Gravitation
Extra Questions
Very Short Answer Type
Question 1.
Define the following terms.
(a) Freefall
(b) Acceleration due to gravity
(c) Mass
(d) Weight
(e) Density
(f) Relative density
(g) Thrust
(h) Pressure
(i) Buoyancy
Answer:
(a) Freefall: Whenever objects fall towards the Earth under gravitational force alone,
we say that the
objects are in free fall.
(b) Acceleration due to gravity The acceleration with which a body falls towards the
Earth due to Earth’s gravitational pull is known as acceleration due to gravity.
(c) Mass: Mass of a body is the quantity of matter contained in it.
(d) Weight: The weight of an object is the force with which it is attracted to the Earth.
(e) Density: The density of a substance is defined as its mass per unit volume.
(f) Relative density: The relative density of a substance is the ratio of its density to
uthat of water.
(g) Thrust: The force acting on an object perpendicular to the surface is called thrust.
(h) Pressure: The thrust per unit area is called pressure.
(i) Buoyancy: When a body is partially or wholly immersed in a fluid, an upward force
acts on it which is called upthrust or buoyant force.
Question 2.
What is the source of centripetal force that a planet requires to revolve around the
Sun? On what factors does that force depend?
Answer:
Gravitational force is the source of centripetal force that a planet requires to revolve
around the Sun. This force depends on the mass of the planet and the Sun and their
separation.
Question 3.
Suppose gravity on Earth suddenly becomes zero then in which direction will the
moon begin to move if no other celestial body affects it?
Answer:
Moon will move tangent to the point of its circular orbit.
Question 4.
Identical packets are dropped from two airplanes, one above the equator and the
other above the north pole, both at height h. Assuming all conditions are identical,
wifi those packets take some time to reach the surface of the Earth. Justify your
answer.
Answer:
No, both packets will take different time. Packet at the pole will take less time
because the acceleration due to gravity at the pole is greater than that the equator.
Question 5.
The weight of any person on the moon is about 1/6 times that on the Earth. A person
can lift a mass of 15 kg on the Earth. What will be the maximum mass, which can be
lifted by the same force applied by the person on the moon?
Answer:
The person can lift six times the weight of an object on the Earth with the same force
on the moon. Hence he can lift 90 kg on the moon.
Question 6.
How does the force of attraction between the two bodies depend on their masses
and distance between them?
Answer:
The force between two masses is directly proportional to the product of the masses
and inversely proportional to the square of the distance between them.
F = Gm1,m2r2
Question 7.
What is the value of acceleration due to gravity on the surface of the moon?
Answer:
The acceleration due to gravity on the surface of the moon is 1.6 m/s 2
Question 8.
Find the weight of a stone on the surface of Earth if its mass is 20 kg. (g -9.8 m/s 2)
Answer:
Mass of stone, m = 20 kg
Weight of stone, w = mg
or, w = 20 x 9.8
∴ w = 196 N
Question 9.
A pressure of 200 pa is acting on an area of 0.5 m 2. Find the thrust exerted on this
area.
Answer:
Given, pressure, p = 200 pa
Area,
A = 0.5 m
Thrust = pressure x area
= 200 pa x 0.5 m2
Thrust = 100 N
Question 10.
In which direction does the buoyant force on an object immersed in a liquid act?
Answer:
The buoyant force on an object acts in an upward direction.
Question 11.
What should be the value of the relative density of a substance so that it may sink in
water?
Answer:
If the relative density of the substance is greater than that of water, then it sinks in
water.
Question 12.
Name the two forces acting on an object floating on the surface of the water.
Answer:
● Gravitational force acts downward.
● The buoyant force acts upward.
Question 13.
The relative density of mercury is 13.6. What does this statement mean?
Answer:
It means that the density of mercury is 13.6 times that of water.
Gravitation Class 9 Extra Questions Short Answer Type 1
Question 1.
Write down SI unit of
(a) Universal gravitational constant
(b) Acceleration due to gravity
(c) Density
(d) Relative density
(e) Mass
(f) Weight.
Answer:
(a) Nm2/kg2
(b) m/s2
(c) kg/m3
(d) no unit
(e) kg
(f) N
Question 2.
If an iron ball exerts a gravitational force F on the rubber ball. How much force will
the rubber ball exert on the iron ball? Explain.
Answer:
The rubber ball also exerts the same force F on the iron ball. According to Newton’s
law of gravitation, both body attracts each other with equal force but in opposite
direction.
Question 3.
Write the expression of acceleration due to gravity of a body of mass m on a planet
of mass M. In which direction does It act?
Answer:
g = GMmr2
M = Mass of planet
Acceleration due to gravity acts towards the centre of the planet.
Question 4.
