# Gravitation Class 9 Science Exam Questions

Please refer to Gravitation Class 9 Science Exam Questions provided below. These questions and answers for Class 9 Science have been designed based on the past trend of questions and important topics in your class 9 Science books. You should go through all Class 9 Science Important Questions provided by our teachers which will help you to get more marks in upcoming exams.

## Class 9 Science Exam Questions Gravitation

Class 9 Science students should read and understand the important questions and answers provided below for Gravitation which will help them to understand all important and difficult topics.

#### 1 Mark Questions

Question. Define thrust.
Answer. Force acting perpendicular to a surface is called thrust.

Question. State a condition for an object to float when placed on the surface of water.
Answer. A body will float if its density is less than that of water.

Question. Explain why sheet of paper falls slower than a coin under gravity through air.
Answer. A sheet of paper has greater surface area, therefore air resistance is more , so it falls slower.

Question. Why does a nail sink in water but a piece of cork floats on it?
Answer. The density of cork is less than water whereas nail has greater density. So cork floats while nail sinks.

Question. A coin sinks when placed on the surface of water. Give reason.
Answer. The coin sinks as its density is greater than water.

Question. State and define SI unit of pressure.
Answer. pressure is defined as thrust per unit area. the unit of pressure is pascal is the pressure exerted when 1 N force acts as a surface area of 1m2

Question. How is pressure related to the thrust exerted on a surface?

Question. Why do buildings have wide foundation?
Answer. Buildings have wide foundations to reduce the pressure on ground. Since larger the area, lesser will be the pressure.

Question. How does the density of fluid affect the magnitude of buoyancy acting on an object immersed in it?
Answer. Weight of fluid displaced = pgV where p is the density. Hence bouyant force increases within increasing density of fluid.

Question. What is value of gravitational force, if distance between two objects is more?
Answer. If distance between two objects is more, force exerted will be less and vice versa.

Question. What is value of gravitational constant G?
Answer. G is Universal gravitational constant = 6.673 x 10-11 Nm2kg-2

Question. Which force is responsible for moon revolving round the earth?
Answer. Gravitational force is responsible for moon revolving around the earth.

Question. State Archimedes principle.
Answer. Archimedes Principle: When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

Question. Give two applications of Archimedes principle.
Answer. It is used in (i) designing ships and (ii) submarines.

Question. Give the SI unit of relative density.
Answer. Relative density has no units.

Question. Relate relative density of a substance to its density.
Answer. Relative density = density of substance / density of water

Question. An object of 40 N weight when immersed in water losses 10 N weight. Will the object float or sink? Why?
Answer. The object will sink as weight of object is greater than the buoyant force.

Question. Density of a solid is 7.5g / cm3 . What is its relative density?
Answer. Relative density = density of substance / density of water
= 7.5/1 = 7.5

Question. State the SI unit of buoyancy.

Question. School bags have broad straps.
Broad straps have greater area and therefore exert lesser pressure.

Answer. Greater the area, less is the pressure exerted. The broad base of dams reduce pressure on the ground.

Question. It is easier to cut an apple with sharp edge of knife.
Answer. Sharp knives have very small area on which the force acts. Hence , pressure is much more and can easily cut an apple.

Question. A mug appears lighter inside water.
Answer. The mug appears lighter as the buoyant force acting in the upward direction reduces the effective weight of the mug when placed in water.

Question. A solid exerts pressure of 20 Pa on a surface of 2 m2. Find its weight.
Answer. Weight = Pressure X Area
= 20 x 2 = 40 N

Question. Relative density of an object is 1.35. Will it float or sink in water?
Answer. As relative density is greater than 1 , the object will sink.

Question. “A coin sinks in water because upthrust acting on it is greater than its weight.” Is the statement correct? If not, correct it.
Answer. The statement is wrong. The correct statement will be ” A coin sinks in water because upthrust acting on it is less than its weight”.

Question. Relate the SI unit of density with its CGS unit.
Answer. 1000 kg / m3 = 1gm / cm3

Question. A body is immersed once in each of the following media:
mustard oil
water
glycerine
petrol
In which ease will there be maximum apparent loss of weight and why?

Answer. Glycerine has highest density so maximum apparent loss will be seen when body is immersed in glycerine.

Question. Differentiate between force and thrust.
Answer. Force can have any direction, but thrust is that force which is acting perpendicular to a surface.

