#### Chapters

Chapter 2: Force and Laws of Motion

Chapter 3: Gravitation

Chapter 4: Work and energy

Chapter 5: Sound

#### Lakhmir Singh Science Class 9 Part 1 Physics

## Chapter 3: Gravitation

#### Chapter 3: Gravitation solutions [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 :

Tire 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.

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 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 x 10^{30} kg and the mass of earth is 6 x 10^{24} If the average distance between the sun and the earth be 1.5 x 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 =8 m/s^{2})

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/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 =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 eart ?

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

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 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 ?

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

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}, 9.2.3 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) 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.

#### Chapter 3: Gravitation solutions [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 statements is true or false :

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

State whether the following statements 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.

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

What name is given to thrust per unit area ?

What is the ss ientific 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 following blank with suitable word :

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

Fill in the following blank with suitable word :

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

Fill in the following blank with suitable word :

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

Fill in the following blank with suitable word :

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

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.

Fill in the following blank with suitable word :

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

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……………………

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 .

Name two common fluids.

What are 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}.

(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 ?

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 y 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 Science Class 9 Part 1 Physics

#### Textbook solutions for Class 9

## 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.

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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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