Chapter : GRAVITATION

NCERT Book, Page 134

Q.1. State the universal law of gravitation.
the universe attrac ts every other body with
CAKs Aonding tu the universal lawof gravitation, every body in
masses and inversely proportional to the squan
fone whch is dirtly proportional to the produt of their
the line joining the centres of the two bodies
of the dstane etween them Ihe direction of force is along
and an object on the
Q.2. Write the formula to find the magnitude of the gravitationalforce between the earth
surface of the earth.
Mx m
Ans. Gravitathonal fore
R
where G- Gravítational constant
M =Mass of the earth
m= mass of the object
and R = Radius of the earth

NCERT Book, Page 136

Q1 hat do you mean by free fall ?
gravitational force of the eart
Ans. The falling of a bodv (or object) from a height towards the earth under the
(with no other forces acting on it) is called free fall.
Q.2 hat do you mean by acceleration due to gravity ?
constant rate. In other words, wher
Ans. When an object is dropped from some height, its velocity increases at a
an object is dropped from some height, a uniform acceleration is produced in it by the gravitational pul o
the earth and this acceleration does not depend on the mass of the falling object. The uniform accelera tior
due tc
produced in afreely falling object due to the gravitational force of the earth is known as acceleration most a
gravity (g). The value of acceleration due to gravity (g) changes slightly from place to place but for
the purposes it is taken to be 9.8 m/s².
NCERT Book, Page 138
Q.L What are the differences between the mass of an object and its weight.
Ans. See Table on page 100of this book.

Q.2 Why is the weight of an object on the moon th of its weight on the earth ?
6
Ars. The rmass and radius of moon are less than that of earth due to which the moon exerts a lesser gravitaional

force on objects (which is th of that exerted by the earth). Since the gravitational force of the moon is
6

that of the carth, therefore, the weight of an object on the moon is th of its weight on the earth
NCERT Book, Page 141
Q.1. Why is it difficult to holdaschool bag having astrap made of athin and strong string?
Fore Now, when a school bag has a strap made ot a thun string, then because
Ans. We know that: Pressure
Area
of this thin string, the weight of ashool bag (or force of school bag) will tall on avery small area of the
shoulder lor hand) of the child. This will produce a large pressure on the shoulder (or hand) of the child. The
large pressuremakes it very painful to carry the heavy shool bag
Q.2. What do you mean by buoyancy ?
Ans. Whenever an object is immersed in a liquid (fully or partially), it experiences an upward force. The upward
force is called buoyant force (or upthrust). The tendency ofa liqujd to exert an upward force on an object
placed in it, is called buoyancy. Even the gases exhibit the property of buoyancy.
240
 BOOK QUESTIONS
AT AND EXERCISES
Why does an object float or 241
When an object is put in sink when placed on the surface of water ?
() weight of the object water, then two forces act on it :
ns.

(i) buoyant force (or acting downwards (which tends to pulldown the
Now,whether arn objectupthrust) acting
will float or sinkupwards (which tends to push up
object), and
the object).
(weight and buoyant force) acting on the in water willdepend on the relative magnitudes
object in opposite directions. of these two forces
(a) If the buoyant torce (or
object wìll float in water.upthrust) exerted by water is equal to or greater than the weight of the
(b) If the buoyant force (or object, the
sink. upthrust) exerted by water is less than the weight of the
object, the object will

).1. You find your mass to be 42 kg on a

NCERT Book, Page 142
weighing machine. Is your mass more or less than 42 kg ?
Ins. Even gases (like air) exert an upward force (or
stand on a weighing machine, then the air exertsbuoy an
ant force) on the objects placed in them. Now,
when we
and makes us slightly lighter than we actually are. So, upward force (buoyant force or upthrust) on our
then our actual mass will be slightly more if a weighing machine shows our mass to be body
than 42 kg. 42 kg,
0.2. You have a bag of cotton and an iron bar, each
indicating a mass of 100 kg when measured on aweighing
machine. In reality, one is heavier than the other. Can you say which one is
Ans. We know that the density of cotton is must less than the density of iron. Due toheavier and why ?
this, 100 kg of cotton has a
much bigger volume than the same mass (100 kg) of iron. Now, due to its bigger volume, 100
will displace much more weight of air (than 100 kg of iron). Due to this, the upward force (or kg of cotton
on 100 kg of cotton will be much more than on 100 kg of iron. This means buoyant force)
that due to greater upward
buoyant force of air acting on cotton bag, the weighing machine will show a much lesser mass of cotton
(than that of iron). So, in reality, 100 kg of cotton weighed in air will be heavier than 100 kg of iron weighed
in air.