#### 2 Marks Questions

Question. An object is suspended with a string which gets stretched. When the object is completely immersed in water, the extension of thread decreases. Explain why it happens.
Answer. The buoyant force exerted by a fluid is equal to the weight of fluid displaced. As a body is lowered in a fluid ,more is the fluid displaced, more is the buoyant force. This buoyant force is maximum when a body is completely immersed in a fluid , The extension of thread keeps decreasing as the net force deceases.

Question. A crumpled sheet of paper falls down faster than a plane sheet, Why?
Answer. Air resistance on sheet of paper will be more then the crumpled sheet as its surface area is greater. The net force acting on it is nearly zero so it moves down slowly.

Question. What factors affecting Air resistance?
1. More the mass, lesser the effect of air resistance.
2. More the surface area of object, more the effect of air resistance.

Question. During a free fall, will heavier objects accelerate more than lighter ones?
Answer. No, during a free fall, heavier objects will not accelerate more than lighter ones because acceleration does not depend on mass of object.

Question. The density of gold is 19 g/cm3. Find the volume of 95 gm of gold.
Answer. Given : Density of gold r = 19 g/m2
mass of gold m = 95 g
then volume
D = m/v
V = M/D = 95/19
V = 5 cm2

Question. Explain why, if a stone held in our hand is released, it falls towards the earth.
Answer. If a stone held in our hand is released, it falls towards the earth because of gravitational force. This force acts between earth and stone, that’s why it falls towards the earth.

Question. Mark the direction of weight of the body and upthrust acting on it, in the following diagram:

FB = bouyant force
W = Weight of object

Question. An object is put one by one in three liquids having different densities. The object floats with 1/9, 2/11 and 3/7 parts of their volumes outside the liquid surface in liquids of densities d1, d2 and d3 respectively. Arrange the densities in ascending order of their magnitude.
Answer. 1/9 = 0.11; 2/11 = 0.18; = 3/7 = 0.43
There are the volume outside of liquids of densities d1, d2, d3 respectively. This means buoyant face is
maximum for d3 and minimum for d2. As buoyant force is proportional to density, d1< d2 < d3.

Question. An object weights 10 N in air. When immersed fully in water, it weights 8 N. Find the weight of liquid displaced by the object.
Answer. Weight of object in air = 10 N
Weight of object in fluid = 8 N
Therefore, Loss of weight = (10 – 8) N = 2 N
Buoyant force = Loss of weight = 2 N

Question. State the principle of floatation for an object of weight W immersed in a fluid.
Answer. Archimedes Principle states when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

Question. (a) State the principle on which the working of a hydrometer is based.
(b) A sharp knife is more effective than a blunt knife. Why ?

Answer. (a) Archimedes Principle : When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
(b) A sharp knife has a smaller area of cross section on which force acts. Thus pressure on sharp knife is more and it is more effective than a blunt knife.
As, Pressurea 1/area

Question. What happens when:
(a) Buoyant force exerted by the fluid is greater than the weight of the body?
(b) Buoyant force exerted by the fluid is equal to the weight of the body?

Answer. (a) The body rises to the surface of fluid.
(b) The body floats.

Question. Why are railway tracks laid on large sized concrete sleepers? Explain.
Answer. Railway tracks are laid on area on large sized concrete slabs to increase the area on which the weight of train will act and hence decrease pressure on the ground.

Question. Two blocks, one of iron and other of wood are immersed in water at same depth. Which one will come upward. Why?
Answer. The wood block will come up because density of wooden block is less than water, where as iron has greater density than water.

Question. Define relative density of a substance. Relative density of silver is 10.8. The density of water is 1000 kg / m3. What is the density of silver in SI units?
Answer. Relative density is defined as the ratio of density of substance to that of water.
10.8 = density of silver/1000
therefore density of silver = 10.8 x 103 kg/m3

Question. Which will exert more pressure 100 kg mass on 10m2 or 50 kg mass on 4m2 ? Give reason.
Answer. P1 = 100/10 = 10 Pa.
P2 = 50/4 = 125 Pa.
The second mass will exert greater pressure.

Question. (a) Explain why a truck or a motor bus has much wider tyres ?
(b) Why do we feel lighter when we swim?

Answer. (a) Bus or truck have wider tyres as they carry heavier loads. To reduce pressure on the ground, the tyres are kept wider.
(b) We feel lighter when we swim due to the buoyant force. Our apparent weight is actual weight.

Question. Account for the statement “Camel walks easily on sand but it is difficult for a man to walk on sand though a camel is much heavier than a man”.
Answer. Camels are much heavier than men but due to their wide feet the pressure exerted on sand is much less, making it easier for them to walk on it.

