure is exerted on the ground by the girl standing on one heel ?

An elephant weighing 40,000 N stands on one foot of area 1000 cm^{2} whereas a girl weighing 400 N is standing on one ‘stiletto’ heel of area 1 cm^{2}.

Which of the two exerts larger pressure on the ground : elephant or girl?

An elephant weighing 40,000 N stands on one foot of area 1000 cm^{2} whereas a girl weighing 400 N is standing on one ‘stiletto’ heel of area 1 cm^{2}.

What is the ratio of pressure exerted by the girl to the pressure exerted by the elephant?

An object weighs 10 N in air. When immersed fully in a liquid, it weighs only 8 N. The weight of liquid displaced by the object will be :

2 N

8 N

10 N

12 N

A rectangle wooden block has length, breadth and height of 50 cm, 25 cm and 10 cm, respectively. This wooden block is kept on ground in three different ways, turn by turn. Which of the following is the correct statement about the pressure exerted by this block on the ground?

the maximum pressure is exerted when the length and breadth form the base

the maximum pressure is exerted when length and height form the base

the maximum pressure is exerted when breadth and height form the base

the minimum pressure is exerted when length and height form the base

An object is put in three liquids having different densities, one by one. The object floats with `1/9` , `2/11` and `3/7` parts of its volume outside the surface of liquids of densities d_{1}, d_{2} and d_{3} respectively. Which of the following is the correct order of the densities of the three liquids?

d

_{1}> d_{2}_{ }> d_{3}d

_{2}> d_{3}_{ }> d_{1}d

_{1}< d_{2}_{ }< d_{3}d

_{3}> d_{2}_{ }> d_{1}

A metal in which even iron can float is :

sodium

magnesium

mercury

manganese

Four balls, A, B, C and D displace 10 mL, 24 mL, 15 mL and 12 mL of a liquid respectively, when immersed completely. The ball which will undergo the maximum apparent loss in weight will be :

A

B

C

D

The relative densities of four liquids P, Q, R and S are 1.26, 1.0, 0.84 and 13.6 respectively. An object is floated in all these liquids, one by one. In which liquid the object will float with its maximum volume submerged under the liquid?

P

Q

R

S

A solid of density 900 kg/m^{3} floats in oil as shown in the given diagram. The oil floats on water of density 1000 kg/m^{3} as shown. The density of oil kg/m^{3} could be :

850

900

950

1050

The density of water is 1000 kg/m3 and the density of copper is 8900 kg/m3. Which of the following statements is incorrect?

`"The density of a certain volume of copper"/"The density of the same volume of water"` = 8.9

`"The volume of a certain mass of copper"/"The volume of the same mass of water"` = 8.9

`"The weight of a certain volume of copper"/"The weight of the same volume of water"` = 8.9

`"The mass of a certain volume of copper"/"The mass of the same volume of water"` = 8.9

The diagrams represent four measuring cylinders containing liquids. The mass and volume of the liquid in each cylinder are stated. Which two measuring cylinder could contain an identical liquid?

W and X

W and Y

X and Y

X and Z

Consider the following information in respect of four objects A, B, C and D :

Object |
Density(kg/m^{3}) |
Volume(m^{3}) |
Mass(kg) |

A | 2 | 4000 | |

B | 8000 | 4 | |

C | 2000 | 1000 | |

D | 4 | 2000 |

Which object would float on water ?

A

B

C

D

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

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.

A body floats in kerosene of density 0.8 x 10^{3} kg/m^{3} up to a certain mark. If the same body is placed in water of density 1.0 x 10^{3} kg/m^{3}, will it sink more or less ? Give reason for your answer.

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.

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 ?

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 ?

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 ?

Give reasons for your answer.

When a golf ball is lowered into a measuring cylinder containing water, the water level rises by 30 cm^{3} when the ball is completely submerged. If the mass of ball in air is 33 g, find its density.

A boy get into a floating boat .

What happens to the boat ?

A boy get into a floating boat.

What happens to the weight of water displaced ?

A boy get into a floating boat.

What happens to the buoyant force on the boat ?

A `1/2` kg sheet of tin sinks in water but if the same sheet is converted into a box or boat, it floats. Why?

## Chapter 3: Gravitation

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

Lakhmir Singh solutions for Science for Class 9 Physics 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 Physics solutions in a manner that help students grasp basic concepts better and faster.

Further, we at Shaalaa.com provide 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 Science for Class 9 Physics chapter 3 Gravitation are Gravitation, Importance of the Universal Law of Gravitation, Free Fall, To Calculate the Value of G, Motion of Objects Under the Influence of Gravitational Force of the Earth, Mass and Weight, Buoyancy, Pressure in Fluids, Why Objects Float Or Sink When Placed on the Surface of Water?, Archimedes' Principle, Relative Density, Universal Law of Gravitation, Thrust and Pressure.

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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