What is the difference between ‘g’ and ‘G’?
Acceleration due to gravity (g):
● The acceleration produced in a body falling freely under the action of the
gravitational pull of the Earth is known as acceleration due to gravity.
● The value of ‘G’ is same at every point on the Earth.
Acceleration due to gravity (G):
● The gravitational force between two bodies of unit masses separated by a init
distance is known as universal gravitational constant.
● The value of is different at different points on the Earth.
Question 5.
The moon is acted by the gravitational pull of the Earth, still, it does not fall onto the
Earth. Explain, why?
Answer:
The gravitational force of the Earth provides centripetal force to the moon for motion
around the Earth.
Question 6.
Mention any two phenomena which successfully explain the basis of the universal
law of gravitation.
Answer:
● Tides in oceans are formed due to the gravitational force between the moon
and the water in oceans,
● The atmosphere of the Earth is possible due to the gravitational force.
Question 6.
On what factors does buoyant force depend?
Answer:
Buoyant force depends on
● Density of fluid
● Volume of fluid displaced
● Acceleration due to gravity at that point
Question 8.
Distinguish between density and relative density of a substance.
Answer:
Density:
● It is defined as mass per unit volume.
● SI unit of density is kg/m3
Relative density:
● It is defined as the ratio of the density of the substance to the density of water
at 4°C.
● Unitless.
Gravitation Class 9 Extra Questions Numericals
Question 1.
The mass of the Sun is 2 x 1030 kg and that of the Earth is 6 x 1024 kg. If the average
distance between the Sun and the Earth is 1.5 x 10 11 m, calculate the force exerted
by the Sun on the Earth and also by Earth on the Sun.
Answer:
Given,
Mass of the Sun, M = 2 x 1030 kg
Mass of the Earth, m = 6 x 1024 kg
Distance between the Sun and the Earth,
r = 1.5 x 1011 m
Gravitational force between two masses is given by,
F = GMmr2
∴ F = 3.6 x 1022N
Question 2.
A planet x has a mass 2 times and radius 3 times that of the Earth. What is the
acceleration due to gravity on the planet, if the acceleration due to gravity on the
Earth is 10 m/s2.
Answer:
Given, Mass of planet, Mp = 2 Me
Radius of planet,
Acceleration due to gravity of Earth,ge = GMeR2e ………(i)
Acceleration due to gravity of planet,gp = GMpr2p=G×2Me(3Re)2
or, gp = 2GMe9R2e
∴ gp = 29 ge
∴ gp = 2.22 m/s2
Question 3.
Calculate the weight of an astronaut of mass 66 kg on the surface of the Earth and
on the surface of the moon. The acceleration due to gravity on the surface of the
moon is 1/6 times the acceleration due to gravity at the surface of the Earth.
Answer:
Given.
Mass of astronaut, m = 66 kg
Weight of astronaut on Earth, we = mge = 66 x 9.8
we = 646.8 N
Weight of astronaut on moon, Wm = mgm = mg/6=66×9.86/6
∴ Wm = 107.8 N
Question 4.
Which will exert more pressure 100 kg mass on an area of 10m 2 or 50kg mass on an
area of 4 m2? (Take g = 10 m/s2)
Answer:
Given, m1 = 100 kg
m2 =50 kg
A1 = 10 m2
A2 = 4 m2
F1 = m1 g = 1000N
F2 = m2g = 500 N
P1 = F1/A1 = 1000N10m2 = 100 pa
P2 = F2/A2= 5004m2 = 125 pa
50 kg mass on 4 m2 exerts more pressure.
Question 5.
The relative density of silver is 10.8. The density of water is 1000 kg/m 3. What Is the
density of silver in SI unit?
Answer:
Given, Relative density of silver = 10.8
The density of water 1000 kg/m3
∴ Density of silver = 10.8 x 103 kg/ms3
Gravitation Class 9 Extra Questions Short Answer Type 2
Question 1.
State following laws or principles
(a) Newton’s law of gravitation
(b) Pascal’s law
(c) Archimede’s principle
Answer:
Newton’s law of gravitation: Every particle in the universe attracts every other
particle with a force, which is directly proportional to the product of their masses and
inversely proportional to the square of distance between the two masses.
Pascal’s law: In an enclosed fluid, if pressure is changed in any part of the fluid, then
this change in pressure is transmitted undiminished to all the other parts of the fluid.
Archimede’s 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 2.
Calculate the average density of the Earth in terms of g, G, and R.
Answer:
Question 4.
Why does an object float or sink when placed on the surface of the water?
Answer:
When an object is placed on the surface of the water, two forces act on the object.
● The gravitational force in the downward direction.
● Upthrust or buoyant force in the upward direction.