Question. Differentiate between density and relative density.
Answer. Density is defined on mass per unit volume. Its unit kg/m3 .
Relative density is the ratio of density of substance to that of water. It is a unit less quantity.

Question. Lead has greater density than iron and both are denser than water. Is the buoyant force on a lead object greater than, or lesser than or equal to the buoyant force on an iron object of the same volume?

Answer. Buoyant force depends on the volume of fluid displaced. It a lead and iron object having same volume are immersed in water, both will experience the same buoyant force.

Question. Give one example each where the same force acting on
(a) a smaller area exerts a larger pressure,
(b) a larger area exerts a smaller pressure

Answer. (a) A man standing on h is feet
(b) A man lying down

Question. State Archimedes’ principle. Write two applications of this principle.
Answer. Archimedes Principle states when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
Application :
1) Designing of ships, submarines
2) Lactometers

Question. A body of mass 20 kg is placed on a 2m2 area. Find the pressure exerted.

Question. An iron ball weights 11 kg in air and 8 kg when immersed in water. Find its relative density.

Question. Thrust of ‘F’ N is exerted on area 2A and thrust of ‘3F’ N is exerted on area A/2. Find the ratio of pressure exerted.

Question. A boy lying on a mattress, stands up on it. He observes that the mattress is now depressed deeper down. Why does this happen?
Answer. The mattress is depressed deeper when the boy stands as pressure increase when area on which
the weight acts becomes smaller. As pressure. As pressure a(1/area) the pressure on the mattress increases

Question. What are fluids? How does upthrust exerted by a fluid on an object immersed in it vary with density of fluid?
Answer. Fluids are matter that can flow i.e. liquids and gases.
upthrust = weight of fluid displaced
= volume of body x density of fluid x acc. due to gravity.
Greater the density, greater will be the upthrust.

Question. Big trucks and tractors have broad tyres. Why?
Answer. Bus or truck have wider tyres as they carry heavier loads. To reduce pressure on the ground, the tyres are kept wider.

Question. An iron nail sinks in the water. But a ship made of iron floats on water. Why?
Answer. A body sinks if its weight is greater than the buoyant force on it like the nail. The ship has a large volume immersed in water. So , buoyant force is equal to its weight. So the ship floats.

Question. Define pressure and state its SI unit. The dimensions of a metallic cuboid are 30 cm x 20 cm x 15 cm and its mass is 30 kg. If the acceleration due to
Answer. Pressure is defined as thrust per unit area. Its unit is Pascal.

Question. A bag of cotton weighs 10 kg and occupies a volume of 2m2 . Find its density. Express this density in CGS. unit.

#### 3 Marks Questions

Question. (a) Do all bodies immersed in a given fluid experience the same buoyant force? Explain.
(b) A 100 = cm3 block has a mass of 395 g. Find its relative density. (Density of water = 1 g/cm3 )

Answer. (a) Buoyant force experienced by a body depends on volume of body immersed and the density of the fluid. If the volume of the immersed part of bodies is same, they will experience the same force.

Question. What is meant by buoyancy? Why does an object float or sink when placed on the surface of a liquid?
answer. Buoyancy is the upward force experienced by an object placed in a fluid.
An object floats or sinks when placed on water depending the net force. If the weight is less than buoyant force it will float, and if more then it will sink.

Question. (a) State the SI units of Thrust and Pressure.
(b) In which situation we exert more pressure on ground when we stand on one foot or on both feet? Justify your

Answer. (a) SI unit of Thrust is Newton and of pressure is Pascal.
(b) We exert more pressure on ground when we stand on one feet than when we stand on both feet because pressure is inversely proportional to the area on which the thrust acts.

Question. Describe a simple activity to prove that objects of density less than that of the liquid float on it.
Answer. Take a block of wood and iron of same volume and put then in a liquid. It is seen that wood floats while iron black sinks. It we find the density of wood it is less than the liquid while that of iron is more. Thus, it is proved that objects having less density than liquid float on it.

Question. The volume of a bag of mass 1250 g is 150 cm3. If this bag is put on water, will it float or sink? Justify your reply. Also, find the volume of water displaced by this bag.
Answer. Density = 1250/150 = 8.33g / cm3 since the density of bag is greater than water it will sink Volume of water displaced = volume immersed x density of water
= 150 x 1 = 150 cm3

Question. A hollow plastic ball is taken to the bottom of a trough of water and released there.
(a) What happens to the ball?
(b) Give reason for this phenomenon.