NCERT Book, Pages 143, 144 and 145

Q.1. How does the force of gravitation between two objects change when the distance between them is reduced
to half ?
Ans. The force of gravitation between two objects is inversely proportional to the square of distance between
1 1
them. That is, F . Now, when the distance between two objects is reduced to half, that is, made then

because =4
the force between them will become 4 times
2

Q2. Gravitational force acts on all objects in proportion to their masses. Why then, a heavy object does not
fall faster than a light object ?
all objects
Ans. We know that: Force= mass x acceleration. Now, it is given to us that gravitational force acts on gravity) is
only if the acceleration (due to
in proportion to their masses, that is, Force oc mass. This is possible constant, theretore, all the objects
constant for a heavy object as well as a light object. Since the acceleration is
(heavy or light) fallat the same speed.
between the earth and a 1 kg object on its surface ?
Q.3. What is the magnitude of the gravitational force earth
the is 6.4 x 106 m).
(Mass of the earth is 6 x 1024 kg and radius of
calculated by using the formula :
Ans. The magnitude of gravitational force is
F=Gx

Nm²/kg²
Now, Gravitational constant, G = 6.7 x 10-l1
Mass of earth, m= 6x 104 kg
Mass of object, my = 1kg of earth
Radius
And, Distance between centre, r=
of earth and object = 6.4 x 10º m
 242 SCIENCE FOR NINTH
Now, putting these values in the above formula, we get:
CLASS : PHYSIC
6.7 x10- x6x104x1 An