When the gravitational force is greater than the upthrust or buoyant force on the
object, the object sinks in water. if the upthrust or buoyant force is greater than or
equal to the gravitational force, then the object floats.
Or
if the average density of the object is greater than the density of water, the object
sinks. If the average density of the object is less than or equal to the density of
water, the object floats.
Question 5.
Write any three applications of Archimede’s principle.
Answer:
Applications of Archimede’s principle –
● To design ships and submarines.
● To design lactometers to test the purity of milk.
● To design hydrometers to find the density of liquids.
Gravitation Class 9 Extra Questions Numericals
Question 1.
A cube of side 20 cm have a mass of 16 kg. Find its relative density.
Answer:
Given,
Side of cube, a = 20 cm = 0.2 m
Mass of cube, m = 16 kg
Question 2.
An object of mass 50 g has a volume of 10 cm3. Will the object float or sink in water?
Give reason.
Answer:
Given, Mass of an object, m = 50 g
The volume of object., u = 10 cm3
= 5 g/cm3
Here, the density of the object is greater than the density of water (1 g/cm 3), hence
the object will sink.
Question 3.
A piece of copper of mass 106 g is dipped in a measuring cylinder containing water
at 22 mL mark. The water rises to 34 mL mark.
Find (a) the volume of the copper piece (b) the density of copper.
Answer:
(a) Volume of copper piece, υ = 34 mL – 22 mL
υ = 12 mL = 12 cm3
(b) Density of copper:
Gravitation Class 9 Extra Questions Long Answer Type
Question 1.
Explain
(a) Why is the weight of a body-less at the equator than poles?
(b) Why is the weight of an object lesson a mountain than at the sea level?
(e) Why is the weight of a man less on the surface of the moon than on the surface
of Earth?
(d) Why is the weight of a body zero at the center of the Earth?
(e) Why does an iron nail sinks in water but a wooden cork floats on water?
(f) Why does a balloon filled with hydrogen gas float in the air?
(g) Why is a bucket of water lighter when in water than when it is taken out of water?
(h) Why is a sharp knife more effective than a blunt knife?
(i) Why do we feel lighter when we swim?
(j) Why does a truck or a motor bus have very wide tires?
(k) Why do nails and pins have pointed ends?
(l) Why are wide wooden sleepers kept below railway lines?
(m) Why does the foundation of a building or a dam have a large surface area?
(n) Why do skiers use flat skies to slide over snow?
(o) Why are broad handles provided in bags and suitcases?
(p) Why a camel walks easily on the sandy surface than a man?
(q) Why is it easier to swim in seawater than ordinary river water?
(r) Why does a ship made of iron float on the water while a nail made of iron sink into
water?
(s) Why does an object experience more buoyant force on the Earth than on the
moon?
(t) Why is it easy to lift a heavy stone underwater?
(u) Why are cutting and piercing tools made sharp?
Answer:
(a) Since the acceleration due to gravity at the equator is less than the acceleration
due to gravity at the poles. So the weight of the body is less at the equator than at
the poles.
(b) When we move away from the center of Earth, acceleration due to gravity
decreases (above the surface of Earth). So the weight of the object at the mountain
is less in comparison to the weight at the surface.
(c) The value of acceleration due to gravity at the surface of the moon is one-sixth
times the value of acceleration due to gravity at the surface of the Earth. So the
weight of a body on the moon is one-sixth times the weight of the body on the Earth.
(d) Acceleration due to gravity at the center of the Earth is zero. Hence the weight of
the object is zero.
(e) The density of an iron nail is greater than the density of water so, it sinks in water.
The density of a wooden cork is less than the density of water, hence it floats on
water.
Or
The weight of an iron nail is greater than the upthrust or buoyant force on the iron
nail, so it sinks in water. On the other hand upthrust or buoyant force on the cork is
equal to the weight of the cork. Hence the wooden cork floats on water.
(f) The density of hydrogen is less than the density of air.
Or
The upthrust on buoyant force acting on a hydrogen balloon is greater than the
weight of the hydrogen balloon. So it floats in air.
(g) A bucket of water appears lighter inside water due to the upthrust or buoyant
force of water.
(h) The pressure is inversely proportional to the area for the same force. Therefore
pressure exerted by a sharp knife îs more than the pressure exerted by a blunt knife.
Therefore, a sharp knife is more effective in cutting an object than a blunt knife.
(i) Due to the upthrust or buoyant force of a water, we feel light.
(j) 8y using a wide tire, the area of contact with the ground is increased and hence
the load is distributed
on a larger area to reduce pressure.
(k) Nails and pins have pointed ends so that these can be fixed with minimum force
because the pressure on the pointed ends would be large.
