GRAVITATION Day 5

1. ARTIFICIAL SATELLITES

3. Aim(s)
By the end of this lesson we will come to:
discuss the importance of Newton's law of gravitation
in understanding the motion of satellites.

4. SCIENCE, TECHNOLOGY AND SOCIETY
CONNECTION
• describe how artificial satellites keep on moving around the Earth due
to gravitational force.

5. 1. An object that revolves around a planet is called
a satellite.
2. The moon revolves around the Earth so moon is
a natural satellite of the Earth.
3. Man-made objects revolve around the Earth.
These are called artificial satellites.
Important Terms

6. •Use of Artificial Satellites
•Most of the artificial satellites,
orbiting around the Earth are
used for communication
purposes.
•Artificial satellites carry
instruments or passengers to
perform experiments in space.
The orbit of artificial satellite is
called geostationary orbit.
A satellite is orbiting
around the Earth at a
height “h” above the
surface of the Earth.

7. Self-Assessment Questions.
1. The height of geostationary satellite from the surface of the earth is about:
(a) 49,260 kilometer (b) 42,300 kilometer (c) 41,000 kilometer (d) 46,300 kilometer
Answer: 42,300 kilometer
2. Weight of an object if value of g on moon is 1.6 ms-2? having mass equals 100 kg is:
(a) 100 N (b) 160 N (c) 1000 N (d) 1600 N
Answer: 160 N
3. The weight of an object in a satellite orbiting around the earth is?
(a) Greater than the actual weight (b) Actual weight (c) Zero (d) Less than the actual weight
Answer: Zero
1. Define satellites .What are natural satellites?
2. What are artificial satellites? Also write the use of artificial satellites.
3. What is meant by geostationary orbit? Also draw the figure of geostationary orbit.

8. MOTION OF ARTIFICIAL SATELLITES
• A satellite requires centripetal force that keeps it to move around the
Earth. The gravitational force of attraction between the satellite and
the Earth provides the necessary centripetal force.
• Consider a satellite of mass “m” revolving round the Earth at an
altitude h in an orbit of radius ro with orbital velocity V0 .
• The necessary centripetal force required is given by equation
o
2
0
c
r
mV
=
F

9. • This force is provided by the gravitational force of attraction between
the Earth and the satellite and is equal to the weight of the satellite
w/ (mg.).
• Thus
gR
=
V
h
+
R
=
r
r
g
=
V
r
g
=
V
r
mV
=
mg
mg
=
W
=
F
0
o
o
h
o
o
h
2
o
0
0
h
h
/
c

10. • Equation above gives the velocity, which a satellite must possess when
launched in an orbit of radius r0 = (R + h) around the Earth.
• An approximation can be made for a satellite revolving close to the Earth
such that R >> h.
• By putting the value of “g” and “R” in above equation we get the value of
“Vo”.
• A satellite revolving around very close to the Earth, has speed “Vo” nearly 8
kms-1 or 29000 kmh-1.
gR
=
V
g
=
g
R
=
h
+
R
0
h

11. Self-Assessment Questions.
1. A satellite is revolving around the sun in a circular orbit with uniform velocity v. If
the gravitational force suddenly disappears, the velocity of the satellite will be:
(a) zero (b) 2v (c) 2v (d) infinity
Answer : v
2. A satellite revolving very close to earth has a speed nearly:
( a) 7 meter per sec (b) 7 meter per sec
( c) 78 meter per sec (d) 10 meter per sec
Answer: 8 meter per sec
3. Calculate the speed of a satellite revolving around close to the Earth.

12. Main points
• 1. An object that revolves around a planet is called a satellite.
• 2. The moon revolves around the Earth so moon is a natural
satellite of the Earth.
• 3. Scientists have sent many objects into space. Some of these
objects revolve around the Earth. These are called artificial
satellites.