(6.4 x 10'
F= 9.8 N
Thus, the magnitude of gravitational force between the earth and a 1kg object on its
surtace is 9.8 newh
O.4. The earth and the moon are attracted to each other by
gravitational
with a force that is greater or smaller or the same as the force with force. Does the earth attract the mo
which the moon attracts the eartt
Why ?
Ans. The earth attracts the moon with the same force with Ar
which the moon attracts the earth. This is beras.
according to Newton's third law of motion, the forces of action and reaction are
So, when earth attracts the moon with a certain always equal and opposi
gravitational force, then the moon attracts the earth with
equal and opposite gravitational force.
0.5. If the moon attracts the earth, why does the earth
not move towards the moon ?
Ans. From Newton's second law of motion, it can be
concluded that the acceleration produced in a body is inversel 0.1
proportional to the mass of the body becausea= Now, due to the very large mass (m) of the earth
gravitational force (F) between the moon and the earth produces very small th A
Actually, the acceleration produced in the earth (by the attraction of moon) is so acceleration (a) in the earth
observed. And hence we do not see the earth move towards small that it cannot b
the moon.
Q.6. What happens to the force between two objects, if :
(i) the mass of one object is doubled ?
(ii) the distance between the objects is (a) doubled,
(iii) the masses of both objects are doubled ?
and (b) tripled ?
Ans. () The gravitational force between two objects is
objects. So, if the mass of one of the objects is directly
doubled,
proportional to the product of masses of the twe
then the force also gets doubled (it becomes
times).
(ii) The gravitational force between two objects is
them. inversely proportional to the square of distance betweer
(a) If the distance between the objects is
doubled (made 2 times), the force between them becomes
1
or 4 (one-fourth).
(b) If the distance between the objects is
tripled (made 3 times), the force between them will
become
or (one-ninth).
(ii) The gravitational force between two
the masses of both the objects are objects directly proportional to the product of their masses. So, if
is
2 x 2 = 4 times. doubled (made 2 times each), the force between them will
become
Q.7. What is the importance of the universal law
Ans. The importance of universal law of of gravitation ?
motion of moon around the earth; and gravitation is that it explains the motion of planets around the sun; the
the motion of
phenomena of rainfall, snowfall, and flow of water inartificial satellites around the earth. also explains the
rivers on the earth.
Q.8. What is the acceleration of free fall ?
Ans. The falling of an object from a height
towards the earth under the gravitational force of earth (with no
forces acting on it) is called free fall. The
freely faling object due to which its speed gravitational force of earth produces a uniform acceleration inother the
(which is commonly known as acceleration due goes on increasing. This is called the
to gravity). The value of acceleration free tall
of
Q.9. What do we call the
gravitational force between the earth and an object ?
acceleration of free fall is 9.8 m/s.
Ans. The gravitational force between the earth and an object is called 'earth's
Q.10. Amit buys few grams of gold at the poles as gravity'.
per the instructions of one of his friends. He
hands over the
 BOOKQUESTIONS AND
HYSIC same when he
EXERCISES
?(Hint. meets him at the 243
The value of g is equator. Will his friend agree
No,the friend at
ollows
: We equator will greater at the poles than at with tthe
know that weight, not agree with the
the weight of gold equator). at the
weight of gola bought ? If not,
a ertain mass ot
gold will Wn
be greater at
xg. Now, since the valuebought poles. This can be explained as
of gis greater at the poles, so the weight of
brought to equator, then the poles (where it iN bought), When the same mass of gold is
its weight will be
certain mass of gold bought found to be less
Why will a sheet of paper at the poles will have lesser weight because the value of gis less at the equator. Thus,
fall slower at the equator
A sheet of
paper has alarger area. Due to than one that is
its large area,crumpled
arth into a ball ?
expenences more resistance when a sheet of paper is dropped from a height, it
sheet of Paper cumpled intofronaball air, its sDeed decteascs and it
falla at aslower rate. On the
has a
si crumple sheet ot paper is dropped from asmaller area Due to its smaller area, when a hallother made
ha
romt
ith and it falls at a faster rate. height, experiences less resistance frorn air, its
it
speed ine
.
Gravitational force on the surface of moon is only 1 as strong as gravitational force on the earth. What is
the weight in newtons of a 10 kg 6
h. tans. We know that the acceleration due object on the moon and on the earth ?
to gravity on thesurface of earth is 9.8 m/s. So, the
ar 1
acceleration due to
Ot
gravity on the surface of nmoon will be of this value, that is, 9.8 x - m/s.
6
() Calculation of weight on the moon 6

Mass of object, m = 10 kg
Acceleration due to, g=9.8 x m/s
gravity on moon
tv
Now, Weight of object, W= mx8
es (on moon) =10x9.8x
6
= 16.3 N (or 16.3 newtons)
(ii) Calculation of weight on the earth
Mass of object, m = 10 kg
Acceleration due to, g = 9.8 m/s²
gravity on earth
Now, Weight of object, W= mxg
(on earth) =10 × 9.8
= 98 N (or 98 newtons)
Q.13. Aball is thrown vertically upwards with a velocity of 49 m/s. Calculate :
() the maximum height to which it rises.
(i) the total time it takes to return to the surface of the earth.
n Ans. () Please note that here the ball is going up against the gravity,so the value of acceleration due to gravity
gis to be taken as negative (with a minus sign).
Here, Initial velocity of ball, u = 49 m/s
Final velocity of ball, v= 0 (The ballstops at top)
Acceleration due to gravity, g= -9.8 m/s² (The ball goes up)
Height, h=? (To be calculated)
And,
Now, putting all these values in the formula :
2= u+ 2gh
(0)= (49) + 2 x (-9.8) x h
we get : 0= 2401 - 19.6 h
19.6 h = 2401
2401
h=
So, 19.6
h= 122.5 m
the ballrises is 122.5 metres.
Thus, the maximum height to which to reach the highest point by using the formula :
(i) We will first calculate the time taken by the ball