Answer. (a) When the ball is released it rises to the surface.
(b) The reason being buoyant force is greater than its weight, so there is a net force in the upward

Question. 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 × 10-11 Nm2 kg-2 )
Mass of bodies M = 120 kg
m = 50 kg
Distance r = 10 m
then gravitational force according to law of gravitation is
F = GMm/r2
= 6.67 x 10-11 x 12 x 5 x 100 / 100 = 6.67 N

Question. Two balls of 1 kg each are placed with their centres 1 m apart, then they attract each other with force. Calculate the force (G = 6.67 × 10-11 Nm2 / kg2 )
Answer. Given : masses of two balls
m = 1kg
m = 1kg
and distance           r = 1m
According to law of gravitation
F = G x m1 x m2 / r2
F = 6.87 × 10-11 x 1 x 1 / (1)2
F = 6.67 × 10-11 N

Question. A stone falls from a buildings and reach the ground 2.5 s later. How high is the building ? (g = 9.8 m/s2)
Answer. Given : Initial velocity u = 0 m/s
gravity acceleration g = 9.8 m/s2
height of building h = ?
time t = 2.5 sec
from 2nd equation of motion
h = ut + (1/2) gt2
h = 0 x 25 + 1/2 x 9.8 x 2.5 x 2.5
h = × 1/2 x 9.8 × 2.5 × 2.5
h = 61.25 × 2 = 30.625 m

Question. How much would a 70 kg man weight on the moon? What would be his mass on the earth and on
the moon? (Acceleration due to gravity on moon (1.63 m/s2)
Mass of man, m = 70 kg
Gravitation acceleration, g = 1.63 m/s2
then mass on moon = 70 kg (constant everywhere)
then weight w = mg
W = 70 × 1.63
W = 114.1 N

Question. The acceleration due to gravity at the surface of the moon is 1.67 ms2 . If radius of moon is 1.74 × 105 m. Find the mass of the moon Given G = 667 × 10-11 Nm1 kg-2.
Gravity acceleration on moon g = 1.67 ms-1
Radius of the moon Rm = 1.7 × 105 m
then mass of moon = Mm = 9Rm/G
= 1.67 x (1.74 x 106)2/16.67 x 10-11
= 1.67 x 1.74 x 1.74 x 1012 / 6.67 x 10-11
= 7.58 × 1022 kg

Question. Derive the expression for attraction force of Earth on object close to its surface?
Answer. When an object is near the surface of earth, the distance between object and centre of the earth will be equal to the radius of earth (i.e. ) because the distance of object is negligible in comparison of the radius of earth.
Therefore.
F = GMEm / R2E
Therefore.
F = m (GME / R2E)
Therefore.
F = mg                          …(i)
Where, g = (GME / R2E)
g = acceleration due to gravity. Value of g = 9.8m/s2

Question. State any two daily life phenomena which are based on Archimedes’ principle. Discuss the role of Archimedes’ principle in industry and defense.
(2) Rising of helium filled balloon in air. In industry Archimedes principle helps in testing purity of materials and designing ships and submarines

Question. A box has dimensions 15 cm x 20 cm x 25 cm. Calculate pressure exerted by box if it is rested on a surface at
(a) 15 cm x 20 cm face
(b) 20 cm x 25 cm face
(c) 15 cm x 25 cm face, given mass of box = 20 kg. Arrange the pressure in ascending order of their magnitude.

Question. Define buoyant force. Name two factors on which buoyant force depends.
Answer. Fluid applies an upward force on a solid which is partially or fully submerged in liquid. This phenomenon is called buoyancy (Up thrust) and applied upward force is called buoyant force.
Two factors on which buoyant force depends:
• Volume or shape of object.
• Density of fluid in which object is immersed.

#### 4 Marks Questions

Question. Define universal law of gravitation? Name the scientist who gave this law?
Answer. The Universal law of Gravitation states that any two bodies having mass attract each other with force directly proportional to the product of their mass and inversely proportional to the square of distance between them. The force acts along the line joining the centres of the objects.

F ∝ m1m2 / d2
therefore. F = Gm1m2 / d2
Here, G is Universal gravitational constant = 6.673 x 10-11 Nm2kg-2 The value of G was found out by Henry Cavendish. Isaac newton gave this law.

Answer. We have studied three equations of motion.
v = u + at
s = u + at
s = ut + 1/2 at2
v2 = u2 + 2as
These equations can be used to calculate the value of velocity, distance, etc by replacing ‘a’ by ‘g’. Nowequations become:
v = u + gt
s = ut + 1/2 gt2
v2 =u2 + 2gs
The value of ‘g’ is taken as positive in the case of object moving towards earth and taken as negative in the case of object is thrown in opposite direction of earth.

