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CH#5
Gravitation
Exercise MCQS
(D)
1000km
(C)
42300km
(B)
Infinity
(A)
6400km
None
Decrease in
altitude
Increase in
altitude
1
g
4
1600N
1
g
3
1000N
1
g
2
160N
increase in
mass of the
body
2g
42300 km
6400 km
1000 km
8000ms-1
800ms-1
Q.NO.
ANSWERS
(i)
B
5.2
Ans.
5.3
Ans.
5.4
Ans.
5.5
Ans.
5.6
Ans.
5.7
Ans.
5.8
Ans.
8ms-1
(ii)
C
(iii)
D
100
850 km
Zero
(iv)
B
Questions
Earth’s gravitational force of attraction
vanishes at
Value of g increases with the
Q.No.
(i)
The value of g at a height one Earth’s
radius above the surface of the Earth is
The value of g on moon’s surface is
1.6ms-2. What will be the weight of a
100 kg body on the surface of the
moon?
The altitude of geostationary orbits in
which communication satellites are
launched above the surface of the earth
is
The orbital speed of a low orbit
satellite is
(iii)
(v)
D
(ii)
(iv)
(v)
(vi)
(vi)
D
Exercise Short Questions
What is meant by the force of gravitation?
The force due to which everybody of universe attracts every other body is called the
force of gravitation.
Do you attract the earth or the earth attracts you? Which one is attracting with a
larger force? You or the earth.
You attract the earth with the same force that the earth attracts you. However, this force
has more effect on you, because you are much lighter than Earth.
What is a field force??
The gravitational pull of Earth acting on the body whether the body is in contact with the
Earth or not is called field force.
Why earlier scientist could not guess about the gravitational force?
Because the concept of gravitation was first of all given by Newton in1665. Earlier
scientist could not guess because there was no concept of gravitation.
How can you say that gravitational force is a field force?
Gravitational force is a non-contact force. For example, the velocity of a body, thrown
up, goes on decreasing while on return its velocity goes on increasing. This is due to the
gravitational pull of the Earth acting on the body whether the body is in contact with the
Earth or not. Such a force is called the field force.
What is meant by gravitational field strength?
In the gravitational field of the Earth, the gravitational force per unit mass is called the
gravitational field strength of the Earth. Near the surface of the Earth, the gravitational
field strength is 10N kg-1.
Why law of gravitation is important to us?
1. It is the gravitational force b/w sun & all the nine planets which make them to move
around the Sun.
2. It is the gravitational force b/w Earth & Moon which makes Moon to move around the
Earth.
3. It is the gravitational pull of Earth which is responsible for all the rain & snow
towards the surface of earth.
5.9 State the law of gravitation?
Ans. “Everybody in the universe attracts every other body with a force which is directly
proportional to the product of their masses and inversely proportional to the square of the
distance between their centres.”
5.10 How mass of Earth can be determined?
Ans.
5.11 Can you determine the mass of our moon? If yes, then what you need to know?
Ans. Yes, we can determine the mass of our moon by using formula M = 𝑔𝑚 𝑅2 if we are
m
𝐺
given the radius of moon.
5.12 Why does the value of g vary from place to place?
Ans. We know g ∝ 1
R2
The value of g is inversely proportional to the square of the radius of the Earth. As we go
up the surface of the Earth, the distance from the centre of the Earth increases & hence
the value of g decreases.
5.13 Explain how the value of g varies with altitude?
Ans. It decreases with altitude.
5.14
Ans.
5.15
Ans.
5.16
Ans.
According to the above equation, we come to know that at a height equal to one Earth
radius above the surface of the Earth, g becomes one fourth of its value on the Earth.
What are artificial satellites?
Scientists have sent many objects into space. Some of these objects revolve around the
Earth. These are called artificial satellites.
How Newton’s law of gravitation helps in understanding the motion of satellites?
Newton’s law of Universal Gravitation determines the motion of satellites, enabling
scientists to calculate their position & velocity at any instant. It also controls the motion
of aircrafts travelling through the solar system to the other planets.
On what factors the orbital speed of a satellite depends?
It depends upon the gravitational acceleration and height of satellite above the
Earth’s surface.
5.17 Why communication satellites are stationed at geostationary orbits?
Ans. Communication satellites take 24 hours to complete their one revolution around
the Earth. As Earth also completes its one rotation about its axis in 24 hours
hence, these communication satellites appear to be stationary with respect to
Earth. It is due to this reason that the orbit of such a satellite is called
geostationary orbit.
PUNJAB BOARDS QUESTIONS
Q.No.1
If R is doubled, then what will be change in equation g =
Ans.
g = 𝟐𝒆
𝑹
Put R = 2R
𝑮𝑴
g/ = (𝟐𝑹)𝒆𝟐
𝑮𝑴
g/ =
/
𝑮𝑴𝒆
𝑹𝟐
.
𝑮𝑴𝒆
𝟒𝑹𝟐
𝟏 𝑮𝑴𝒆
g = (
𝟒
𝟏
𝑹𝟐
)
g/ = g
𝟒
Q.No.2 What is the numerical value of “g" at Sun & Mars?
The value of g at Sun is 274.2ms-2 & at Mars is 3.73ms-2.
Ans.
Q.No.3 What is satellite & geostationary satellite?
An object that revolves around a planet is called a satellite. The
Ans.
geostationary satellites are the artificial satellites which are placed in
geostationary orbits. The height of a geostationary satellite is about 42,300
km from the surface of the Earth. Its velocity with respect to Earth is zero.
Q.No.4 Define gravitational field?
The field in a region in space in which a particle would experience a
Ans.
gravitational force is called gravitational field.
Q.No.5 What is difference b/w natural & artificial satellite?
Difference b/w natural & artificial satellite is that natural satellite is a
Ans.
satellite that originated in space whereas artificial satellite originated on
earth & was put into orbit by humans.
Q.No.6 Write the value & unit of gravitational constant “G” in SI units?
Ans.
G = 6.673× 𝟏𝟎−𝟏𝟏 Nm2𝐤𝐠 −𝟐
Q.No.7 What are artificial satellite? Write its one use?
Scientists have sent many objects into space. Some of these objects revolve
Ans.
around the Earth. These are called artificial satellites. Most of the artificial
satellites, orbiting around the Earth are used for communication purposes.
Q.No.8 Write the value of Mass of Earth with unit?
Ans.
M = 6.0× 𝟏𝟎𝟐𝟒 kg
Q.No.9 Define weight & write its unit?
Weight of a body is equal to the gravitational force between the body and
Ans.
the Earth. Its unit is Newton (N).
Q.No.10 What is the direction of gravitational field?
A gravitational field exists all around the Earth. This field is directed
Ans.
towards the centre of the Earth.
Q.No.11 Define geostationary orbit?
The orbit in which Geostationary satellite revolves around the earth is
Ans.
called geostationary orbit.
Q.No.12 What is meant by GPS?
Global Positioning System (GPS) is a satellites navigation system. It helps
Ans.
us to find the exact position of an object anywhere on the land, on the sea or
in the air.
Q.No.13 Why we cannot feel force of gravitation around us?
Due to small value of G, the gravitational force of attraction between
Ans.
objects around us is very small and we do not feel it.
Q.No.14 What is meant by gravitational force?
Mutual force of attraction b/w objects is called gravitational force.
Ans.
Q.No.15 What is height & speed of GEO stationary satellite from the surface of
the earth?
The height of a geostationary satellite is about 42,300 km from the surface
Ans.
of the Earth. Its velocity with respect to Earth is zero.