(l) Wide wooden sleepers are kept below railway lines to reduce pressure on the
railway tracks and prevent them from sinking into the ground.
(m) The foundation of a building or a dam has a large surface area so that the
pressure exerted on the ground is less. This is done to prevent the sinking of the
building or dam into the ground.
(n) To increase surface area and decrease pressure on snow.
(o) Using broad handles increases area and hence decreases pressure.
(p) A camel walks easily on the sandy surface because the area of the camel’s feet
is large as compared to the area of a man’s feet. So the pressure exerted by the
camel on the sandy surface is very small as compared to the pressure exerted by
the man. Therefore the feet of the camel do not sink much in the sand due to small
pressure.
(q) The density of seawater is greater than the density of river water. Upthrust or
buoyant force acting on a body is more in seawater.
(r) An iron nail sinks in water because the density of iron is greater than that of water.
So the weight of the nail is more than the upthrust of water on it. On the other hand,
a ship made of iron does not sink. This is because the ship is hollow and the empty
space contains air which makes the average density of the ship less than that of
water and hence it floats.
(s) The buoyant force acting on a body depends on acceleration due to gravity.
Acceleration due to gravity on the Earth is more than that on the moon.
(t) When a stone is placed in water, an upthrust or buoyant force acts on it due to
which its apparent weight or net downward force on it decreases.
(u) To exert more pressure with the same force as pressure is inversely proportional
to area. With increasing sharpness area decreases and hence pressure increases.
Gravitation Class 9 Extra Questions HOTS
Question 1.
The gravitational force between two identical bodies is 200 N. Mass of both bodies
becomes twice and separation becomes half. What will be a new force between
them?
Answer:
Given, Force between two bodies, F = 200 N
Force between two identical bodies, F = Gm2r2
F = Gm1m2r′2
Now the mass of both bodies becomes twice and separation becomes half, New
force
F’ = G(2m)(2m)(r/2)2=Gm2r2 = 16 F
Force becomes sixteen times.
New force, F’ = 16 x 200 = 3200 N
Question 2.
The gravitational force between two objects is 100 N. how should the distance
between the objects be changed so that force between them becomes 50 N?
Answer:
F = Gm1m2r2=100N ……….(1)
F’ = Gm1m2r2=50 ……….(2)
From (1) and (2), we get
r2r2=2
∴ r’ = 2–√r
Question 3.
A body weighs 5 kg in air and 3 kg when fully immersed in water.
1. Find the apparent loss in weight of the body.
2. The upward thrust on the body.
3. The volume of the body. (take g = 10 m/s2)
Answer:
Given, Mass of the body in the air,
ma = 5 kg
Mass of body when immersed in water,
mw = 3 kg
1. The apparent loss in weight = weight of the body in air – weight of the body in
water
= 50 N – 30 N = 20 N
Apparent loss in weight = 20 N
2. Upthrust on the body = loss in weight of the body = 20 N
3. The volume of the body = volume of liquid displaced
Question 4.
Explain why a gas bubble released at the bottom of a lake grows in size as it rises to
the surface of the lake.
Answer:
The pressure at the bottom is greater than at its surface. Therefore, as the bubble
rises up, the pressure of water on it decreases as a result its volume increases.
Question 5.
A block of wood is placed inside the water and the water is then heated. What
change will occur in the state of floatation of the block?
Answer:
The density of water decreases as temperature increases. Hence the volume of the
block immersed in water increases.
Gravitation Class 9 Extra Questions Value Based (VBQs)
Question 1.
Sumeet’s uncle often complained that the milk supplied to him was not pure. But the
man, who was supplying milk said that it was pure. Sumeet knew how to test the
purity of milk. He brought a lactometer and tested the milk.
● On what principle, does the lactometer works?
● Write down the values shown by Sumeet.
Answer:
● Lactometer works on Archimede’s principle.
● Sumeet is an intelligent boy and has good knowledge of science.
Question 2.
Raman went to Goa and he saw tides in the sea. He asked his teacher about the
reason behind the formation of tides in sea. His teacher explained the phenomenon
of the formation of tides.
Answer the following questions:
● Explain the reason for the formation of tides in the sea.
● Write down the values shown by Raman and his teacher.
Answer:
● The reason for the formation of tides in sea is the gravitational attraction of the
moon.
● Raman is a curious student and his teacher is a cooperative person.
Question 3.
Ayush went to buy a school bag. There he selected a bag with broad straps for more
comfort.
Answer the following questions on the basis of the above statement.
● Why are broad handles provided in bags and suitcases?
● Write down the values shown by Ayush.
Answer:
● Broad handles are provided in bags and suitcases so that the area of contact
increases. This reduces the pressure exerted by the weight of the bag or the
suitcase.
● Ayush is a knowledgeable boy.