Question. What is the relation between mass and weight of an object?
Weight of an object is the gravitational force applied by the earth on that object.
The relation between mass and weight of an object is:
W = m × g
Where
W = Weight of object
M = mass of object
g = Gravitational acceleration
The weight of a body changes from place to place, depending on mass of object.

Question. With help of two examples define Buoyancy & Buoyant Force?
Answer. Fluid applies an upward force on a solid which is partially or fully submerged in liquid. This phenomenon is called buoyancy (Upthrust) and applied upward force is called buoyant force.
Experiment – 1:
Take a empty bottle and float it on water. If we push it down in the water partially or fully and then release, bottle suddenly jumps upward. This is because the water applies an upward force on bottle.
Experiment- 2:
Hang a rubber string. Attach a stone to the rubber string. String will elongate. Submerge the stone in water – filled bucket. The elongation in string decreases. This happens due to upward buoyant force of water.

#### 5 Marks Questions

Question. The mass of sun is 2 × 1030 kg and the mass of earth is 6 × 1024 kg. If the average distance between the sun and the earth be 1.5 × 108 km. Calculate the force of gravitation between them.
mass of sun M = 2 × 1030 kg
mass of earth m = 6 × 1024 kg
Average distance r = 1.5 × 108 km
= 1.5 × 108 × 1000 m
= 1.5 × 1011 m
then gravitation force

Question. 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/s2)
Height h = 20
Initial velocity u = 0m/s
gravity acceleration g = 10 m/s2
(i) Time to reach the ground from 2nd equation of motion
h = ut + (1/2)gt2
20 = 0 × t + 1/2 × 10 × t2
20 = 5t2
t2 = 20/5 = 4
t = √4 = 2 sec
(ii) Speed when it hits the ground t = 2sec from 1th equation of motion
V = u + gt
= 0 + 10 × 2 = 20 m/s

Question. 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/s2)
Initial velocity u = +20 m/s
Find velocity = 0 m/sec
Gravity acceleration g = 9.8 m/s2
Distance s = ?
From 3rd equation of motion
V2 – u2 = 2as
02 – (+20)2 = 2 × 9.8 × s
+ 400 = 2 × 9.8 × s
s = -400/2 x 9.8 = – 20.4 m
Negative sign shows direction of displacement and gravity acceleration are opposite direction.
the distance = 20.4 m

Question. An astronaut on the moon measures the acceleration due to gravity and finds it to be 1.67ms-2 . He knows that the earth is about 80 time more massive than the moon. what is h if estimate of the radius of the earth to that of the moon? (g = 9.8 m/s2)
Gravity acceleration on moon a = 1.67 m/s2
Gravity acceleration on earth g = 9.8 m/s2
From formula of gravity acceleration
on earth g = GM/R2. . . (1)
On moon a = GMm/R2. . . (2)
From equation (1) & (2)

Question. A rectangular wooden block has mass of 4 kg the length, breath and height of the wooden block are 50 cm, 25 cm and 10 cm respectively. Find the pressure on the table top.
(a) When the wooden block is kept with its surface measuring 50 cm × 25 cm on the table
(b) When the wooden block is kept with its surface measuring 25 × 10cm on the table
Mass of wooden block m = 4kg
gravity acceleration = 10 m/s2
the weight W = m × g = 4 × 10 = 40 N
(a) Force F = 40 N
area a = 50cm × 25 cm
a = (50/100)m × (25/100)m
a = 0.5m × 0.25 m
a = 0.125 m2
then pressure = force/Area = 40/0.125 = 320 Nm
(b) Force F = 40 N
Area A= 25 cm × 10 cm
= (25/100)m x (10/100) m
= 0.25m × 0.1 m
= 0.025 m2
Pressure = force/Area = 40/0.25 = 1600Nm2

Question. If the distance between two masses is increased by a factor of S, by what factor would the mass of one of them have to be altered to maintain the same gravitational fore would this be an increase or decrease in the mass?
Let M and m be the masses of two object.
and distance between them = d
then gravitation force
F = GMm/d2 …….(1)
If the distance between two masses is increased by 5 time , then d’ = 5d
Now gravitation force –

Question. Suppose a planet exist whose mass and radius both are half those of the earth. Calculate the acceleration due to gravity on the surface of this plant.
G = 6.67 x 10-11Nm2kgr2
Mass and radius both are half those of the earth
Mass of planet

Question. The value of a g at six distances A, B, C, D E and F from the surface of the earth are found to be 3.08m / s2